CarTech®
CarTech®,Inc.
39966GrandAvenue
NorthBranch,MN55056
Phone:651-277-1200or800-551-4754
Fax:651-277-1203
www.cartechbooks.com
©2014byMikeMavrigian
Allrightsreserved.Nopartofthispublicationmaybereproducedorutilizedinanyformorbyanymeans,electronicormechanical,including
photocopying,recording,orbyanyinformationstorageandretrievalsystem,withoutpriorpermissionfromthePublisher.Alltext,photographs,
andartworkarethepropertyoftheAuthorunlessotherwisenotedorcredited.
Theinformationinthisworkistrueandcompletetothebestofourknowledge.However,allinformationispresentedwithoutanyguaranteeon
thepartoftheAuthororPublisher,whoalsodisclaimanyliabilityincurredinconnectionwiththeuseoftheinformationandanyimplied
warrantiesofmerchantabilityorfitnessforaparticularpurpose.Readersareresponsiblefortakingsuitableandappropriatesafetymeasures
whenperforminganyoftheoperationsoractivitiesdescribedinthiswork.
Alltrademarks,tradenames,modelnamesandnumbers,andotherproductdesignationsreferredtohereinarethepropertyoftheirrespective
ownersandareusedsolelyforidentificationpurposes.ThisworkisapublicationofCarTech,Inc.,andhasnotbeenlicensed,approved,
sponsored,orendorsedbyanyotherpersonorentity.ThePublisherisnotassociatedwithanyproduct,service,orvendormentionedinthis
book,anddoesnotendorsetheproductsorservicesofanyvendormentionedinthisbook.
EditbyPaulJohnson
LayoutbyMonicaSeiberlich
ISBN978-1-61325-207-9
ItemNo.SA334
TitlePage:Inordertoavoidappearanceissuesandtoincreasesystemlongevity,eitherceramic-coatedsystemcomponentsorusinga
100-percentstainlesssteelsystemisrequired.Theonlythingtoconsideristhataceramiccoatingprotectstheexteriorofthepipes,
butlong-termcorrosionontheinsideofthepipescantakeplace.Ahigh-gradestainlesssteelpipeprovidesanti-corrosionprotection
onbothinsideandoutsidewalls.
BackCoverPhotos
TopLeft:Top-qualityexhaustpipemanufacturerstakethetimetodeveloplocatingjigsfortheplacementofhangerrodstoprecisely
matchOEMfit.Whilecheapoffshorepipesmayormaynotfitproperly,leadingU.S.exhaustpipemakersstrivetoproducepipesthat
exactlymatchtheOEMdesigns.
TopRight:Mergecrossoversareprecisioncutforflow,thencarefullyTIGweldedtosealallseams.Thisresultsinasmoothflow
transitionwithminimalexhaustflowturbulence,incontrasttoastraightbalancepipethat’sweldedat90degreestothemainpipes.
MiddleLeft:Whenaslipjointisfabricated,theendneedstobeexpanded.Inthisphoto,a3-inchpipe’sendhasbeenexpandedto
allowaslip-fittoamufflerthatfeaturesa3-inchentrypipestub.
MiddleRight:Atimesavingapproachtoheaderprototypedesignistotackweldandassembleaseriesoftubesectionsinaspecific
vehicle,ratherthanusethetime-consumingtaskofmakingnumeroustestbends.
BottomLeft:Oncetheprimarytubesforaspecificapplicationhavebeenbenttoshapeandcuttolength,theindividualtubesare
assembledinaweldingjigthatholdsthetubesinplaceattheflangeandgroupedtoachievetheproperarrangement.
BottomRight:Thisbankwasmockedupusingplasticblocks.Shownhereisonemetalprimarytubethatwasreplicatedusingthe
modelingblocks,clampedtogether,andtackwelded.Thecollectorisattached,keepingtubesinthecorrectlocation.(PhotoCourtesy
Icengineworks)
CONTENTS
Acknowledgments
Introduction
Chapter1:Theory,DesignandPrinciples
IntakeAir
Carburetors
EFIThrottleBodies
MassAirflowSensors
FuelInjectorSizing
IntakeManifolds
CylinderHeads
Camshafts
SpecialFiring-OrderCamshafts
ExhaustManifolds
PumpingLossReduction
TheFour-StrokeEngineCycle
OxygenSensorTechnology
Chapter2:ExhaustSystemComponentDesign,FlowandFunction
ExhaustManifolds
ExhaustHeaders
ExhaustManifoldandHeaderCoatings
ExhaustPipes
Mufflers
CatalyticConverters
Chapter3:ExhaustPipe
Materials
ExhaustPipeSizing
PipeBending
PipeShape
PipeMaterialandCoatings
CrossoverPipes
ConvertingtoDualExhaust
ExhaustSystemSupports
PipeConnections
SampleExhaustSystemInstallations
2014Chevy/GMC1500Cat-BackInstallation
2011FordMustangGTXO-PipeInstallation
Chapter4:HeaderDesignandFunction
SelectionConsiderations
HeaderStyles
Materials
HeaderCoatings
PrimaryTubes
FlangeWelding
TubeBending
HeaderFabrication
PortMatchingandScavenging
GasketsandFasteners
DesignYourOwnHeaders
Chapter5:MufflersandCatalyticConverters
Mufflers
Resonators
CatalyticConverters
Cat-BackSystems
DiagnosingCatalyticConverters
TestingtheConverter
Chapter6:SuperchargedandTurbochargedSystems
Supercharging
FuelSystem
Turbocharging
EngineHigh-BoostUpgrade
TurbochargerTemperatureManagement
HeadersversusCast-IronManifolds
ExhaustPipeDiameters
TurbochargerLocation
Intercoolers
Chapter7:ExhaustSystemMath
FindingSweptVolume
CalculatingPrimaryTubeSize
PipeCFM
ExhaustWaves
PrimaryTubeLength
CollectorDiameterandLength
SourceGuide
ACKNOWLEDGMENTS
During the research for this book, a select number of exhaust industry experts were kind enough to
provideguidance.Mythankstothefollowingfortheirassistance:
George Rumore, Stainless Works; Jim Robinson, Ultimate Headers; Brian Carruth, Trick Flow
Specialties;JimLaughlin,SpiralTurboExhaust;DanLopez,CorsaPerformanceExhaust;VictorFranco,
Icengineworks; Vince Roman, Burns Stainless; Chase Knight, Crane Cams; Scott Gressman, Gressman
Powersports;TonyLombardi,RossRacingEngines;BobFall,FallAutomotiveMachine;DavidBorla,
Borla; Don Miller, Addison-McKee; Andy Shapiro, Flowmaster; Eric at Classic Chambered Exhaust;
Dan Swain, Swain Tech Coatings; Rob Griffin, Cerakote Ceramic Coatings; Christy Johnson, Cherry
Bomb; Henry Hippert, Eastern Catalytic; Bill Tichenor, Holley Performance; Cheryl Harrik, Jet Hot
PerformanceCoatings;andCustomExhaust,Wooster,Ohio.
INTRODUCTION
Theexhaustsystemisavitalcomponentofenginepackage,butalltoooftenexhaustsystemdesign,
components, and function are an afterthought. Owners arbitrarily bolt on a header, resonator, catalytic
converter, and muffler combination that may or may not be suited to the chassis or properly tuned to
deliverthebestperformance.Whenbuildingahigh-performanceengineapplication,youneedtoconsider
thefunctionandoperationasanintegratedsystem.Inparticular,thecarb,intake,andcamdirectlydictate
how much fuel is put into the engine. Moreover, the air/fuel mixture that is put into the engine must be
evacuated from the engine. The engine needs to evacuate exhaust gas from the combustion chamber
efficiently and quickly. However, it also effectively scavenges and returns unburned to the combustion
chamber during the scavenging process. An exhaust system that’s ill suited to the engine degrades
performancepureandsimple.
Thebookisyourhands-onguidetoachievetheultimateinexhaustperformance.It’snotaguidethat
provides advanced physics and flow dynamics to explain theory and function of exhaust. Instead, it
provides practical advice and easy-to-understand math so you gather all the right components for your
engine.Itallowsyoutounderstandyourengine’sexhaustsystemandselectexhaustsystemcomponents,
withregardtofunctionandengineperformanceaswellasappearance.Thediscussionsincludeexhaust
header design and sizing, exhaust pipe sizing, exhaust pipe crossovers, bending, flanges, clamping and
hanging, muffler designs, catalytic converter design, troubleshooting and break-in, exhaust component
materials and specialty coatings, exhaust system math, exhaust design and sizing for turbocharging and
supercharging,cylinderheadexhaustvalvesandportdesign,theintakesystemthataffectsexhaustoutput,
camshaftdesignthataffectstheexhaustpulse,andmore.
Completeengine-to-tipsystemsareavailableformostpopularvehicleapplications.
Choosing an aftermarket performance exhaust system, or individual system components, such as
headersormufflers,canbeadauntingtaskbecauseofthewiderangeofofferings.Thisbookmakesthe
processofselectingindividualcomponentsoracompletesystemmucheasier.Whetheryou’reassembling
an exhaust system using off the off-the-shelf components or designing an exhaust system for special
applications,thegoalofthisbookistoguideyoufromplanningstagetofinalinstallationsoyouhavea
properlydesigned,welltuned,andsafeexhaustsystem.
Specifically, this book delivers insight regarding how exhaust headers, pipes, and mufflers are
designed and manufactured, how various engine components affect the exhaust stream, and tips on
selectingsystempiecesaswellasvariousinstallationtips.Itprovidesathoroughexplanationofengine
design,function,airflow,andofcourseexhaustflow.
I delve into the principles of exhaust gas flow so you understand the function and purpose of each
component and can apply them to your particular project. I also discuss each component in the exhaust
system,itsfunction,andhowitrelatestoothercomponentsinthesystem.Ishowyoudown-to-earthmath
soyoucandeterminetheidealexhaustsystemandcomponentsforyourparticularapplication.Keepin
mindthatallcomponentsareinterrelatedandtoachievemaxperformanceallthecomponentsneedtobe
compatibleandcomplementary.
I show you how to plan, fit, bend, and fabricate pipes to fit a particular engine and chassis
combination.Exhaustgasesareroutedintotheheadersastheyimmediatelyleavetheengine,andheader
design, tube length, and tube diameter have an immense impact on performance. Resonators, catalytic
converters,andmufflersareallpartofthehigh-performanceexhaustequation,andIrevealeachpertinent
aspectofselectingthebestexhaustcomponentsforyourparticularsystem.
Chapter 1 provides a comprehensive overview and explanation of engine components that are
involvedinintakeandexhaustflow,includingdiscussionsofintakeair,carburetors,EFIthrottlebodies,
mass airflow sensors, fuel injector sizing, cylinders head design and function, camshaft technology
(including explanations of such areas as intake and exhaust valve lift, duration, lobe separation angle,
centerline, cam timing), and an explanation of special-firing-order camshafts. Also included in this
chapter are discussions of exhaust manifolds, an explanation of engine pumping losses, engine intake,
exhaust,compressionandpowerstrokeevents,andinformationregardingoxygensensors.
Chapter 2 focuses on exhaust system components, including exhaust manifolds and tubular headers,
exhaustpipe,mufflers,andcatalyticconverters.
Chapter 3 offers specific information about exhaust pipe sizing and bending, including an
understandingofvariousbendingmethodssuchascrushbending,wrinklebending,andmandrelbending,
aswellasvariousbendingmethodssuchasCNCandmachine-assistedmanualbending.Thischapteralso
includes a discussion of dual-exhaust system crossovers, exhaust pipe support hangers, and pipe
connections,aswellashelpfulinformationregardingconversionsfromasingle-toadual-exhaustsystem.
Chapter 4 contains detailed information that reveals the correct system and exhaust header type for
yourengine.Thisincludesprimarytubesizingandlength,tubingmaterialsandbending,portmatchingand
exhaustheaderscavenging,theuseandadvantagesofspecialtyexhaustheadercoatings,andmore.
Chapter5delvesintothemufflerandcatalyticconvertersthataresuitableforparticularsystemsand
applications.InthischapterIexaminespecifictechnology,materials,andintendedmufflerandcatalytic
converterapplication.
Chapter 6 covers exhaust system design and applications specifically for forced-induction
applicationssuchasturbochargingandsupercharging.
Chapter7offersavarietyofrelativemathematicalformulassoyoucandeterminethecorrectdesign,
size,andcomponentsofyourexhaustsystem.
At the end of the book, a Source Guide lists selected manufacturers of automotive performance
exhaustsystemsandcomponents.
CHAPTER1
THEORY,DESIGNANDPRINCIPLES
The components involved in the engine’s air and fuel intake, combustion, and exhaust process are
revealedinthischapter.Inordertobetterunderstandthefunctionoftheexhaustsystem,ithelpstofirst
understandhowtheairandfuelchargeenterstheengineandhowitisinfluencedbythechainofevents
that include intake air, carburetor or throttle body, intake manifold, fuel metering, cylinder heads,
camshaft,exhaustmanifoldortubularheaders,andtheengine’sintake,compression,power,andexhaust
strokes.
Aruleofthumbistodetermineengineintakevolumeandthenapproximatelymatchthisvolumeforthe
exhaust. This begins by selecting the appropriate intake manifold and carburetor size, as well as other
variablessuchascylinderheadintakeportvolume,camshaftprofile,andcompressionratio.Factorssuch
as these can affect your choice of header primary tube diameter and exhaust pipe diameter. In simple
terms,asyougeneratemorecylinderpressureandhorsepower,additionalexhaustvolumelikelyneedsto
increaseinordertoaccommodatetheengine’scapabilitytobreathe.
IntakeAir
Cool intake air is denser than warm air. Higher air density means more oxygen molecules, which
providesmoreintakeairchargeandmorepower.Obviously,themoreairyoucandrawintoanenginethe
more horsepower it generates. A cold-air intake refers to an air inlet system that feeds from air that is
coolerthantheairinsidetheenginecompartment.Thefartherawayfromtheengine’sheat,orthemore
isolatedtheairisfromengineheat,thecoolertheairthat’savailabletofeedtheengine.
Acold-airfilterconnectstotheengineintakewithducting,relocatingthefilterelementfartherawayfromtheengineheatsource,
placingthefilterclosetoacolderfresh-airsource.Cold-airsystemsareofferedforspecificvehicleapplications,withacold-air
shroudthathelpstocapturemoreairdirectlytothefilter.(PhotoCourtesyK&NEngineering)
Oneoften-ignoredaspectisusingacold-airintake.Itisanopen-elementairfilterthat’slocatedclose
totheground,butitrunstheriskofcreatingahydrolockintheengine.Thiscanoccuriftheairfilteris
exposedtoexcessivewater(forinstance,whendrivingthoughhigh-standingwaterwherewatermaybe
drawn directly into the intake system). If water enters the engine and makes its way to the combustion
chambers,itispossibleforthewatertobuildtothepointwhereitpreventsthepistonsfromreachingtop
dead center (TDC), creating a hydraulic lock. Remember that while you can compress air, you can’t
compressaliquid.Ifhydro-lockoccurs,itcantaketheengineoutinabigway,includingbentconnecting
rods,destroyedrodbearings,bustedpistons,andmore.Unlessthevehicleistobestreetdrivenonlyin
dryweather,orusedonlyonadragstrip,locatingtheairfilter(s)lowandclosetothegroundisn’tan
issue.Ifyouplantooperateinwetconditions,beawareofthis.
Theairintakesystemonthisdrag-racecar’sLS7engine,builtbyHutterEngineeringinChardon,Ohio,isbothefficientand
gorgeous.Theductingthatfeedsairtothethrottlebodiesisfabricatedusingcarbon-fibertubing.Coldairisroutedfromthenoseof
thecardirectlytotheengine,withnoengine-bayhotairenteringtheintakeairstream.
Acone-orbarrel-styleairfiltermayalsoallowestablishingacold-airintakeforuniversalapplications,suchasinracecarengine
bays.Whilestillwithintheconfinesoftheenginebay,thisfilterhasbeenlocatedfartherawayfromtheengine.
Airfilterelementsideallyshouldbeaslarge(intermsofsurfacearea)aspossible,limitedprimarilybytheconfinesoftheengine
bay.Inexpensiveairfilterssometimesprovideexcessiverestriction.Eveniftheyofferacceptableflow,cheappaperelementsmaynot
providemuchinthewayofusablelife.It’sbesttobuysuperiorfilterswhenthebudgetallows.Somefiltersaredesignedtobe
reusable;theyrequirewashinganddryingbeforeplacingthembackintoservice.(PhotoCourtesyK&NEngineering)
Theintakesystem,includingairintakeandcarburetor(orfuelinjectionsystem),hasadirectimpact
onairflowthroughtheengineandsubsequentlytheflowcapabilitiesoftheexhaustsystem.Alwayskeep
inmindthatwhatenterstheenginealsoneedstoleavetheengine.Engineanalysissoftwaretools,suchas
Ricardoandothers,provideassistanceindevelopingasystemtodeterminecrosssectionsandvolumes
fromtheairfilterinlettotheexhaustcollectoroutlet.
Carburetors
A carburetor’s job is to deliver the air/fuel mixture that the engine requires for any given state of
engineoperation.Thecarburetormixesfuelandairinthecorrectratio,orproportion.Inordertocreate
combustion,thefuelchargeneedstobemixedwithairtoatomizethemixture.
Acarburetor’sinletsystemfeaturesafuelbowl,whereaspecificamountofliquidfuelisstored.A
floatinsidethebowlfeaturesataperedneedlethatengagesintoaseatorifice.Theadjustmentlevelofthe
float,alongwithfuelpressure,maintainsthecorrectfuellevelinthebowl.Asthefuelleveldrops,the
floatdrops;thismovestheneedleawayfromitsseat,allowingfueltoenterthebowl.Whenthefuellevel
risestoitsadjustedsetting,thefloatrises,movingtheneedletowarditsseat.Thissimplefloat-activated
needle and seat system maintains the required level of fuel within the bowl, providing the adequate
amountoffueltobedeliveredtothecarburetor’smeteringsystem.
Fuelinletpressure,thepressurecreatedbythefuelpump,directlyaffectsfloatlevel.Iftoomuchfuel
pressureispresent,thefloatrises,causingtheenginetoruntoorich,withexcessfuelspilling/ventinginto
the carburetor’s air inlet. If fuel pressure is too low, the float drops and lowers the bowl fuel level,
resultingindecreasedfueldeliverytothemainjets,promotingaleancondition.
A fuel-injected system requires a high fuel pressure, often in the 30- to 40-psi range, in order to
provideinstantfueldeliverywhentheinjectorsopen.Ontheotherhand,acarburetedfuelsystemrequires
arelativelylowfuelpressure,commonlyinthe4-to6-psirange.Inacarburetedsystem,too-highfuel
pressureoverwhelmsthefloat,causingthefloattorisetoofar.Wheneveranelectricfuelpumpisused,
youneedtopayattentiontoitsratedpressureoutput.Ifthepumpisratedtoproduceinexcessofabout6
psi, a fuel pressure regulator must be installed in the fuel line. The best setup is to use an adjustable
pressureregulatoralongwithapressuregauge,toallowyoutoadjustandverifyfuelpressure.
A4-barrel,orsecondary-type,carburetoroffersmorefuelthana2-barrelwhenthesecondariesareopened.Theprimarysideisused
forlightthrottledemandsandthesecondariesdelivermaximumair/fueldeliveryforheavyacceleration.
Fromthebowlarea,fuelflowsthroughmainjets,whichcontroltheflowoffuelintothecarburetor’s
meteringsystem.Mainjetsizesarebasedonventurisize,atmosphericpressures,andambientoperating
temperatures.Theventuriprovidesapointofconstrictionforincomingair,acteduponasthedown-stroke
ofthepistonscreatesavacuumsignal.Asairrunsthroughtheventuri,itincreasesspeedandthencreates
apressuredropasitexitstheventuri.Thispressuredroppromotesfuelfromthebowlintothisvacuum
pull,pullingfuelthroughthedischargenozzleintheboostventuri.Themixtureoffuelwithairatomizes
thefuel.
Apassageinthecarburetorappliesvacuumtoapowervalvelocatedbetweenthebowlandmetering
system.Atengineidlespeed,vacuumishighest.Atidle,highvacuumkeepsthediaphragminthepower
valve closed. As the throttle opens during increased demand, vacuum drops, which allows the spring
inside the power valve to overcome vacuum and opens the diaphragm. This allows added fuel to flow
throughthevalve,richeningthefuelmixturetoaccommodatetheincreaseinthrottle-openingairdemand.
Inotherwords,whenyouopenthethrottle,thepowervalveprovidesaddedfuel.
An accelerator pump, located at the bottom of the bowl, serves as a mechanically operated fuel
injectorthatsuppliesanextrashotoffuelwhenthethrottleisopenedsuddenly/quickly.Thisaddedshotof
fuelreducesoreliminatesthechanceofastumbleorlaguponsuddenthrottleopening.Theaccelerator
pumpfeaturesaleverthatisactuatedbythethrottlelinkage.
A4-barrel,orsecondary-type,carburetorbroadensthepoweroutputpotentialbyaddingmoreairand
fueldeliverywhenenginedemandsrequiremoredelivery.Asecondary-typecarburetorisessentiallytwo
carburetorsinasinglepackage.Boththeprimaryandsecondarysidesfeaturetheirownmeteringsystems.
The primary side is used for light throttle demands; the secondary side is designed to operate when
additional or maximum air/fuel delivery is required. The secondary side activates either mechanically,
via the carburetor throttle linkage, or by vacuum, utilizing a diaphragm that opens the metering circuit
based on engine vacuum. A vacuum-operated secondary-type carburetor is more forgiving because the
secondaries open only as required according to the engine’s vacuum signal. Therefore, this type of
carburetor offers more latitude, allowing the use of a slightly larger CFM rating in contrast to a
mechanicallyoperatedsecondarythatiscontrolledbythedriver’sthrottlecontrol.
ChoosingCarburetorSize
Manyaretemptedtorunabiggercarburetor,intermsofCFMrating,assumingthatthisautomatically
resultsinaddedpower.Someevenchoosealargercarburetorsimplyforbraggingrights.Likesomany
otherenginecomponents,biggerisn’tnecessarilybetter.Thevolumeofthecarburetormustbematchedto
theengine’sneeds.Youneedtomatchthecarburetoraccordingtotheengine’svolumetricrequirements.
Volumetricefficiency(VE)referstotheengine’sabilitytobreathe.VErepresentsaratiooftheweight
oftheincomingambientairtothetheoreticalvolumeofairthattheenginecanconsumeattheanticipated
engineRPMatwhichitmakesmaximumtorque.VEisexpressedasaratioofthesetwofactors.Astock,
low-performance engine likely features about 80-percent VE at its maximum torque range, while a
modified, better breathing performance engine may feature a VE in the 85- to 90-percent range. If you
refertocarburetorCFMsizeforagivenenginedisplacementandpeak-torqueRPM,youcandetermine
whatsizecarburetorisappropriate,basedonatheoretical100-percentVE.YouthenmultiplypeakRPM
bytheengine’sVEinordertodeterminewhattheactualcarburetorsizeshouldbe.
Thischartaidsinselectingacarburetorsizeforanticipatedwide-openthrottle(WOT)attheengine’slowestRPM.(PhotoCourtesy
HolleyPerformanceProducts)
Thischartaidsincarburetorselectionbasedonengine-operatingRPM.Forexample,a400-cienginetheoreticallyrequiresabout
800cfmwhenoperatingatabout6,800rpm.Factorssuchascylinderhead,intakemanifold,andcamspecificationsarevariables
thatcomeintoplay.Thechartprovidesagoodstartingpointforcarburetorselection.(PhotoCourtesyHolleyPerformanceProducts)
Thehighertheengine’sVE,thebiggerthecarburetoritcanutilize.VEcanbeincreasedbyincreasing
theengine’sbreathing,whichcanentailusingacamshaftwithmoreduration,choosingamoreefficient
andfreer-flowingintakemanifold,improvingtheexhaustsystemflow,cylinderheadporting,andreducing
theengine’sparasiticlossesbyaccurizingallclearances,achievingpropercylinderboresurfacesfinish,
enhancedrotatingassemblybalancing,andtakingadvantageofspecializedanti-frictionandoildrain-back
coatings,etc.
Inthemostbasicterms,thelargertheenginedisplacementandthehighertheenginespeed,themore
airitcanconsume;therefore,thelargerthecarbcanbe.
SimpleCarbFormula
The following formula may be used to roughly determine carburetor size based on engine
displacementandmaximumenginespeed:
MaximumCarbCFM=(CI÷2)×(maximumRPM÷1,728)
Where:
CFM=cubicfeetperminute
CI=cubicinchesofdisplacement
RPM=revolutionsperminute(enginespeed)
1,728=mathconstant
Asanexample,a403-ciengineproducespeakpoweratamaximumof6,500rpm.Usingtheformula:
[(403÷2)×(6,500÷1,728)]201.5×3.76=757.64
Here,acarburetorsizeof700to750cfmisappropriate.
A vacuum-secondary carburetor is somewhat forgiving, allowing you to use a slightly larger
carburetor. A carburetor with mechanical secondaries is not so forgiving. If in doubt when deciding
between two sizes in the required range, it’s often better to choose the smaller carburetor. Selecting a
mechanicalsecondarycarburetorthatfeaturesadouble-pumperdesignthatistoolargefortheapplication
canresultinsagsorbogsuponaccelerationiftheengineusesupthepumpshotbeforethemainfuelshot
isdelivered.
Forced-inductionenginesutilizingeithersuperchargingorturbochargingcantakeadvantageoflarger
carburetors,sinceforced-inductioncantypicallyincreaseVEtoapointwellover100percent.
EFIThrottleBodies
EFIisanactivesystemthatadjustsfueldeliveryaccordingtotheoperatingconditionsoftheengine
while a carburetor is a passive system that controls both air and fuel intake. The engine control unit
(ECU)managesanelectronicfuel-injectionsystem’sfueldelivery,anditpreciselycontrolsfueldelivery
timinganddurationviatheinjectors.Icouldrefertoathrottlebodyastheengine’sairvalve.Thethrottle
body’sonlyjobistoallowairtoentertheengine.Intheory,themoreairdelivered,themorepowerthe
engine can produce; of course, when mixed with the appropriate ratio of fuel. Changing an original
equipmentmanufacturer’s(OEM)throttlebodyforalargeraftermarketthrottlebodyisnecessaryinorder
to introduce more air, but there is a point of diminishing returns. Going with the largest throttle body
availableisn’tnecessarilythebestmove.
FactorsofCarburetorSize
Listedherearegeneralizationsofrelativecarburetorsize(cfm)applications.
SmallerCarburetor
• SuitedformoretorqueatalowerRPMrange
• Betterforautomatictransmissionsandlowerstall-speedtorqueconverter
• Appropriateforlowercompressionratios
• Accommodateslesscamshaftduration
• Suitedforlowerdrive-gearratio
LargerCarburetor
• SuitedforhigherhorsepowerenginesathigherRPMrange
• Suitedformanualtransmissionorautomaticwithhigherstall-speedconverter
• Appropriateforgreatercamshaftduration
• Suitedforhigherenginecompressionratio
• Suitedforhigherdrive-gearratio
Throttlebodysize,intermsofthroatdiameter,needstobeproperlysizedtothedisplacementofthe
engine.Ifthethrottlebodyistoosmall,airvelocityisfaster,butvolumeisofteninsufficient.Ifthethrottle
bodyistoolarge,theairvelocityistypicallytooslow,butmoreairisdrawnintotheengine.
An accepted formula for determining throttle body size in millimeters is based on engine
displacement,measuredincubicinchesofdisplacement(ci):
ThrottleBodySize(mm)=√[(ci×196.3×RPMatmax.hp)÷67,547]
Where:
196.3=mathconstant
67,547=mathconstant
Forinstance,let’ssaythattheenginefeatures408ci,andthisengineisexpectedtoreachmaximum
horsepowerat6,500rpm.Usingtheformula:
Forthisnaturallyaspiratedengineexample,athrottlebodyfeaturingan88-to90-mmthrottlebodyis
approximately the correct size. Slightly increasing the size provides a small cushion for maximum
horsepower.Inthisexample,athrottlebodysizeofaround90mmdoesnotrestricttheairneededforthis
engineataspeedof6,500rpm.Whileyoucouldgowithanevenlargerthrottlebody,airspeed(velocity)
mayslowdownandnotfillthechambersquicklyenough.
Throttlebodiesforelectronicfuelinjectionsystemsmanageincomingaironly.Movingtoalarger-volumethrottlebodycanoften
improveperformance,aslongastheenginecanusetheincreasedairvolume.(PhotoCourtesyBBKPerformance)
If you grossly oversize the throttle body, you introduce a lot of air very quickly, which may be too
much air for the engine to handle during quick or snap-throttle activation, possibly making the vehicle
undrivableonthestreet,whileatthesametimenotimprovinghorsepower.
Ifyou’rerunningaforced-inductionsystem,throttlebodysizebecomesslightlylesscritical,sincethe
supercharger or turbocharger forces air into the engine. A Roots-style supercharger sucks air into the
enginethroughthethrottlebody,whileaturbochargerpushesairthroughthethrottlebody.Thegoalhere
isnottocreateatoo-smallbottleneckrestrictionforairflow.
Airpressure,aswellasairdensity,isapproximatelytwiceashighinaturbosetupascomparedtoa
naturallyaspiratedengine,sointheoryyoucanuseasmallerthrottlebodyforaturboascomparedtoa
naturallyaspiratedsetup.
MassAirflowSensors
A variety of methods of monitoring airflow have been used in production vehicles over the years,
including now-outdated van airflow meters, Karman Vortex airflow meters, and speed density systems
thatrelyonahostofenginesensorstodetermineairflow.Themostcommonsystemstodayuseeithera
hot-wire or a hot-film mass airflow (MAF) sensor. A hot-wire or hot-film MAF sensor allows the
electroniccontrolmodule(ECU)todirectlymeasureintakeairtemperature,humidity,andairdensity.A
hot-wire MAF sensor features a thin platinum wire located in the airstream that is heated to a
programmed temperature using a reference voltage of around 5V supplied from the ECU. As intake air
passesthewire,thewireiscooled,resultinginachangeinresistance.Asthewiretemperaturedrops,the
voltagechanges.TheECUseesthisdropandmanagesthevoltagesignalforoperatingconditions.Based
ontherequiredvoltagechanges,theECUisabletodeterminethequalityoftheairmass.
ThisMAFsensorismountedtoasectionofairintakeducting.Ahot-wireorhot-filmMAFsensorallowstheECUtodirectlymeasure
intakeairtemperature,humidity,andairdensity.Ahot-wireMAFfeaturesathinplatinumwirelocatedintheairstreamthatis
heatedtoaprogrammedtemperatureusingareferencevoltageofaround5VsuppliedfromtheECU.Asintakeairpassesthewire,
thewireiscooled,resultinginachangeinresistance.Asthewiretemperaturedrops,thevoltagechanges.TheECUseesthisdrop
andmanagesthevoltagesignalforoperatingconditions.Basedontherequiredvoltagechanges,theECUisabletodeterminethe
qualityoftheairmass.
A hot-film MAF sensor operates in a similar manner, featuring a heated film. One side of the film
maintainsaconstantreferencetemperature,whiletheothersideisexposedtointakeair.Thedifferencein
temperature allows the ECU to determine incoming air quality. The ECU uses this information, in
combinationwithothersensors,todeterminefuelinjectiondelivery.
Dirt, oil, spider webs, etc. can contaminate a MAF sensor wire (or film). If the MAF sensor is
contaminatedbyoil,thecauseisusuallyoilvaporfromthePCVsystemintermsofblowbyoilthatmakes
itswaypastthepistonrings.
AMAFsensorislocatedbetweentheairintake/filterandthethrottlebody,anditmeasurestheflowrateofairthatentersanengine.
NotetheplatinumwireinthisMAFsensor.Foreignmattereasilycontaminatesthesefragilewires.NeverassumethataMAFsensor
isfaulty.Inspectthecontactterminalstomakesurethattheconnectorisfullyseated.Thatisfirstthingyoucandotomakesureitis
operatingproperly.
It’s recommended that you clean the MAF sensor every time you service your air filter. The MAF
sensorislocatedintheengineairduct,betweentheairboxandthethrottlebody.Thesensorisgenerally
secured with two screws. Disconnect the wiring harness connector from the sensor and remove the
sensor. Do not touch the platinum wire or film with your fingers. This can leave residue on the sensor
wiresurface.CleanthesensorairpassageandwireusingonlyaMAFsensorcleanerspray.Don’tusea
brake cleaner or other type of solvent, as this can damage or further contaminate the sensor. Once the
sensorisdry,re-installit.AfouledMAFsensorcancausearoughidle,hesitation,orpoorperformance
atwide-openthrottle(WOT).
FuelInjectorSizing
The formula below that can be used to estimate fuel injector rating, based on injector pounds-perhour.Whilenothavingadirecteffectonyourexhaustsystemdesign,properlymatchinginjectorsizesto
the engine is one step in optimizing power and efficiency. In an injected system, information obtained
from the oxygen and air/fuel sensors in the exhaust stream is used by the ECU to help establish the
requiredair/fuelmixture.Iftheinjectorsarenotsizedproperlyfortheengine,efficiencyandpowerisn’t
maximized,regardlessoftheexhaustsystemdesign.Oneofthefactorsyouconsiderisbrakespecificfuel
consumption(BSFC).
Thisistheratiobetweentheengine’sfueluseandenginepoweroutput.Atengineidle,theratiois
high due to the throttle being closed. At peak torque, BSFC is low during this point of maximum fuel
efficiency.BSFCincreasesastheenginerevsbeyondpeaktorquetowardmaximumhorsepower.BSFCis
reducedasengineefficiencyincreasesduetolessfrictionandlessdrag,suchaswhentheenginefeatures
lower tension piston rings, an improved oil scavenging system and/or when drainback coatings are
appliedtothecrankshaftandconnectingrods,theuseofanelectricwaterpumptoreducecrankshaftdrag,
etc.
Another factor for calculating injector size is injector duty cycle. Most typically, this is about 80
percent.
InjectorRating=(engineHP×BSFC)÷(numberofinjectors×injectordutycycle)
For example, using the formula for an 8-cylinder engine that produces 500 hp and features a
turbocharger:
500×.5=250
8injectors×.8=6.4
250÷6.4=39.06Ibs/hr
Inthisexample,fuelinjectorswitharatingofabout39lbs/hrshouldbeadequateandprovideagood
startingpoint.IftheBSFCis.6,thesuggestedinjectorsizeisaround46lbs/hr(500×.6÷6.4=46.875).
FindingBSFC
BSFCcanonlybeaccuratelydeterminedonanenginedynamometer.However,thefollowingisa
roughestimateforillustrativepurposesoftypicalBSFCratiosforvarioustypesoffour-stroke
automotiveenginesatpeakhorsepower:
EngineType
BSFC
Fuelinjectedandlowcompression
Fuelinjectedandhighcompression
Nitrousinjection
Forced-induction
.4
.4
.5to.6
.5to.7
IntakeManifolds
Theintakemanifolddirectsairflowfromthecarburetor/throttlebodytothecylinderheadintakeports.
The choice of intake manifold style and size of the runners affects the horsepower and torque range,
influencedbyfactorssuchascrosssectionalareaandrunnerlength.Generallyspeaking,asingle-plane
intakemanifoldisbestforhigherRPMpower,whileadual-planedesignisbestsuitedforlower-RPM
andtorqueatlowerRPM.
Intermsoftheplenumlocation,againingeneralterms,tallerrunners,andhigherplenumlocationsare
bestsuitedtoaccommodatehigherengineRPMandpowerathigherRPMbands.
Youalwaysneedtobeaware;air/fuelthatcomesinmustbeabletogetoutoftheenginesoyoumust
selectyourexhaustsystemaccordingtothesefactors.Shorterexhaustprimariesdumptheengineexhaust
pulsesmorequicklytobetteraccommodatehigherRPM,whilelongerprimarytubescanimproveexhaust
scavengingtobenefitalower-RPMpowerband.Headerprimarytubediameterandlengthaffecthowthe
exhaust gases are pulled out of the cylinder head exhaust ports, as scavenging of exhaust can create a
vacuumeffect,notonlypullingexhaustoutbutaidinginpullinginadditionalintakecharge.
Asingle-planeintakemanifoldfeaturesasingleopenplenumthatfeedsalleightcylinders.Runnersaretypicallyshorterthana
dual-planemanifoldforamore-directshotateachcylinder,whichisgenerallybetterforhigher-RPMpower.Single-planemanifolds
alsousuallyfeatureabitofanairgapbetweentherunnersandblock,whichaidsinreducingtheair/fueltemperature.Asingleplanemanifoldislikelyabetterchoiceastheengineusuallyrunsaboveabout2,500rpm.Ifyouplantooperatetheengineprimarily
at2,500rpmorless,adual-planemanifoldmaybethebetterchoice.
The configuration of the intake manifold runners directly affects power and where maximum torque
occurs.Withoutdiscussingaspecificengineapplicationintermsofdisplacement,cylinderheaddesign,
and camshaft profile, I address intake manifolds in general terms. In addition to intake manifold runner
length, you need to consider runner volume, including length and cross section area. A larger crosssection area and shorter runner length is better suited to higher-RPM ranges and larger-displacement
engines, while a smaller cross-section area and longer runner length is better suited to smallerdisplacementenginesandlower-RPMranges.Mostengineintakemanifoldsareacompromisebydesign,
abalancebetweenenginepoweroutputandlow-RPMtorquetosuitetheintendedapplicationanddriving
conditions.
Adual-planeintakemanifoldfeaturesasplitplenum,providinganintakechargeevery180degreesofcrankshaftrotation.(They
areoftenreferredtoas180-degreemanifolds.)Intakerunnersareusuallylongerthanonasingle-planemanifold.Adual-planeis
generallybettersuitedtolow-endtorqueandpowerforstreetuse.Regardlessoftheapplication,payattentiontotheadvertised
engineRPMoperatingrangeforboththemanifoldandthecamshaftthatyouplantouse.Trytomatchthemanifoldtothecambased
onthoseratings.
Ifthecross-sectionareaistoolargefortheapplication,areductioninpeaktorquecanoccurandthe
RPMrangeofwherepeaktorqueiscreatedcanincrease.Onanenginedynograph,peaktorquegenerally
indicateswhereintheRPMrangeVEisthegreatest.
Notethatthesegeneralizationsapplytonaturallyaspiratedengines.Whenyouconsideranenginefed
byforcedinduction,whetherthatinvolvesaturbochargerorsupercharger,intakemanifoldrunnerdesign
becomes less critical because the air charge is forced into the cylinders under a positive pressure.
Matching the intake manifold runners to the selected cylinder heads is the goal, in terms of port crosssectionarea,inordertomaximizeefficiencyintermsofairflow.
Port matching (where the intake manifold exit ports match the size and shape of the cylinder head
intakeports)isanimportantsteptomaximizeairflow.Theintentistopreciselymatethelocation,shape,
andsizeoftheintakemanifold’sairexittothecylinderheadintakeportentry.Iftheportsaremismatched
(forinstanceiftheportwallsarenotalignedoriftheintakemanifoldportsarelargerthanthecylinder
headintakeports),incomingairhitstheexposedwalls,creatingunwantedturbulence.Mountingapoorly
matched intake manifold to the best set of cylinder heads can easily reduce power and prevent taking
advantage of the engine’s maximum potential. Intake manifold runners are commonly designed with a
slighttapertopromoteairflowvelocity,enhancingairflowspeedonitswaytothecylinderhead.
Manysingle-carburetedintakemanifoldsfeatureunequal-lengthintakerunners,andthusthefrontand
rear runners are longer and narrower than the center runners, which may be shorter and wider in
comparison. In theory, this equalizes or balances the volume of air distributed to the cylinders. In
addition, airflow velocity must be considered. As a result, the shape of the intake manifold runners
(runnertaperingandthedegreeofturnsintherunner)maybetunedinordertoequalizeairflowvelocity
toallcylinders.
Becauseofitssplitdesign,adual-planemanifoldisgenerallybettersuitedtolower-RPMoperation,enhancingoveralldriveability.
Thatdoesn’tmeanthatadual-planemanifoldisn’tsuitedforhigh-performanceuse,however.Itdependsonthepowerband.In
generalterms,lower-RPMuseislikelyabetterchoiceforadual-planeandhigher-RPMuseisbetteraccommodatedwithasingleplanemanifold.
Thiscutawayviewofacarburetedintakemanifoldprovidesagoodviewoftheplenumdividers.(PhotoCourtesyHolleyPerformance
Products)
High-risetunnelrammanifoldsarebestsuitedforextendedhigh-RPMuseandprovidemoretorqueandhorsepoweroveralonger
range(duringhigherenginespeed).Modularhigh-riseintakemanifolds(oftencalledtunnelramsbecauseofthelongsingle-plane
intakerunners)areavailablethataccommodateeitherasingle4-barrelcarburetorordualcarbs.ThisexampleofaHolleyHi-Ram
intakemanifoldisfittedwithanupperplenumthatallowsmountingapairofcarburetorssideways,orin-line,dependingonthe
carburetordimensionsanddesiredthrottlelinkagesetup.
High-riseintakesystems,byvirtueoftheirdesign,raisethecarburetor(s)wellabovetheengine.Hoodclearanceissuesare
commonlysolvedbytheinstallationofatallhoodscoop.
Certainelectronicfuelinjectionintakemanifolddesignsfeatureacentralforward-mountthrottlebodyandaseriesofequal-length
intakerunnersdirectlytoeachcylinder.(PhotoCourtesyBBKPerformance)
Inadditiontocast-aluminumEFIintakemanifolds,lighter-weightmanifoldsarealsoavailablemadefromadvancedpolymer
materialforbetterheatdissipation.Becauseoftheirdesignandconstruction,thesearetwo-piecemanifoldswithlowerandupper
sections.Acoolerintakechargeprovidesadenseraircharge,whichinturnpromotesincreasedpower.Anotherbenefitofthe
polymermanifoldisitsabilityfordesignerstocreatesmootherandmore-laminarairflowmoreeasily.(PhotoCourtesyFAST/Comp
Cams)
Carburetedintakemanifoldscommonlyfeaturedividerwallsintheplenumtoaidindirectingtheair/fuelchargetothecylinders.
Thesewallsarealsodesignedtohelpinbreakingup,oratomizing,thechargetopreventcreationoflargefueldroplets.
Customsheet-metalintakemanifoldsareconstructedofbilletaluminumtubesandpanelsandwelded
together while stock manifolds are made of heavier-wall cast aluminum. Material and weld strength
becomes a critical concern when dealing with a forced-induction setup, due to the repeated cycles of
pressurizedairwithinthemanifold.Thisrequireshigh-strengthaluminum,suchas6061billet.
In addition, when using a welded intake manifold, the plenum box volume becomes critical for a
naturally aspirated application. Therefore, the plenum volume should ideally match the engine
displacement to eliminate the possibility fuel starvation for individual cylinders. For instance, if the
enginedisplacementis408ci,theplenumvolumeshouldprovide408ci.Ifplenumvolumeistoosmall,
upper-RPMpowercansuffer.Thisbecomesmoreofacriticalissueforcompetitionenginesincontrastto
street-drivenengines.
CylinderHeads
Cylinderheadsareakeycomponenttotheoverallairflowsystemofanengineandshouldbeoneof
the first hard parts to consider when creating a plan for a new engine build. They are responsible for
conditioning the air/fuel intake charge into the cylinder, aid in the conversion of chemical and thermal
energyintomechanicalenergy,andprocessspentexhaustgasesintotheengineexhaustsystem.Oncethe
cylinder heads have been determined for an engine build, the selection of support components, such as
intakemanifolds,pistons,exhaustheaders,andcamshafts,becomeslesscumbersometochoosetheright
combinationofpartstoachievethedesiredoutcome.
Each component of the engine airflow system must work together to achieve an efficient flow path,
whileminimizingflowrestrictions,basedonagivenenginevolumedisplacement.MostperformanceV-8
cylinderheadsareofoverhead-valve(OHV)design,wheretheintakeandexhaustvalvesarepositioned
abovetheenginecylinderboreformaximumintakeandexhaustefficiency.
ChamberSizeforEngineDisplacement
ThesearetypicalexamplesforGMLS–typeengineswherethecross-sectionareaoftheintakeports
hasbeenmaximizedforparticularapplications.
Engine
4.8/5.3/5.7L
Chamber(cc)
195to205
5.7L
6.0/6.2L
205to220
225to255
Asyouincreasetheintake-sideflow,youmustalsoincreasetheexhaust-sideflowtoavoidexhaust
restriction.Asyoupackincreasedairflowandfeedmorefuelintotheengine,theexhaustsystemmustbe
abletoevacuateexhaustgasesaccordingly.Theneedforaless-restrictiveexhaustsystemandtheability
for the exhaust system to pull, or scavenge, the exhaust becomes increasingly important. This is where
moving to an appropriate tubular-header system provides a distinct advantage in contrast with a set of
OEMcastexhaustmanifolds.
SelectingtheRightCylinderHeads
Intheperformanceaftermarket,thereisanoverabundanceofcylinderheadchoicesformostpopular
engine applications. Consumers can purchase complete brand-new performance aftermarket cylinder
heads or rework OEM cylinder head castings through hand porting or by high-tech CNC (computer
numericcontrol)porting.Whatevertheenginegoal,cylinderheadsmustbesizedproperlytothevolume
displacement of an engine, operate in the intended RPM range of the vehicle application, physically fit
intothedesiredchassisspace,andfunctioneffectivelywithsupportcomponentssuchasintakemanifolds,
exhaustheaders,andcamshafts.
Most aftermarket cylinder head manufacturers rate performance cylinder heads based on engine
volumedisplacement,RPM,andapplication(on-roadonly,raceonly,orboth).
Generally,asmallerintakeportvolumerangecreatestorqueatalowerRPMcomparedtoalarger
intake port volume for the same engine application. Always consult the cylinder head manufacturers’
product data for specific engine builds and support components for comparison between intake port
volumes.
IntakePort
Cylinder head intake port shape, cross-section area, and volume can be considered limiting factors
whendiscussingengineperformanceinnaturallyaspiratedapplications.Generallyspeaking,mostengines
that perform well in naturally aspirated form perform even better when forced-induction strategies or
nitrousoxideisadded.
Intake port shape is dictated by the envelope of space given by the overall design of an engine,
valvetrain layout, and intended vehicle application. In terms of pushrod-type engines, the intake port
widthmustnotbemuchlargerthanthedistancebetweenthepushrods,generallycalledthe“pinchpoint”
of an intake port. There are several strategies for increasing the distance between pushrods, including
offsetlifters,offsetrockerarms,andcompoundvalveangles.
Valveangleisreferencedrelativeto90degreesfromtheblockdecksurface.Asmallerintakeportvolumegenerallycreatestorque
atalowerengineRPMcomparedtoalargerintakeportvolumeonthesameengine.
Thelengthofacylinderheadintakeportisestablishedbyitslocationrelativetothecenterlineofthe
cylinder bore and intake manifold mating flange. Height of an intake port is determined by space
constraints of matting components such as intake manifolds, valve covers, and valvetrain hardware.
Generally speaking, intake ports with larger corner radii outperform port designs with smaller corner
radii, based on the cross-section shape transition from the intake port opening to the valveseat sealing
surface.
Intakeportcross-sectionareaisoneofthemostimportantdimensionswhendeterminingthecylinder
head airflow requirements for a particular engine size. Equations derived from fluid dynamics science
tells you that a given cross-section area is only going to flow so much fluid based on its speed, mass,
temperature, and compressibility. Most computer engine analyzer software programs require an intake
port minimum cross-section area input to calculate potential torque and horsepower estimates of an
engine.
Intakeportvolumeisbasicallyafunctionofthelength,width,andheightoftheintakeportshapethat
fitsintoestablishedspaceconstraints.Marketingtrendsintheperformanceaftermarketcanbemisleading
because cylinder heads are categorized in most cases by their intake port volume rather than minimum
cross-sectionarea.Technically,acylinderheadwithanintakeportvolumeof180cccouldoutperforma
cylinderheadwitha195-ccintakeportifthecross-sectionareaofthe180-ccportislarger.
As mentioned earlier, whenever you increase the engine’s intake flow, the exhaust flow must be
increasedtoavoidexhauststreamrestriction.High-performanceaftermarketcylinderheaddesignsoften
featureraisedexhaustports.Raisingtheexhaustportlocationonthecylinderheadgenerallyresultsinan
increaseofexhaustflow,sincethisprovidesamore-directpath.Aftermarketperformancecylinderhead
manufacturers invest quite a bit of time and testing in an effort to develop improvements in power.
Raised-portcylinderheadsareofferedforapplicationswherethishasprovidedabenefitbasedontheir
extensivetesting.
ExhaustPort
Cylinderheadexhaustportsfunctioninreversecomparedtointakeportsinthattheexhaustgasflow
enterstheexhaustportfromthefacesideofthevalve,ratherthanthestemsideofthevalveinanintake
port. Generally, exhaust ports are smaller in volume and cross-section area compared to intake ports,
flowing15to40percentlessonaverage.Thisisbecauseexhaustgasesarepushedoutofthecylinderby
the piston, and also pulled out at the same time by the exhaust header system, referred to as exhaust
scavenging.
Exhaust port shape is dictated by space constraints of exhaust headers, spark plugs, and
vehicle/chassisclearance.Generallyalongerexhaustportshapeoutperformsashorterexhaustport,with
bothhavingthesamecross-sectionarea.Thisisbecauseturningtheexhaustgasesoutofthecylinderhead
andintotheexhaustsystemismoreefficient.Inmostcases,thediameteroftheexhaustheaderprimary
tube needs to be at least the same or larger than the diameter of the exhaust valve; otherwise a flow
restrictioncouldbecreated.Thebulkvolumeofflowinanexhaustportgenerallyfollowsthelongerside
of an exhaust port, which is why there are trends to “D-shaped” exhaust ports, with the straight side
towardthebottomoftheport.
Exhaustportshavesmallervolumeandcross-sectionareathanintakeportsbecauseexhaustgasesarepushedoutbythepistonsand
pulledoutbyexhaustsystemscavenging.
IntakeandExhaustValveDiameters
Intakeandexhaustvalvediametersareafunctionofthedesignoftheintakeandexhaustports.
Generally,exhaustvalvediametersare70to80percentoftheintakevalvediameterbecauseofthe
differenceinflowrequirementsoftheintakesideversustheexhaustside.
Thesearetypicalvalvediametercombinationsofaftermarketsmall-blockcylinderheads:
Intake(inches)
1.940
2.000
2.020
2.055
2.165
Exhaust(inches)
1.500
1.575
1.600
1.600
1.600
CombustionChambersandValveAngles
Combustionchambersincylinderheadscomeinavarietyofdifferentshapesandsizesdependingon
the engine application. The combustion chamber’s main functions in a cylinder head is to aide in the
intake cylinder filling, compressing the air/fuel mixture, controlling flame travel during ignition, and
processingexhaustgasesintotheexhaustport.
Combustionchambervolumeandshapedesignhasamajoreffectonthestaticcompressionratioinan
engine, the amount of ignition timing advance that can be added, and the output emissions through the
exhaustsystem.Somemodernenginesusetwosparkplugspercylindertoachievemaximumefficiency,
whilereducingexhaustemissionsbymorecompletecombustion.
Valve angles and combustion chamber volume are directly related to one another. Generally, valve
angleismeasured90degrees(perpendicular)fromthecylinderheaddeck.InthecaseofOEMGMLS
cathedral-port cylinder heads, the valve angle is 15 degrees. For the sake of comparison, early smallblockChevroletcylinderheadsfeature23-degreevalveangles.
Numerically lower valve angles allow for a shallower combustion chamber, resulting in lower
combustion chamber volume. Numerically higher valve angles require a deeper chamber, resulting in a
largercombustionchambervolume.
Thecombustionchamberaidsintheintakecylinderfilling,compressingtheair/fuelmixture,controllingflametravelduringignition,
andprocessingexhaustgasesintotheexhaustport.
CitingtheGMLScathedral-portcylinderheadsasanexample,valveangleisestablishedat15degrees,comparedto23-degree
valveanglesfeaturedinearlysmall-blockChevroletcylinderheads.Numericallylowervalveanglesallowforashallower
combustionchamber,resultinginlowercombustionchambervolume.
Itisimportanttonotethatvalveanglesaffecttheamountofclearancebetweenthevalvesandpistons.
Choosingtherightcombinationofcamshaft,piston,andcylinderheadiscrucialtothereliabilityofthe
engine build. A general rule of thumb for valve-to-piston clearance is .080-inch intake and .120-inch
exhaust,measured+/-15crankshaftdegreesfromintaketopdeadcenter.
Camshafts
Throughout this book, I talk about exhaust gas scavenging. This refers to the engine’s system being
abletopulltheexhaustgasesoutoftheengine.Themoreexhaustvolumepulledout,andthefasterit’s
pulledout,themoretheair/fuelmixtureisallowedtobedrawnin.Efficientlyburningmoreairandfuel
meansmorepower.
Positive exhaust pressure runs from the exhaust ports to the end of the exhaust pipe. The pressure
wave collapses at the exit, and a negative pressure wave is created that tries to return to the cylinder
head’s exhaust port. Ideally, you want the negative pressure wave of the exhaust gas to hit the exhaust
valvejustbeforethevalvecloses.
Duetovalveoverlap,theintakevalvestartstoopenwhiletheexhaustvalveisstillopen(withthe
exhaustvalveoffitsseatbeforeitcloses).Thishelpsreducecylinderpressure,allowingamoreefficient
intakestroke.Asthepistonmovesupduringtheexhauststroke,exhaustgasispushedout.Whentheintake
valvestartstoopenjustbeforethepistonhitsTDC,andwiththeexhaustvalvestillopen,theexhaustgas
helpstopulltheairandfuelchargeintothecylinder.
Particularlyinanaturallyaspiratedengine,whereforcedinductionisn’tafactor,thisvalveoverlap
aidsinbothexhaustpushandintakechargeentry.ThehighertheplannedengineRPM,themoreintakeand
exhaustvalveoverlapisneeded.
CamshaftTiming
Whenthecamshaftisadvancedtheintakevalveopenssoonerandtheenginedeliversmorelow-end
torque. Advancing the camshaft also decreases intake valve-to-piston clearance and increases exhaust
valve-to-pistonclearance.
Whenchoosingyourcamshaft,payattentiontolobeseparationangle(LSA).Atighter(smallernumber)LSAtendstomovetorqueto
alowerRPMrange,whileincreasingmaximumtorque.Awider(largernumber)LSAtendstomovethetorquetoahigherRPM
range.
Retardingthecamshaftkeepstheintakevalveopenlaterandthusdelaystheintakeclosing.Thishelps
generate power at higher engine RPM. Retarding the camshaft also increases intake valve-to-piston
clearance,whiledecreasingexhaustvalve-to-pistonclearance.
LobeSeparationAngle
The lobe separation angle (LSA) refers to the number of degrees between the centerlines of the
camshaft’s intake lobe and the exhaust lobe. Differences in LSA have an effect on engine performance.
Forexample,acamshaftmayfeaturea108-,110-,114-,or118-degreeLSA.
ThetightertheLSAis,thesmallertheLSAnumber.TighteningtheLSAtendstomoveenginetorqueto
a lower RPM and increases maximum torque with a narrower powerband. A tighter LSA also builds
highercylinderpressureandincreasestheengine’seffectivecompression.Theincreaseincompression
also increases the possibility of detonation/knock, which may require the use of higher-octane fuel.
Engine vacuum at idle is decreased, with a degradation of idle quality, and valve-to-piston clearance
tightensup.AtighterLSAmovestorqueatalowerengineRPM,increasesmaximumtorque,andprovides
anarrowerpowerband.Inaddition,itincreasescrankingpressure,reducesidlevacuum,increasesvalve
overlap,anddecreasesvalve-to-pistonclearance.
AwiderLSAtypicallyprovidesabroaderpowerbandandimprovesvacuumatidleandidlequality.
Engine torque is slightly reduced and moved to a higher RPM range. Cylinder pressure and effective
compressionisreducedandthechanceofdetonationislowered,makingawiderLSAcamshaftslightly
more accommodating for today’s fuels. A wider LSA moves the torque band to a higher engine RPM
range, reduces cranking pressure, increases idle vacuum, creates a wider powerband, decreases valve
overlap,andincreasesvalve-to-pistonclearance.
Here’sacomparisonoflobeseparationangles(LSAs).AtighterLSA(left)tendstogeneratemoretorqueatalowerRPMrange.A
widerLSA(right)tendstolovethetorquebandofahigherengineRPM.
Acamshaft’sbasecircleisthediameterofthecamcoreatthecenterlineofthecore.Thelobelift(orheight)representsthedistance
fromthebasecircletothelobepeak.(IllustrationCourtesyLunati)
Timingofthevalveopeningandclosingeventsiscriticalforair/fuelintakeandexhaustevacuation.(PhotoCourtesyCrowerCams
andEquipment)
Overlap
The goal with camshaft timing is to maximize cylinder-fill by using an earlier intake valve opening
during the intake stroke, in combination with a later exhaust valve opening in order to maximize the
benefitofthecombustionprocess.Asenginespeedincreases,thisscavengingeffectincreases.
Valve overlap is measured in degrees, from the intake-valve opening event to the exhaust-valve
closingevent(attheendoftheexhauststrokeandthestartoftheintakestroke).Inotherwords,thisisthe
periodwherebothvalvesareopenatthesametime.Thiseventhelpstheexhaustgasvelocitypullmore
oftheintakeair/fuelchargeintothecylinders.Mosthigh-performancestreetenginesbenefitfromvalve
overlapinthe50-to75-degreerange,whileahigh-outputdragracingenginemayuseoverlapinthe100degreerange.Inverygeneralterms,thehighertheengine’spoweroutput,themoreitrequiresincreased
valveoverlap.Youcanthinkofoverlapasascavengingeffectthataidstheexhaustgastodrawagreater
chargeintothecylinder.
Referringtothecamshaftspecificationsinamanufacturer’scatalogorthecamshaft’scamcard,you
cancalculatevalveoverlapbyaddingtheintake-openingeventindegreesbeforetopdeadcenter(BTDC)
totheexhaustvalveclosingindegreesaftertopdeadcenter(ATDC).Forexample,iftheintakeopensat
27degreesBTDCandtheexhaustvalveclosesat25degreesATDC,youhave52degreesvalveoverlap
(27+25).Alwaysrefertothecamshaft’sadvertiseddurationtimingnumbers,notat.050-inchduration.
Overlap is affected by lift, duration, and LSA. An increase in lift or duration increases overlap. If
LSA is decreased, overlap is increased. Increasing valve overlap tends to increase top-end power, but
canreducelow-endpowerandcandegradeidlequality.
ExhaustValveLift
Liftisthemaximumamountofopenvalveliftthatoccurswhenthepeakofthelobecontactsthelifter.
On your camshaft specification card, lobe lift and valve lift are listed. Lobe lift refers to the distance
betweenthecamshaftbasecircleandthelobepeak.Actualvalveliftismagnifiedbytherockerarmratio.
Here’stheformulafordeterminingtotalvalvelift:
ValveLift=camshaftlobelift×rockerarmratio
For example, if the camshaft features a lobe lift of .367 inch and the rocker arm features a ratio of
1.7:1,theformulaworksoutto.6239inchlift(.367×1.7).
Ifyoumovetoa1.8:1rockerarmratio,thesamecamshaftachievesatotalvalveliftof.6606inch
(.367×1.8).
Acamshaft’sexhaustvalveliftshouldaccommodatesyourplannedexhaustsystem.Asyouincrease
valvelift,there’smoreroomfortheexhaustgasestoleavetheengine.Thisinturnmeansthatyouhave
moreareatofillviatheintakesystemandpasttheintakevalve.Initially,youcanassumethatthemoreair
youmove,themorepoweryoumake.However,ifvalveliftisexcessive(toohigh),theexhaustvalves
areopenduringthecombustionprocess,whichreducespower.
Whilewe’rediscussingtheexhaustvalvelift,considertheexhaustvalveheaddiameter.Anyonewho
haslookedatanassembledcylinderheadhasnoticedthatexhaustvalvediametersaresmallerthanthe
head’s intake valve diameters. Because an engine has an easier time pushing air out of the engine than
drawing air into the engine, the exhaust valve area doesn’t need to be as large as the intake valve.
Typically,theexhaustvalvediameterisabout57percentoftheintake-valvediameter.
Duration
Acamshaft’sdurationrepresentshowlongthevalvesstayopeninrelationtodegreesofcrankshaft
rotation.Longer-durationcamshaftsallowtheenginetobreathebetterathigherRPM.Shorterdurations,
alongwithshortervalvelift,speeduptheintakeandexhaustflow.Withacylinderheadthatfeaturesa
somewhatrestrictiveexhaustside,thisgenerallyrequiresgreaterexhaustdurationtohelppulltheintake
chargeintothecombustionchamberandcylinder.
Thisisthebasictheoryofvariablevalvetiming,sinceavariablevalvetimingsystemmaximizesthe
intakeandexhaustvalveevents.Thedurationdoesn’tchange,butthetimingofthevalveoverlapevents
change.Thishelpsexplainwhyhigherlift,combinedwithlargerdiametervalvesandlongerduration,is
selectedforenginesthataretomakemaximumpowerathigherenginespeeds.
Durationisrecordedintwoways:advertiseddurationanddurationat.050inch.Advertisedduration
represents the angle of crankshaft degrees relative to a position of the crankshaft, but different
manufacturersmayusedifferentpointsofreference.Forthisreason,it’sbesttocomparecamshaftprofile
durationsbasedonreferenceto.050-inchlifterrise.
By increasing camshaft duration, the valve remains open for a longer period, which promotes peak
poweratahigherenginespeedrange.TheintakevalvebeginstoopenatapointBTDCandclosesata
pointafterbottomdeadcenter(ABDC).Theexhaustvalvebeginstoopenatapointbeforebottomdead
center(BBDC)andclosesatapointATDC.Rememberthatthedistance,ornumberofdegrees,between
TDCandBDCis180degrees.Here’stheformulaforfindingtotalduration:
Duration=openingdurationatBTDC+closingdurationatATDC+180
Using a specific camshaft as an example, let’s say that the cam causes the intake valve to begin to
openat17.5degreesBTDCandclosestheintakevalveat58.5degreesABDC.Usingtheformula,youget
anintakedurationof256degrees(17.5+58.5+180).
Ifthesamecamshaftbeginstoopentheexhaustvalveat69.5degreesBBDCandclosestheexhaust
valveat14.5degreesATDC,theexhaustdurationis264degrees(69.5+14.5+180).
Therefore, in this example, the camshaft features a duration of 256 degrees intake and 264 degrees
exhaust.Suchasplit-durationcamshaft(whereintakeandexhaustdurationaredifferent)isgenerallybest
usedwithacylinderheadthatfeaturesamore-restrictiveexhaustside.
With a cylinder head that has a more restrictive exhaust side, you can increase camshaft exhaust
duration and increase the exhaust primary tube diameter to effectively increase the flow of the exhaust
portinthecylinderhead.Simplyput,addingabitmoredurationhelpsindealingwithaless-than-ideal
cylinderheaddesign.
CamshaftCenterline
Camshaftcenterlinereferstothepointhalfwaybetweentheintakeandexhaustvalvecenterlines.The
intake centerline refers to the position of the peak of the intake lobe in crankshaft degrees ATDC. The
exhaustcenterlinereferstothepeakoftheexhaustlobeincrankshaftdegreesBTDC.
CamshaftforForced-InductionApplications
Asmentionedearlier,camshaftoverlapisusedtohelpthenegativeexhaustpressurewavetoaidin
pulling the air/fuel charge into the cylinder. With a forced-induction setup, the turbocharger or
supercharger creates a boost to the air/fuel charge. With a supercharger, too much overlap can be
detrimental,forcingexhaustgasoutexcessively,whichreducesoreliminatesthescavengingeffect.
For a turbocharged system, a smaller camshaft is preferred, compared to a cam designed for a
naturally aspirated engine. The turbocharger is already packing in a higher-density air charge, so less
durationisneeded.Theexhaustdrivestheturboinordertopackinmoreintakecharge,soexhaust“pull”
isn’tneededtohelppullintheintakecharge.However,withnitrousoxideadded,insomecasesalonger
exhaustdurationwithawiderLSAmaybeneededtorelievethehigherexhaustheatthat’sgenerated.
Advancing/RetardingCamshaft
AdvancingorretardingcamshafttimingmovesthetorquebandtowardlowerorhigherengineRPM
operation.Advancingthecamshaftenhanceslow-endpowerwhileretardingthecamshaftenhancestopend power and sacrifices low-end torque. Advancing or retarding the camshaft moves the valve events
ahead or behind the piston travel. In a conventional clockwise-rotation engine, moving the camshaft
farther clockwise advances the valve events, while moving the camshaft counterclockwise retards the
valveevents.
Advancingthecamshafttimingresultsinanearlierintakeopening,whiledecreasingintakevalve-topistonclearanceandincreasingexhaustvalve-to-pistonclearance.
Retardingthecamshaftcausestheintakevalvetoopenlater,whileincreasingintakevalve-to-piston
clearanceanddecreasingexhaustvalve-to-pistonclearance.
Ifyouplantoadvanceorretardthecamshaft,youneedtocheckvalve-to-pistonclearancewiththe
camshaft in the changed position. Therefore, if you advance the camshaft, pay attention to intake valve
clearance.Ifyouretardthecamshaft,payattentiontoexhaustvalveclearance.
SpecialFiring-OrderCamshafts
Compared to the factory camshaft firing order, special firing-order (SFO) camshafts are sometimes
usedinracingapplications.Thisdifferentfiringorderisalsofeaturedonsomelate-modelOEMengines,
suchasGM’sLSseries.
Thereasonthatadifferentfiringorderissometimesusedistoobtainasmootherrunningengineand
improvedcylinder-to-cylinderfueldistribution.GMadoptedaspecialfiringorderinitsGenIIIandGen
IVLSengineseries,whichfeaturea4/7and2/3swapforthesamereasons:tosmoothouttheharmonics
in the pursuit of greater engine durability and to potentially generate more power. Changing the firing
orderinthismannercanalsoofferthebenefitofreducingharmoniceffectsanddeflectionforcesatthe
crankshaftandthemainbearings.Anotherpotentialbenefitliesinreducingisolatedhotspotsinadjacent
cylinderwalls.
Specialfiring-order(SFO)camshaftsalteratraditionalfiringorderinordertosmoothoutenginepulsesandtoreducevalvetrain
harmonics.Strategicallyswappingthecamshaftfiringorderhelpstoachieveasmoother-runningengine,whichreducesharmonic
effectsatthecrankshaftandthemainbearings.Italsoreducesheatbyeliminatingtwoadjacentcylindersfiringinsuccession.This
benefitsenginesthatoperateatornearpeakRPMforextendedperiodsoftime.InaV-8enginea4/7swapandpossiblya2/3swap
(dependingontheengine)isthemostcommonapproachwithanSFOdesign.
As an example, let’s consider the common small-block and big-block Chevy engines, which have a
firingorderof1-8-4-3-6-5-7-2.Duringeachengineoperation,everycylinderhasa“companion”inthe
firing order. Both companion cylinders reach TDC at the same time, with one cylinder on the power
strokeandonecylinderontheexhauststroke.Thesecylinders(pairedas1/6,2/3,4/7,and5/8)maybe
interchanged,orswapped,inthefiringorderwithouttheneedtomodifythecrankshaft.
Acommonpracticeamongraceenginebuilderswhofollowthistheoryistoswapcylinders4and7,
creatinganewfiringorderof1-8-7-3-6-5-4-2.Whilesomebuilderswhohavetriedthisprocessreport
noperformancegains,othersclaimtohavepickedupasmuchas10hpindoingso.
To maximize the exhaust scavenging effect, the header primary tube length is affected by the engine
firingorder(basedonwave-tuningtheory).Experimentationonanenginedynoorchassisdynoisuseful
when determining header primary tube length when using a special firing-order camshaft. Race-engine
buildersfrequentlyexperimentwithdifferentprimarytubelengths;thishelpstodeterminethebestsetup
toaccommodateanSFOcamshaft.SomebuildersexperimentingwithSFOcamsreportsubstantialgains
whileothershaveobtainedminimalimprovements.
ExhaustManifolds
Exhaustmanifoldsarecommonlymadeofcastiron,althoughafewexistasshortrunsoftubularsteel
thatmergeintoacommonexit.Cast-ironexhaustmanifoldshavebeenincommonuseamongcarmakers
for decades for two primary reasons: They’re less expensive to manufacture, consisting of a one-piece
casting rather than a tubular header that requires assembly and welding; and because of their compact
size,theyareeasiertoinstallonaproductionline.
Ifyouwanttoruncast-ironexhaustmanifoldsbutaren’tthrilledwiththedesignand/orappearanceofastockmanifold,custom
aftermarketmanifoldsareavailableforalimitednumberofpopularapplications.Aftermarketmanifoldsaregenerallydesignedto
providesuperiorexhaustflowalongwithamuch-improvedappearance.
Tubularexhaustheadersprovidededicatedprimarygasroutingforeachcylinder,eventuallymergingintoacommoncollector.
Tubularheaders,availableinbothmildsteelandstainlesssteel,alsoprovideasubstantialweightsavingscomparedtocast-iron
manifolds.
Today, the only reasons that a hobbyist, street rodder, or car collector chooses a cast-iron exhaust
manifoldratherthanasetoftubularexhaustheadersareoriginality,easeofinstallation,and/orbudget.If
the vehicle is being restored in terms of historical accuracy, whatever type of exhaust system was
originallyfeaturedonthevehicleispreferred,andthatincludescast-ironorcaststainlesssteelexhaust
manifoldswhereapplicable.Theonlyreasontoswitchfromheavycast-ironmanifoldstotubularheaders
involvesthegoalofincreasinghorsepower.
Thevariouschoicesofheaderprimarytubediameterandlengthprovideadegreeoftuning,interms
ofselectingvariousheaderdesigns,whereasastockcast-ironexhaustmanifoldoffersvirtuallynoengine
tuningpotential.Ifyourgoalistoextractadditionalhorsepowerandtorque,tubularheadersofferchoices
thatallowyoutotunetheexhaustsystemtotheengine,whereasOEMcastexhaustmanifoldsprovidesize
and design limitations. The choices available with tubular headers provide tuning capabilities that are
simplynotpossiblewithcastmanifolds.
Onthepositiveside,cast-ironexhaustmanifoldsaremorecompactandthereforeeasiertoinstalland
generallyrequirelessunderhoodspace.Inaddition,becauseofthethickermaterial,theytendtocapture
heatabitbetter,andtendtolowerexhaustnoisebecauseofthemoreinsulatingpropertiesofthethickand
heavy cast-iron construction. On the negative side, they’re heavy, so if weight is a consideration,
switchingtotubularheadersisaplus.
Ascastironagesandissubjectedtoheatcyclesovertime,ittendstobecomemorebrittleandcanbe
pronetostresscracking.Thepotentialforstresscrackingisexaggeratediftheexhaustpipingandmufflers
arenotsupportedproperly,astheleverageandvibrationappliedastheremainderoftheexhaustsystem
movesplacesincreasedstressonthecastiron.Fromaperformancestandpoint,asageneralrule,castironexhaustmanifoldscanactasbottlenecks,withabruptexhaustpassageanglesrestrictingflow.
Inessence,thepurposeofasingle-piececast-ironexhaustmanifoldistoallowtheexitofthespent
exhaust gases, with little or no regard to engine performance. With a performance-built engine that
featuresacamshaftwithmoreoverlap,it’simportanttoseparatetheprimaryexhaustpaths,whichisyet
anotherreasontochooseindividualtubesfeaturedinexhaustheaders.
Withthissaid,cast-ironexhaustmanifoldsstillhavetheirplace.Asalreadymentioned,forthosewho
desireanoriginalappearance,theuseoforiginal-styleexhaustmanifoldsismandatory.Ifyourgoalisto
obtainfunctionandoriginalappearanceratherthanmaximizingengineperformance,theuseofcast-iron
exhaustmanifoldsisacceptable.
If you’re trying to salvage a set of original cast-iron exhaust manifolds that are cracked or have
pinholes, be aware that welding cast iron can be tricky. Cracks may be addressed either by pinning or
welding. Pinning involves drilling a small hole at each end of the crack in order to provide stopping
pointstopreventthecrackfromincreasinginlength.Thisisfollowedbydrillingaseriesofadditional
holesalongthecrack,witheachholethenfittedwithascrew-inpinthatisdesignedtopullthecrackback
together.Dependingonthelocationofthecrack,pinningcanprovetobeverysuccessful.Anexcellent
sourceforcrack-pinningrepairmaterialsisfromLockN’Stitch.
Another method of repair involves an actual cast-iron welding process. This involves careful Vgrindingthecrack(s),pre-heatingthemanifoldtoaspecifiedhightemperature,thensprayingaspecialized
cast-ironpowderintothecrack,allowingthematerialtofusetogether.AnexcellentsourceforthisisCast
WeldingTechnologies.
There’susuallyanexceptiontoanygeneralrule,andwithregardtocast-ironexhaustmanifolds,this
involvesaturbochargerinstallation.Theuseofcast-ironexhaustmanifoldsisoftenpreferredinaturbo
setup because the turbocharger is often directly mounted to the cast-iron manifold. The size of these
manifoldsmustbeconsideredaswell.Inaconfinedarea,thecompactcast-ironmanifoldsimplybetter
accommodates a turbo because it takes up much less space than a tubular header. In addition, the
supportive strength of a cast-iron manifold may better suit the weight of a turbocharger. Moreover, the
thickerandheaviercast-ironmaterialbettercapturestheexhaustheat.
PumpingLossReduction
Pumping loss in a gasoline internal combustion engine essentially refers to anything that resists
crankshaftrotation.Pumpinglossesrefertothework,orenergy,requiredtomoveairintoandoutofthe
cylinders. These are losses other than normal frictional factors, such as piston ring drag, timing chain
drag,andtheeffortrequiredtocompressthevalvesprings.Thisalsoincludesdrivingaccessories,suchas
water pump, alternator, power steering pump pulleys, etc. Pumping losses can also be viewed as
“negativetorque”effectsthattrytoresistcrankshaftrotation.
Theenginesuffersapumpinglosswhentheairintakeisrestrictedwhiletheintakevalveisopen.This
occursduringtheintakecyclewhenthepistonismovingdownthecylinderbore.Anythingalongthepath
oftheincomingairchargecanrestrictairflow,suchastheairducting,airfilter,intakemanifoldrunners,
andcylinderheadintakeports.Thethrottlebladeandassemblymaybetheprimaryrestriction.Whenthe
throttle,whetherthisinvolvesacarburetororfuelinjectionthrottlebody,ispartiallyopenorclosed,it
createsaflowrestrictionagainstwhichthepistonistryingtopullair.Thisiswhytheengineproduces
morevacuumatidlethanopenthrottle.
Suchpumpinglossisunavoidable,butselectingtheappropriatesizecarburetororthrottlebodycan
minimizeit.Becauseacarburetorneedstobesizedforbothairpassageandfueldelivery,sizeselection
mustbebasedonthedisplacementanddemandsoftheparticularengine.Withacomputer-managedEFI
system,movingtoalargerthrottlebodyreducesthisairchargerestrictionbecausetheECUonlydelivers
the amount of fuel needed to maintain the correct air/fuel ratio, regardless of the throttle body size. In
otherwords,youcangetawaywithalargerthrottlebodywithoutoverfueling.
Intakesystempumpinglosscanbedrasticallyreducedorcompletelyeliminatedwiththeuseofforced
inductionbecauseairisbeingdeliveredunderpositivepressureasthethrottleisopenedtowide-open
position. This added cylinder pressure provides added force to help the piston move downward. Of
course,sincethecamshaftexpendsenergytodrivethesupercharger,thereisadegreeofparasiticloss.
Althoughaturbochargertakesadvantageofexhaustgasesinordertogeneratetheforcedairchargeand
requiresnocrankshaftenergy,theturbochargeritselfposesanexhaustrestrictionpumpingloss.Thereis
no way to completely eliminate pumping losses, but you can reduce this wasted energy by careful
selectionoftheintakesystem,cylinderheaddesign,camshaftprofile,crankcaseventilation,andexhaust
flow.
Inordertoimproveenginevacuumwhenusingahigh-liftlong-durationcam,anexternalbelt-drivenvacuumpumpisoftenused.
Thiscanreduceexcesscrankcasepressure,whichreducespistonringblowbyandcrankwindage.
Here’sanexampleoffour-strokeengineevents.Ontheintakestroke,thepistonmovesdownward,theintakevalveopens,andthe
exhaustvalveisclosed.Duringthecompressionstroke,thepistonmovesupward,withbothintakeandexhaustvalvesclosed.During
thepowerstroke,thesparkplugfires,thepistonmovesdownward,andbothvalvesareclosed.Duringtheexhauststroke,thepiston
movesupward,theintakevalveisclosed,andtheexhaustvalveisopen.
It’sobviousthatapistongeneratescylinderpressureasitrisesduringthecombustionstroke,pushing
and compacting air upward. Less obvious is the pressure that the underside of the piston creates in the
crankcase as it moves downward, acting like a cup, pushing air down into the crankcase. A positive
crankcaseventilation(PCV)valveallowsintakevacuumtohelppullthispressureoutofthecrankcase.
An external vacuum pump accomplishes much the same effect, in scavenging pressure out of the
crankcase,reducingtheparasiticenergyloss.
TheFour-StrokeEngineCycle
Eachstrokeoftheenginecyclehasadifferenteffectontheexhaustsystem.Tobetterunderstandintake
andexhausteventsintheengine,youneedtounderstandthefour-cycleevent,whichincludestheintake
stroke,compressionstroke,powerstroke,andexhauststroke.
IntakeStroke
The intake stroke begins at the end of the previous exhaust stroke. Before the piston reaches intake
TDC, the exhaust valve is still open while the intake valve starts to open. This is referred to as valve
overlap.
Duringoverlap,bothvalvesareopen,allowingasmallamountofintakechargetobepulledintothe
combustion chamber by the closing of the exhaust valve. This is referred to as the intake scavenging
effect.
Furthermore,asthepistonmovesdownitdrawsthebulkoftheair/fuelcharge.Pumpinglossesare
createdbyanyrestrictionsintheairintakepath,includingthepositionofthethrottleplate.Thepistonis
tryingtodrawairthrough,by“pulling”againstanyrestrictionsalongtheway,whichincreasesmanifold
vacuum.
As manifold vacuum increases, pumping loss increases as the crankshaft works to overcome this
negativepressure.Thiseffectisnotedmostatlowenginespeedandathighenginespeedevenwiththe
throttlewideopen,whenthevolumeoftheairintakelimitstheamountofairthatcanbepulledintothe
cylinder.
CompressionStroke
Thepistonmovesupwardandcompressestheair/fuelcharge.Theair/fuelchargeisignitedbeforethe
pistonarrivesatTDConthecompressionstroke,withpeakcylinderpressuretakingplacejustafterTDC
onthepowerstroke.PumpinglossisgreatestatWOTduringthecompressionstroke,whichisincreased
withahighercompressionratioandwithamore-denseaircharge.
Moreforceisrequiredtocompacttheaircharge,somorepumpinglossisencountered.However,the
pumpinglosscreatedduringthecompressionstrokeisnegatedduringthepowerstroke,sothepumping
lossgeneratedduringthecompressionstrokereallyisn’taproblem.
PowerStroke
Onceignitionhasoccurred,thepistonispusheddownward,applyingpowertorotatethecrankshaft.
As the piston moves downward on the power stroke, cylinder pressure drops substantially and
progressively as the piston moves farther down in its bore. The power stroke does not create pumping
loss.
ExhaustStroke
Thepistonmovesupwardastheexhaustvalveisopen,pushingtheexhaustoutofthecylinderhead.
Thepumpinglossvariablesthatoccurduringtheexhauststrokearedependentonrestrictionintheentire
exhaust system. If the engine is equipped with nitrous injection, the added oxygen and fuel creates an
increaseinexhaustpressure.Asthepistonfightsthisaddedpressureduringtheexhauststroke,pumping
loss increases. To evacuate the exhaust more efficiently, a camshaft with a longer exhaust duration and
openingtheexhaustvalveabitearliercanhelp.
Pumpinglossesarenormalandcannotbeeliminated.Thebestwaytoreducetheenergywastedby
pumping losses is to let the engine breathe, through the use of low-restriction air intakes, relieving
crankcasepressure,valvetimingthatallowstheexhaustgastoleaveandpullairintothecylinders,and
exhaustsystemsthatreducerestrictionandscavengeexhaustpulsesoutoftheengine.
OxygenSensorTechnology
AllproductionvehiclesfortheU.S.marketmadein1996andlaterfeatureOBD-IIdiagnosticsand
requiretwooxygensensors:onebeforethecatalyticconverterandoneaftertheconverter.TheECMuses
theoxygensensorlocatedbeforetheconvertertoadjusttheair/fuelratio.Theoxygensensorlocatedafter
theconverterisprimarilyusedforcatalyticconverterefficiencycontrolandmonitoring.
Whenyouseeoxygensensorsinacatalogorrepairmanual,youmaynoticethetermsS1andS2.The
S1indicatesthattheoxygensensorislocatedbeforethecatalyticconverter,andtheS2indicatesthatthe
oxygen sensor is located after the converter. On a V-type engine, the bank location of the sensor(s) is
indicatedbyB1orB2(bank1orbank2).Forexample,anoxygensensoridentifiedasB1S1indicates
thelocationasbank1andbeforetheconverter.Bank1usuallyreferstotheleft(driver-side)bankand
Bank2astheright(passenger-side)bankofcylinders.
The ECM uses the information provided by the oxygen sensor signal to manage the air/fuel ratio.
Using this signal, the ECM’s fuel management program adjusts the amount of fuel injected into the
cylinder. The two most common types of oxygen sensors are the narrow-range oxygen sensor and the
wide-rangeoxygensensor,whichisalsocalledtheair/fuel(orA/F)sensor.
Althoughanoxygensensorfunctionsdifferentlythananair/fuelsensor,thegoalisbasicallythesame:tomonitortheamountof
oxygenintheexhauststream,sothattheelectroniccontrolmodule(ECM)canadjusttheengine’sair/fuelratiobyricheningor
leaningthemixture.
TheA/Fsensorappearssimilartotheoxygensensor,butfeaturesdifferentconstructionwithdifferent
operatingcharacteristics.TheA/Fsensorisreferredtoasawide-rangesensorduetoitsabilitytodetect
air/fuelratiosoverawiderange,sothattheECMcanmoreaccuratelymeterthefuelinanefforttoreduce
emissions.Whileoxygensensorsoperateataround750degreesF,theA/Fsensoroperatesatabout1,200
degreesF.AnA/Fsensoralsochangescurrentamperageoutputinrelationtotheamountofoxygeninthe
exhaustsystem,providingtheECMwithmoreaccurateair/fuelratioinformation.
The A/F sensor is calibrated for stoichiometry, the theoretically ideal air/fuel ratio of 14.7:1. The
ECMusesanydeviationfromthisidealtoadjustthefuelmixtureandfuelinjectiontime/duration.When
the vehicle features a three-way catalytic converter, the main heated oxygen sensor is temperaturecontrolledbytheECM.Whentheairintakevolumeislowandthetemperatureoftheexhaustgasislow,
currentflowstotheheatertoheatthesensorforaccurateoxygencontentdetection.
Abungfittingcaneasilybeinstalledonaheadercollectorforoxygensensororair/fuelsensorinstallationeithertoaccommodatean
electronicfuelmanagementsystemorsimplytomonitoranengine’sair/fuelratioforfine-tuningpurposes.
CHAPTER2
EXHAUSTSYSTEMCOMPONENTDESIGN,FLOW
ANDFUNCTION
Youneedtherightplan,exhaustsystemdesign,andcomponentselection.Youcannotsimplyselecta
randomcollectionofexhaustpartsandexpecttorealizetopperformance.Youmusttakeintoaccountyour
engine displacement, head size, cam timing, intake design, carb size, and other aspects of the engine
package. You also need to consider intended operating RPM, application, transmission type, and other
factors. This chapter examines specific exhaust system components, including cast exhaust manifolds,
tubularexhaustheaders,exhaustpiping,mufflers,sound-tuningresonators,andcatalyticconverters,along
withtherolethateachplays.
ExhaustManifolds
Insimpleterms,anexhaustmanifoldissomewhatsimilarinfunctionasanintakemanifold.Anintake
manifoldreceivesincomingfeedairfromacentralpoint,whetherthatinvolvesacarburetororthrottle
body.Theairchargeisthendistributedtotheindividualcylinders.Anexhaustmanifoldallowsindividual
cylinderexhaustgasestoexitthecylinderhead,immediatelyintoasinglecollectivepath,orchamber.
Anexhaustmanifoldisnotthemostefficientdesign,butitisthemostsimple,mostdirect,andleast
expensivemethodofroutingtheengine’sexhaustpulsesoutoftheengine.Mostexhaustmanifoldsdonot
provideanequallengthofexhaustflowcylinder-to-cylinderbeforetheindividualcylindergasescombine
andentertheexhaustpiping.
ThisOEMcast-ironexhaustmanifoldwastakenfromanLQ45.3LGMLStruckengine.Notethemetalheatshieldthatthefactory
installedtoprotectenginebaywiringharnesses.Dependingonthesourceoftheexhaustmanifold,aheatshieldmayormaynotbe
present.Ifaheatshieldisfeaturedandyouwishtoeliminateit,itisgenerallyeitherboltedorrivetedinplace.Ifyouplantohavean
oldmanifoldcleanedandceramiccoated,theheatshieldmustberemoved.Ifyouopttoretaintheheatshield,itcanbecleanedand
coatedseparately,thenreinstalled.
A typical exhaust manifold, because of the commonly used cast-iron construction, is also heavy.
Althoughweightisn’taconsiderationforatypicalstreetengine,it’scertainlyafactorforacompetition
engine. Compared to exhaust headers, which feature either a steel or stainless steel flange and tubes, a
cast-iron exhaust manifold can be prone to stress cracking resulting from thermal changes and/or
mechanicalstress.Thesecrackscanbecausedbyimproperfastenertighteningorasaresultofoperating
vibrationsintheremainderoftheexhaustsystem,sinceanironcastingismorebrittlethansteel.Whilea
cracked cast-iron manifold can be repaired, welding cast iron properly is fairly tricky and requires an
experiencedwelder,oftenusingapowder-basedweldingprocedure.
Ascast-ironexhaustmanifoldsgo,LSunitsaren’ttoobad,fromappearanceandfunctionstandpoints.Themanifolddoesnothave
anysevererestrictions.Fromaflowstandpoint,thedesignisquiteefficientwithsmooth-flowingrunners.Thecastingqualityisalso
exceptionallyhigh.
EventhoughtheflowdesignonthisLSexhaustmanifoldisrelativelyunobstructed,thedownfallofanymanifoldisobvious:The
primaryrunnerlengthsdifferfromthefrontofthecylinderheadtotherear.Atubularexhaustheaderwithequalornear-equal
primaryrunnerlengthsimprovesenginepowerandtorquebyhelpingtoevenouttheenginepulsesandenhancingexhaust
scavenging.
Mostlater-modelexhaustmanifoldsfeatureaball-stylecouplingsurfaceforexhaustpipeconnection.Thisprovidesmoreflexibility
forachievingagoodsealattheinstalledangle.Alwaysreplaceusedstudswithnewstuds.Stubbornstudsmayberemovedmore
easilybytorch-heatingtheironandcontactingtheheatedbaseofthestudlocationwithwax,whichseepsintothethreadsasthe
metalexpands.Thewaxpenetratesanyvoidsandservesasalubricant.Whenyouencounterareallystubbornstud,trythisbefore
yousnapthestudoffandhavetodrillouttheremainingmaterial.
Anyexhaustmanifoldorheaderflangeshouldprovideastraightsurfacetoproperlymatetothecylinderhead.Slightdeviationsin
theneighborhoodof.010inchorsoinflatnessofthematingflangemightbesolvedbytheexhaustflangegasket.Whileminimal
deviationsinflatnessofthemountingflangemightbeminimizedduringbolttightening,keepinmindthataheader’ssteelflange
maybesomewhatmoreconformablethanthatofacast-ironmanifold.Castironisbrittle.Anypronounceddeviationinflange
flatnessresultsinstressesplacedontheironduringbolttightening,leadingtoanincreasedpotentialforstresscracking.
Ausedcast-ironexhaustmanifoldhasbeen“seasoned”bythermalchangesandhaslikelysettledintoamoreconsistentstate.
Checkthematingsurfaceoftheflangewithastraightedge.Anydeviationsintermsofwarporwavinessbeyondabout.004inch
shouldbecorrectedbyproperlyresurfacingtheflange.Resurfacingbyhandshouldbeavoidedbecauseitmaymakethecondition
evenworse.Havethisdonebyamachineshoponaresurfacingmachineoronamillingmachine.
OEMcarmakersarenotconcernedwithoptimizingexhaustflowinmostproductionengines.Thisusedexhaustmanifoldshowsclear
evidencethatthetopandbottomofthemanifold’sportflangefacehasbeenexposedtocombustionwithexcessflangematerial
blockingthepathofexhaustflowfromthecylinderhead.Ifthismanifoldweretobesalvagedandusedinaperformanceengine,it
wouldbewisetoportmatchthemanifoldtothecylinderheadbygrindingtheupperandlowerexposedportionsoftheportopening.
Evenslightobstructionstotheexhaustpathcanresultinexhaustreversionbacktowardthevalves.
ThisLSenginefromaCorvetteZ06featuresacenter-dumpexhaustmanifold.Itisareasonablyefficientdesign,asfarascast
manifoldsareconcerned.Theinvestmentcastingfeaturesafairlysmoothflowthrougheachrunner.
Center-dumpcast-ironexhaustmanifolds,suchasthisLSexample,provideanear-equalprimaryrunnerlengthandaresimilarin
designtoacenter-dumpblock-huggerexhaustheader.Theheavy-dutyconstructionofthecast-ironunitmakesitagoodcandidate
forturbochargermounting.
With this said, you can use cast-iron exhaust manifolds if you’re not concerned about maximizing
engine power. However, tuned (tube diameter and length) tubular exhaust headers are preferred for
enhancing horsepower and torque. Properly functioning manifolds “get the job done” and are certainly
appropriate for anyone on a tight budget, or for those interested in maintaining a factory-original
appearancefromarestorationstandpoint.Basically,cast-ironexhaustmanifoldsarerelativelycompact,
durable,andprovideenginefunction,butarelimitedintermsofmaximizingenginebreathing.
ExhaustHeaders
Theuseoftubularheaders,ratherthancast-ironexhaustmanifolds,offerstwoadvantages:reducing
vehicleweightand,moreimportant,extractingadditionalhorsepowerandtorquebyprovidingindividual
exhaust paths for each cylinder. Individual primary tubes are less restrictive, in contrast to the unequal
exhaustpathsfoundinmanifolds.Becauseexhaustmanifoldstypicallyfeatureverysharpflowpathsthat
result from packing a cylinder head’s exhaust bank into one compact unit, the use of tubular headers
providethedistinctadvantageofamuchlessrestrictiveflowforeachindividualcylinder.
Althoughtubularheadersareavailableinarangeofconfigurations,somewithunequal-lengthtubes
(for ease of fitment) and others with equal-length tubes, the option of using equal-length primary tubes
offerstheadvantageofmoreefficientbalancingofexhaustpressuresandflow.
Tubular header construction also allows increasing primary tube length before the cylinder gases
combineatorafterthecollector.LongerprimarytubestendtoresultinincreasedtorqueatlowerRPM,
andtuningbyalteringprimarytubelengthprovidesadistinctadvantageoverusingashort-pathexhaust
manifold.Whileacast-ironmanifoldtypicallyfeaturessharpandmore-restrictivepathsfortheexhaust,
tubular header primary tubes that are mandrel-bent provide drastically improved flow by eliminating
sharpandrestrictivebends.
Althoughshortyexhaustheadersmaynotfeatureequal-lengthprimarytubes,theadditionalrunnerlengthsstillprovideadistinct
advantageovertheuseofanOEMexhaustmanifold.Shorty,orblock-hugger,headersareapopularchoicewherespaceisata
premiumforastreet-drivenvehiclewheremaximizinghorsepowerisn’tapriority.
Headerflangesmustmateflushwiththecylinderheadsandgaskets.Paycarefulattentiontotheflatnessoftheflange(s).
Center-dumpheaders,oftenreferredtoasblockhuggers,areoftenagoodchoicewherespaceislimited.Theprimarytubesare
roughlyequalinlength.Theshortprimarytubedesigncanbeagoodchoiceforlow-RPMtorqueforastreetapplication.
Athickheaderflangeof3/8inchprovidesanicelyrigidmountingsurface.Aone-piecedesign,asshownhere,canpotentiallybe
machinedorgroundflatifwarpingoccurs.
Potential disadvantages of tubular exhaust headers include a typically higher price tag, especially
when moving into stainless steel construction. However, the increased cost is fully justified when
consideringtheperformanceadvantageovermanifolds.Dependingonthespecificvehicle,theinstallation
oftubularheaderspresentsanaddedchallengeduetospaceconstraintsintheenginebay,butthisisoften
unavoidable simply because of the available clearances. Again, depending on the vehicle and engine,
installationcanrangefromaneasydrop-intoaknuckle-bustingordeal.However,thoseofuswhoinsist
onmaximizingengineperformancearewillingtoacceptthischallenge.
Compared to cast-iron exhaust manifolds, tubular headers tend to produce more noise due to the
thinner wall construction of the tubes. The thick cast-iron construction has better sound-insulating
properties.Thesepotentialissuesareoflittleconcernwhenyouconsidertheperformanceimprovements
thattubularheadersoffer.
ExhaustManifoldandHeaderCoatings
Regardlessofyourchoicebetweencast-ironexhaustmanifoldsandtubularheaders,fromastandpoint
of appearance and longevity, either style is easily installed. If you’re dealing with cast-iron exhaust
manifolds,acoatingcanbeappliedwitheitheraheat-resistantpaintoraceramic-basedcoating.
Heatpaintshavebeenaroundfordecades,andwhileformulationsmayhaveimprovedovertheyears,
maintenance is very likely needed in order to keep a fresh appearance. This means removing the
manifolds, cleaning and stripping back down to bare metal, and re-coating. Depending on the specific
application,acoatingofheatpaintmayretainanacceptableappearanceforaslongasseveralyears,or
asshortasafewweeks.
Amuch-preferredapproachistotakeadvantageofaceramicthermalbarriercoatingasofferedbyJet
Hot, Swain, Polydyn, Calico, and others. Today’s ceramic coatings come in a variety of colors and
finishes(flat,satin,andgloss),andifproperlyapplied,typicallylastforthelifeofthemanifolds.
If you’re contemplating tubular exhaust headers, you have three basic options: heat paint, ceramic
coatings,orstainlesssteelconstruction.Heatpaintisbetterthannothing,butit’ssimplynotthebestlongtermsolution.
Forsteelheaders,aceramiccoatingisbyfarthebetterwaytogo.However,keepinmindthatwhile
theoutersurfacesarecoated,theinsidewallsofthetubesarelikelynotcoated.Somecoatingservices
mayattempttoalsoapplythecoatingtotheinnerwalls;thisisoftendifficultifnotimpossibletoachieve.
Leadingheadermanufacturershavetraditionallyproducedexcellentfit,clearance,andperformance.Exhaustheaderquality
overallhasimprovedgreatlyinrecentyears,thankstotheadventofCNCtubingbendersandCNClasercuttersforflanges.
Payattentiontotheterm“stainlesssteel.”Althoughqualityaftermarketexhaustmakersusehighgradesofstainlesssteelthat
withstandsthetestoftimeandtheelements,manyOEMsystemsthatareclaimedtobemadeofstainlesssteelareactuallycomposed
ofextremelylow-alloycontent.ThisOEMsystemservesasaperfectexample.Itwasremovedfromaone-year-oldcarthatwas
exposedtoamereonemonthofwinterdriving.
Ceramichigh-heatcoatingsbenefitanyexhaustcomponents,includingcast-ironexhaustmanifolds,tubularheaders,andexhaust
pipe.Headersareavailablealreadycoatedfrommostleadingheadermanufacturers,suchastheselong-tubeheadersfromHooker.
Incontrasttousinghigh-heatspray-onpaint,ceramiccoatingsaremuchmoredurableintermsofmaintainingappearance,aswell
asincreasingthermalefficiency.Evenstainlesssteelheaders(whicharemorethermallyefficientthanmildsteel)canbenefitby
reducedthermalheatmanagement.
An option is to choose tubular headers that are made with high-quality stainless steel, which resist
corrosion both inside and out. Stainless headers are offered in a natural satin finish or can be fully
polished. Also consider that even though stainless steel headers do not require a coating in order to
prevent corrosion, applying a thermal barrier ceramic coating improves heat management, lowers
underhood temperatures, and improves exhaust heat scavenging. A thermal barrier ceramic coating
providesthesamebenefitstoanymaterial(castiron,steel,orstainlesssteel).
ExhaustPipes
Exhaust pipes route exhaust gases from the manifold or header collector to a planned termination
point,whichmayinvolveroutingthroughmufflersandconverters.Anyexhaustpipethatisshapedtofit
thespecificvehicleroutestheexhaustgases,butinordertomaximizeengineperformance,youneedto
payattentiontopipeconstructionfromarestriction,orbackpressure,standpoint.
RoutingOptions
Shapingthepipebendscanbeaccomplishedbycrushbending,wrinklebending,ormandrelbending
(allthreeapproachesareexplainedinChapter3).Whilecrushorwrinklebendingresultsinrestrictions
and/ordeviationsinexhaustflow,mandrelbendingmaintainsauniformdiameterandsmootherflowpath.
Foranyperformanceexhaustsystem,mandrelbendingistheonlyacceptableshapingmethod.
ForthemajorityofcommonV-8engineapplications,theexhaustpipediameterstoconsiderareinthe
2.5- to 3.5-inch range. If the pipe diameter is too small, the exhaust gas is restricted at higher engine
RPM.Iftoolarge,itcanreduceexhaustflowvelocityandinturnthiscanreducethescavengingeffect.
This can result in reduced low-end torque as well as a potential for an annoying “droning” noise at
cruisingspeeds.
Quality-mindedexhaustpipemanufacturerstakethetimetodeveloplocatingjigsfortheplacementofhangerrodstoprecisely
matchOEMfit.Whilecheapoffshorepipesmayormaynotfitproperly,leadingU.S.exhaustpipemakersstrivetoproducepipes
thatexactlymatchtheOEMdesignsandoftenimplementimprovementswhereneeded,asaresultofon-goingfit-testing,
development,andlisteningtocustomerfeedback.
Althoughacrush-orwrinkle-bentexhaustpipegetsthejobdone,nothingbeatsthesmoothappearanceandfree-flowing
performanceofmandrelbending.Mandrelbendingcreatesasmoothtransitionwithoutdeformationofthepipe,maintaininga
relativelyconsistentpipediameterbecauseajointedmandrelisplacedinsidethepipeduringbending,preventingpipewallcollapse.
Thismaintainsaconsistentdiameterthroughoutthebend,alongwithapleasingappearance.
ThisexhaustsystemfeaturesasimpleH-pipecrossoverwiththesameODasthematingpipes,soinsteadofweldingthebuttjoints,
thisbuildersimplyusedwidebandclamps.
Exhaust pipe is most commonly available in either steel or stainless steel. Steel pipe is commonly
available with zinc or aluminized surface treatments that resist corrosion, but if you’re concerned with
both appearance and longer-term corrosion resistance, pipes can be ceramic coated with the same
processutilizedforheadercoating.Fortheultimateindurability,high-gradestainlesssteelexhaustpipe
isavailablefromperformanceexhaustmanufacturers.
Widebandclampsthatfeatureastep-downtransitionallowconnectingpipestomufflerswherethepipeODandmufflerinletor
outletfeaturesanIDthatdoesn’tpermitaproper-fittinglapjoint.ThesteppedbandclampisthenselectedbasedontheODofboth
thepipeandthemufflerneck.
Whereyou’refacedwithtwopipesorapipe-to-mufflerconnectionwherebothODsarethesame,abutt-jointconnectioncanbe
achievedbyusingabandclampwiththesameIDfromendtoend.Bandclampsareavailableinavarietyofdiameters.
For turbocharged systems, high-temperature silicone coupler sleeves are commonly used to connect
pipes to intercoolers or at any pipe connection where flexing and/or thermal expansion is a concern.
These couplers are commonly attached to exhaust pipes with broad band clamps, usually of the T-bolt
type.
SupportHangers
Thermal expansion is an issue that many people are unaware of. As exhaust temperature rises, the
metalheatsandexpands.Asaresult,theexhaustpipetendstogrowinlength.Thismaynotbevisually
evident,butsmallchangesinpipelengthcancreatestresswhenthisgrowthexperiencesanyresistance.
Forexample,ifsolidsupporthangersrigidlysecuretheentireexhaustsystemtothechassis,thermal
growthstressesmightbeappliedtopipeconnections,flanges,flangebolts,etc.Inextremecases,thiscan
result in cracked exhaust manifolds, distorted flanges, etc. The exhaust system that follows the exhaust
manifoldorheadersshouldbeallowedtomoveslightlyasneeded,topreventstress-relateddamageand
to help isolate the exhaust system from the chassis. This will help to minimize bothersome harmonic
vibrationthatisotherwisefeltandheardinthepassengercompartment;rubberorotherdampingmaterial
isusedonOEMexhausthangersareusedforthisreason.
Ifcustomaftermarkethangers(suchasthosemadefrombilletaluminumorstainlesssteel)aredesired,
some form of damping isolation should be featured, such as high-temperature silicone or elastomer
bushingsatthehanger-to-chassisboltlocation.
Fumes
Exhaustpipesmustterminateoutsideofthevehicletopreventtoxicfumesfromenteringthepassenger
compartment.Pipesmustexitattherearoratthesidesofthebody.Whilesomedrag-racevehiclesmight
featurelong-tubeopenheadersthatterminateunderthechassis(wherethecarbuilderhasdeterminedthe
best exhaust plan for maximum power and torque at the track), consider that the vehicle is only being
operatedfora1/8-or1/4-milerunatatime.
Rear-exitingtailpipesand/ortipsmustextendbeyondtherearbodyfascia.Iftheydon’t,exhaustfumescaneasilyrollunderthe
body,migratethroughthetrunkorhatcharea,andenterthepassengerarea.
Foranyvehiclethatwillbedrivenforanyextendedamountoftime(atidleoratanyvehiclespeed),
itiscriticaltoroutetheexhaustgasesawayfromthevehicle’sundersidetoavoidthepotentialfordeadly
carbonmonoxidetomigrateintothevehicleinterior.
This1964FordThunderboltclonewasoutfittedwithstraightpipesrunningrearwardfromtheheadercollectorstoapairof45degreeelbows.Theexhausttipsoneachsideexitedaheadoftherearwheelwells.Eachtipwasslashcuttoachieveanangle
paralleltothebody.Whetheryou’rerunningtheexhaustwithrearorsideexits,thetipsmustprotrudebeyondthebodytoavoid
carbonmonoxidefumesfromenteringthepassengercompartment.
Joints
Whenconnectinganexhaustpipetoamuffler,lapandbuttjointsarethetwotypesofconnectionsto
consider.Alapjointfeaturesonepipeinsertedintothematingpipe.Thisconnectioncanthenbesecured
byweldingorwiththeuseofaclamp.Asaddleclamp(alsoknownasaU-boltclamp)pinchesthelap
jointtogether.Thiscreatesasolidconnectionbutmakesitdifficulttodisassemblethejointinthefuture.
A band clamp provides a wide footprint over the joint, with one side of the clamp diameter
accommodating the exhaust pipe and the other side of the clamp fitting the muffler neck side. This is
referredtoasalapjointbandclamp.
AbuttjointisusedwhentheODoftheexhaustpipeandmufflerneckareidentical.Thistypeofjoint
can be connected by either welding or by using a wide band clamp that features the same diameter on
eachendoftheclamp.
CalculatingDiameter
Whensizingexhaustpipeforanaturallyaspiratedenginetheexhaustsystemshouldbeabletoflow
about2.2cfmforeveryunitofhorsepower.
Terminology
Theterms“pipe”and“tube”areofteninterchangedandmisunderstood.Intheory,atubeisa
structuralpieceandapipeisintendedtotransferfluidsorgases.Pipeisusuallymeasuredbyits
insidediameter(ID)andtubingismeasuredbyitsoutsidediameter(OD).
Although it may fly in the face of engineering standards and terms, the automotive industry
typically refers to exhaust pipe by its OD and muffler necks by ID. When fitting exhaust pipe to a
muffler,youcommonlyneedtomatchthepipeODtothemufflerneckID.
Theselectionofaftermarketexhaustpipetipsisextremelybroad,withplentyofdesignsandfinishestosuitanytaste.Highly
polishedstainlesssteeltipsarethemostpopular.
ExhaustPipeSizing
Thischartprovidesaroughestimateforexhaustpipediametersizing.
Forexample,iftheengineproduces425hp,theexhaustsystemshouldbeabletoflowabout935cfm.
For a single exhaust system, exhaust pipe diameter should be around 3.25 to 3.5 inches. For a dualexhaust system, pipe diameter should be in the 2.5-inch range. This is only theory. Actual real-world
sizingcandependonwhatRPMrangetheengineisexpectedtomaketoptorque.
AslightlysmallerdiametercanmovetorquetoalowerRPMandaslightlylargerdiametercanmove
torquetoahigherRPMrange.Enlargingthesystemtoofarcandecreaseexhaustgasvelocityandcanbe
detrimentaltogasscavenging.
Hereisasimpleformulathatprovidesaroughestimateforexhaustpipediameter:
ExhaustPipeDiameter=RPM÷1,000×enginedisplacement÷2
For example, if you expect the maximum torque to hit at around 4,800 rpm and the engine
displacementis408ci,theformulagivesyouadiameterof979.2cfm(4,800÷1,000×408÷2).
Thisrequiresanexhaustpipediameter(foradualsystem)tobearound3.5inches.
Mufflers
Thepurposeofamuffleristoaidinmanagingtheengine’sexhaustsoundlevel,andto“tune”exhaust
tone. While some view the use of mufflers as simply a necessary evil, in terms of controlling engine
exhaustnoiselevel,mufflersprovideanadditionalenginetuningaidintermsofhorsepowerandtorque,
primarilyforstreet-drivenvehiclesthatmustoperateandperforminawiderangeofvehicleandengine
speeds.
Forthesakeofillustrativecomparison,considerafirearm’ssilencer.Thisdeviceattachestothegunbarrel exit in order to decrease the audible noise created by the explosive discharge. Contrary to the
popularterm,thisdevicedoesnot“silence”thedischarge.Rather,itsuppressesthenoise,sotheproper
termforthisdevicewouldbeasuppressor,notasilencer.Thesameholdstrueforanautomotiveexhaust
system’smuffler.Insteadofsilencingoreliminatingtheexhaustnoise,themufflersuppressestheexhaust
note.Thedesignofthemufflerhasadirecteffectonboththelevelofnoiseandthetoneoftheengine’s
exhaust.
Round-profilecanister,orbullet-style,mufflersofferbothexcellentstraight-throughflowandareducedprofileforexhaustrouting
wherespaceisatapremium.(PhotoCourtesyBorlaPerformanceIndustries)
Performancemufflerdesignersinvestanincredibleamountoftimeindevelopingmufflersthatreducenoiselevelswhileproviding
minimalflowrestriction.(PhotoCourtesyFlowmaster)
The combustion process that takes place in the engine’s cylinders creates sound waves that travel
from the cylinder head through the exhaust system. Along the way, these sound waves run through the
muffler. Depending on the design of the muffler, these sound and pressure waves are altered by soundabsorbinginsulation(suchasfiberglass)orbyinterruptionsasthewavesinteractbybafflesand/ortubes
thatmayfeatureperforations,anyorallofwhichservetocausethesoundwavestoloseenergy.Inahighperformance muffler, the objective is to alter these sound waves while minimizing flow resistance and
backpressure.
Mufflerswithoffsetinletsareoftenneededformufflerlocationstoaccommodateunder-bodyordriveshaftclearances.
Muffler designs vary from simple straight flow-through tubes with perforations where the sound
wavesaredampenedbysound-absorbingmattingthat’swrappedaroundtheperforatedtube,todesigns
thatuseacomplexseriesofangledbafflewallsandcarefullydesignedinternaltubebendsthatdivertthe
waves while promoting both efficient and low-resistance flow and a specific noise level and “tone
flavor.” Achieving efficient flow and developing a specific sound is a carefully orchestrated balancing
act.
Thetotal“recipe”ofamufflerdesign,includinginternalvolumeofthemufflerbody,thethickness(or
gauge)ofmetal,andthetypeandshapeofinternalbaffling,canresultinarangeofsounds.Thesoundcan
befromquiettothroatytoraspy.Theresultscanvaryandcanbetunedfordifferentenginespeedandload
conditions.
The muffler sound, or exhaust note, varies widely depending on the style, size, design, and
manufacturer.Generallyspeaking,itshouldcomeasnosurprisethatamufflerthatfeaturesfewerexhaust
pathdeviations,sound-absorbingchambers,andfrequency-alteringaspectsproducesalouder,orhigherdecibel,exhaustnote.
In addition to choosing a muffler based on low restriction, for a street application, you need to
considerthetypeofsoundthatyoudesire.Apartfrommeetingacceptabledecibellevelsappropriatefor
street use, the muffler’s effect on engine exhaust note influences the buying decision for a particular
mufflerbrandandstyle.Somedriversmaypreferaquietsoundoutputatidleandcruising,andathroaty,
loudernoteduringacceleration,whileothersmaypreferalouder,morenotablesoundatallengineRPM.
Without having the luxury of trying a variety of muffler designs, the only practical approach is to
consider how the muffler manufacturer describes a muffler’s sound, listen to the exhaust sounds of
vehiclesandenginessimilartoyoursatlocalcarshows,andsearchvehicle-specificInternetforumsfor
theopinionsofothers.Asaresult,youarebetterequippedtomakeyourselectiondecision.
Somemanufacturersofferdirect-replacementmufflers;someoffermufflersspecificallydesignedfor
powerandsoundenhancementforperformance-mindedapplications,suchasBorla,Corsa,Flowmaster,
SuperTrapp,SpiralTurbo,andMagnaflow.Manufacturerssuchasthesespecializeindevelopingmufflers
thatmaximizeenginepowerwhileprovidingthepreferredtypeofsound.
MufflerInlet/OutletSizing
Followingisaroughestimatechartformufflerinlet/outletsizingbasedonenginedisplacement.
Keep in mind that a competition muffler’s requirements involve maximizing engine power while
meetingaparticularracesanctioningbody’ssounddecibelrequirements,whereapplicable.Forastreet
application,thedesignparametersaremorerestrictive.Performancemufflermanufacturersmustdesign
mufflersthatnotonlyenhancepower,butalsoprovidethetypeofsounddesired,whileavoidingannoying
droningorboomingsoundsthatmaybeobjectionabletothedriverandpassengers.
Thereisnosuchthingastheidealmufflerforeveryapplicationbecausepersonaltastesvary.Some
driverspreferaraspysound,whileotherspreferalow-frequencyor“mellow”sound,whileothersmay
preferaveryquietexhaustnoteatidleandcruisingspeed,withanotableincreaseinsoundunderhard
acceleration.Amuffler’ssounddiffersdependingontheenginetype,displacement,andpowerband.With
somanyvariablestoconsider,therealityintermsofselectingamufflerforagivenapplicationistoread
eachmanufacturer’sdescriptionsofitsmodels,andgatherinputfromotherswhoownvehiclessimilarto
yours.
While the Internet is full of information both useful and false, vehicle-specific forums offer a good
starting point to gauge the opinions of others who have tried a certain muffler brand and model with a
specificvehicleandenginecombination.Leadingperformancemufflermanufacturersinvestanextensive
amountofresearch,development,andtestingtoprovidejusttherightsoundthatvariouscustomersdesire.
Mufflertechnologyismuchmorecomplicatedthanmostpeoplerealize.
Althoughdifferent-diameterpipe-to-mufflersizescanbeadaptedusingstep-uporstep-downadapters
(orbyswedgingasmallnecktoalargerdiameter),it’sbesttomatchdiametersatallconnectionsinorder
tomaintainaconstantdiameter.Slip-togetherconnectionsrequireapipeODtomatchthemufflerIDneck.
CatalyticConverters
Catalyticconvertersareinstalledintheexhauststreaminordertolowerharmfulemissions.They’re
designedtochangethreeharmfulpollutants,hydrocarbons(HC),carbonmonoxide(CO),andoxidesof
nitrogen(NOx),intowatervapor(H2O),carbondioxide(CO2),andreducedlevelsofoxidesofnitrogen.
Hydrocarbons and carbon monoxide are formed as the result of incomplete combustion of the fuel.
Ambientair(theairthatwebreathe)thatisdrawnintotheengineaspartoftheair/fuelmixcontainsa
highlevelofnitrogen.Whenthenitrogenburns,itresultsinoxidesofnitrogen.Bothhydrocarbonsand
oxidesofnitrogenareconsideredpollutantsandcontributetosmog.Carbonmonoxidecontaminatesthe
body’sbloodstream,reducingdeliveryofoxygentovitalorgans.
The air we breathe consists of a mixture of several gases, including 78-percent nitrogen, 20.99percentoxygen,and.03-percentcarbondioxide,inadditionto.94-percentargon,.01-percenthydrogen,
.00123-percentneon,.004-percenthelium,.00005-percentkrypton,and.000006-percentxenon.
HowTheyWork
Inthesimplestterms,theenginetakesintheharmlessnitrogen,oxygen,andcarbondioxide;burnsair
with fuel to create harmful hydrocarbons, carbon monoxide, and oxides of nitrogen; then the catalytic
converter “scrubs” the exhaust in an attempt to convert emissions back into harmless nitrogen gas and
watervapor.Tosummarize,youcanviewthisasachaineffectof“inclean,burndirty,exitclean.”
Catalyticconvertersactaschemicalconversionunitsthatchangetheseharmfulemissionsinorderto
lower oxides of nitrogen levels and to transform hydrocarbons and carbon monoxide into harmless
emissions. Today’s catalytic converter features a ceramic or metal core called the substrate, which is
washcoated with a catalyst of precious metals that provide chemical reactions to convert the harmful
emissions.
Scientists discovered that certain precious-metal materials provide the reactions needed to
accomplish this feat, including how hydrocarbons and carbon monoxide react with oxygen and how
carbonmonoxidereactswithoxidesofnitrogen.Thepreciousmetalscommonlyinuseincludeplatinum
(Pt),palladium(Pd),andrhodium(Rh).Platinumoffersgoodoxidationcharacteristicsintermsofhelping
toconverthydrocarbonsandcarbonmonoxideintowatervaporandharmlesscarbondioxide,butnotso
great at reducing oxides of nitrogen. Palladium offers good conversion for all three harmful emissions.
Rhodiumisthemostexpensiveofthewashcoatmaterialsbutprovidesthebestperformanceforremoval
ofhydrocarbons,carbonmonoxide,andoxidesofnitrogen,butagain,it’sveryexpensive.
Exhaustgasespassthroughthecatalyticconvertersubstrate,whichiswashcoatedwithplatinum(Pt),palladium(Pd),orrhodium
(Rh).Oxidesofnitrogen(NOx),carbonmonoxide(CO),andhydrocarbons(HC)enterthecatalyticconverterintheexhauststream
andreactwiththesubstrate.Exitingtheconverterarenitrogengas(N2),carbondioxide(CO2),andwatervapor(H2O).(Photo
CourtesyEasternCatalytic)
Toachievethebestemissionsconversionperformance,it’scommonfortoday’scatalyticconverters
tofeatureacombinationofrhodiumandpalladium.
The chemical reaction that occurs inside the catalytic converter must take place under high
temperatures. The process of the chemical reaction, along with exhaust heat, results in increased
temperatureswithintheconvertertoaround1,300degreesF.Asaresult,converterstendtorunveryhot.
This is the reason that heat shields are often required for vehicle installations, in order to protect
surrounding panels, fluid lines, and wiring harnesses, and to reduce the chances of the hot converter
igniting any flammable materials on the road surface. A quick and easy way to tell if a converter is
workingistomeasuresurfaceheatbeforeandaftertheconverter,sincethelevelofheatshouldbehigher
attheexitoftheconverter.
ConverterDesign
Therearethreetypesofcatalyticconverters:two-way,three-way,andthree-waywithair.Two-way
converters were employed in the early days of converter installation, until about 1981. The two-way
converter, so-named because it handled only two combustion by-products, was relatively good at
converting carbon monoxide and hydrocarbons, but not oxides of nitrogen. This early style usually
featuredprecious-metal-coatedpellets.Theywerereplacedwiththree-wayconvertersaround1981.
Three-wayconvertersfeaturehoneycombedsubstrateswashcoatedwithspecificpreciousmetalsand
handled carbon monoxide and hydrocarbons as well, but also tackled the issue of oxides of nitrogen
reduction. The term “three-way” refers to the handling of all three types of emissions. Three-way
converters are used on electronic fuel management, computer-controlled engines in conjunction with
oxygensensorstocontrolair/fuelratiomoreprecisely.
Three-waywithairconvertersarethree-wayconvertersthatalsofeatureanairinjectiontubelocated
betweenthetwoseparatebrickcellsthatfeedextraairtohelpreduceoxidesofnitrogenemissions.These
arereferredtoasthree-way-plusconverters,notfour-wayconverters.Theyfeaturetwochambersections
ofsubstrate.Thefrontchambersubstrateislikelycoatedwithrhodiumandpalladiumtohandleoxidesof
nitrogenreduction,andthesecondchambersubstrateislikelycoatedwithacombinationofplatinumand
palladium.Theairinjectiontubeislocatedbetweenthetwochambers,providingaddedoxygenforthe
secondchambertohandleconversionofhydrocarbonsandcarbonmonoxide.
PerformanceCatalyticConverters
Catalyticconvertersarerequiredbylawforstreet-drivenvehiclesthatwereoriginallyequippedwith
these emissions-control devices. Even though your vehicle may require one or more converters, that
doesn’t mean that you’re stuck with the OEM factory designs that in many cases feature undesirable
exhaustflowrestriction.It’sobviousthatforaperformanceapplication,youwanttoreducebackpressure
whenandwhereitmakessense.Inordertoprovideabetter-breathingconverterforthego-fastcrowd,
several aftermarket manufacturers now offer fully functioning catalytic converters that feature reduced
backpressurecomparedtoOEMconverters.Reducedbackpressureconvertersfeaturelargercells(bigger
honeycombholes)oragreaternumberofcellsinordertolowerflowresistance.
Athree-waycatalyticconverterfeaturesanairinjectiontubethatprovidesadditionaloxygentoenhancethechemicaloxidation
reactionprocess.Twosubstratechambersectionsarefeatured;thefrontchamberisdesignedtohandleoxidesofnitrogen,whilethe
secondchamberdealswithreductionofhydrocarbonsandcarbonmonoxideemissions.(PhotoCourtesyEasternCatalytic)
Performancecatalyticconvertersareavailableasdirect-replacementfitsforOEMsystems.TheexampleshownhereisEastern’s
TruPerformanceconverter.(PhotoCourtesyEasternCatalytic)
Eastern Catalytic is one good example of a manufacturer that caters to the performance market; it
offers two types of performance converters. The Tru Performance model features either polished 304
stainlesssteelorsatin-finish409stainlesssteelbodieswith300-cells-per-inchceramiccatalystandsteel
meshV-ringretainerstoresistblowoutunderhardthrottle.TheBulletCatconverterfeaturesall-tubular
stainless steel construction (looks like a really short glasspack) and is available with either a 200-cell
metallicfoilor400-cellceramicsubstrate.Eithermodelaccommodatesenginesupto8.0Ldisplacement.
Ahigh-strengthheat-resistantcorrugatedfoiliswoundintoacylindricalshape,thenwrappedwitha
layer of non-corrugated foil that’s brazed to the cylinder. The foil structure is washcoated with a
proprietary NANO coating that meets OBD-II requirements. The foil construction is better able to
withstand extreme pressure and vibration compared to ceramic substrates, which is ideal for forcedinductionandhigh-horsepowerapplications.
Round-profile/tubularbullet–stylecatalyticconvertersarealsoavailable,offeringaspace-savingandstreamlinedadditiontothe
exhaustsystem.(PhotoCourtesyEasternCatalytic)
Converters such as the Tru Performance model are appropriate for any naturally aspirated
performanceapplication,whiletheBulletmodelprovidesahigherlevelofdurabilitysuitableforforcedinduction applications. I’ve cited Eastern’s products as good examples. High-performance catalytic
converters are also available from manufacturers including Magnaflow, Flowmaster, Dynatech, SLP,
Walker,Vibrant,andDynoMax.
Inessence,acatalyticconverterthatisreferredtoasaperformanceconvertershouldfeatureamorerobustconstruction,alongwithreducedflowrestriction,whetherthisisachievedbylargercells in the
bricks or a larger number of cells for increased surface area. If your vehicle is street driven and is
requiredtomaintainoneormorecatalyticconverters,youdefinitelywanttomoveuptoaquality-built
high-performanceconverterdesignthatoffersaslittleflowrestrictionaspossible.Ifyourenginefeatures
forcedinduction,especiallyturbocharging,andyoumustrunaconverter,itisimperativetoupgradetoa
sturdilybuiltconverterthatprovidesthedurabilityneededinaforced-inductionapplication.
Insidethebullet-styleconverterisasteelfoilthathousesahoneycombsubstratewithlessrestrictionforimprovedflow.Eastern’s
BulletCat,asoneexample,isavailablewitheithera200-cellmetallicfoilor400-cellceramicsubstrate.Theactivewashcoatis
designedtowithstandextremepressuresandvibrations,makingthisidealforforced-inductionapplications.(PhotoCourtesyEastern
Catalytic)
Thisisaviewofametallicsubstrate,washcoatedinaproprietarycoatinginanextreme-dutyBulletCathigh-performanceconverter.
(PhotoCourtesyEasternCatalytic)
Eventhoughyoumaybeforcedtoretainacatalyticconverter,beawarethatlessrestrictiveandmoredurableconvertersarereadilyavailable.Providingthattheconverterisdesignedforhighperformance
with reduced restriction, choosing a performance catalytic converter is easy. Simply order the selected
high-performance converter that features the appropriate size inlet and outlet to mate to your exhaust
pipinginordertomaintainauniformdiameter.
Flowrestrictionisreducedinaperformanceconverterbyeitherincreasingthesizeofthesubstrateorincreasingtheholepassage
sizesofthecellsinthesubstrate.(PhotoCourtesyEasternCatalytic)
Enginecat-backsystemsthatfeatureperformanceconvertersarealsoavailableasbolt-ondirectreplacementsforcertainOEM
vehicleapplications.Shownhereisadirect-replacementsystemforaFordMustang.NotethecrossoverH-pipeconnection.(Photo
CourtesyEasternCatalytic)
CHAPTER3
EXHAUSTPIPE
Inthischapter,Idiscussthepipethatroutesexhaustgasesfromtheheaderstotheterminationofthe
exhaustpath,aswellaspipesizing,materials,bendingmethods,pipeclamping,andpipesupport.
Materials
To select exhaust pipe for a given application, first consider your budget. Expense factors include
materials,quantityofpipe(singleversusdualsystem),theapplicationofaspecialtycoating,andpipebendinglabor.
Stainlesssteelismoreexpensivethanmildsteel.Theadvantagesofusingstainlesssteelarelonger
life,intermsofcorrosionresistance,andsuperiorheattransfer.Ifyourbudgetistight,ifyoudon’tplanto
operatethevehicleinavarietyofclimates,andyou’renotoverlyconcernedwithsurfacecorrosion,mild
steeliscertainlyanoption.
Pipesmadeofmildsteelareavailableasbaresteel,butarealsocommonlyofferedwithcorrosionresistantaluminizedorgalvanizedsurfacetreatments.Ifyourbudgetallows,stainlesssteelispreferred,
especiallyfromstandpointsoflongevityandlong-termappearance.
FitandClearanceConsiderations
Intermsoffitmenttoavehicle,youcancobbleasystemtogetherbypurchasingaselectionofpipe
shapes(straightsections,pre-bent45-degreeand90-degreesections,etc.)andtrimmingtolength.Pipeto-pipesectionsmustbeeitherweldedorclampedtogether.
Another option is to purchase a pre-formed kit that is already shaped to fit your specific vehicle.
Many manufacturers offer complete systems for popular vehicles that include pipe and mufflers that
generallyrequireonlyassemblyandinstallation.
Yet another option is to have your entire exhaust pipe system custom-bent for your specific
application.Thecustomapproachisoftenrequiredforuniqueapplications,suchascustomstreetrods,
wherenopre-designedsystemmaybeavailable.Thechoiceofyourpipesystemisbasedonbothcost
andhowmuch,ifany,oftheworkyouprefertodoonyourown.
Precisemandrelbendingallowsaheadermanufacturertoachieveaprecisionfit.TheexampleshownhereisforaCobrareplica.
Yourselectionofpipediametershouldbebasedonenginesizeandexistingorplannedhorsepower
level.Typically,aperformance-orientedV-8engineapplicationrequirespipediameterinthe2.5-to3.5inch range. Keep in mind that for any given application, pipe diameter should be slightly larger for a
singleexhaustsystemthanforadualsystem.However,biggerisnotalwaysbetter.Ifthepipeistoolarge
indiameter,youloseexhaustflowvelocity,whichcanactuallyhurtengineperformance.
Payattentiontotheentireundercarriageforclearanceissues.Placingpipestooclosetothefloorcan
lead to excessive heat transfer to the floor and carpet. Also, if the pipes are close enough to the floor,
frame, or subframe they can make contact, resulting in annoying banging or irritating resonance noise.
Remember that the exhaust system is connected to the engine and moves in relation to the engine’s
movementonitsmounts.Unlikesomeracecarsthathaveasolid-mountedengine,streetvehiclesfeature
engineandtransmissionmountsthatprovideabitofcompliance.
Makesurethatthepipesclearthetransmissionanddriveshaftproperly.Ifthepipesaretoocloseto
the transmission, the transmission fluid temperature could elevate, which could potentially result in
transmissiondamage.Generallyspeaking,Imaintainatleasta1-inchclearancebetweenanyexhaustpipe
andthetransmissionorthetransmissioncoolinglines.
Payattentiontodriveshaftclearancetomakesurethatthepipescannotcontactthedriveshaft.When
trial-fitting the pipes, support the vehicle on a lift or stands and allow the rear suspension to hang
unloaded.Ifyouplantouseacrossoverpipewithadual-pipesystem,makesurethatthecrossoverclears
thedriveshaftevenwiththerearsuspensionunloaded.
Unlessyou’reusingaside-exhaustlayout,yourpipesroutetotherearbumperarea.Asaresult,the
pipesneedtorouteoverorundertherearaxle.Ifthepipesrunovertheaxle,sufficientclearancebetween
thepipesandtheaxlehousingiscritical.Withthevehicle’ssuspensionloaded,compressthesuspension
byhavingahelperbouncetherearofthevehicleupanddownwhileobservingbody-to-axleclearance.If
thepipesrouteundertheaxle,checkclearancewiththerearsuspensionunloaded.
Any vehicle’s exhaust system must exit beyond the body to avoid exhaust fumes from entering the
vehicle interior. If you plan to run a full-length exhaust system with the pipes exiting the rear, the tips
shouldbeflushwithorslightlybeyondtherearbumper.Ifthepipe(s)isnotlongenough,orifyouwantto
enhanceappearance,addastainlesssteelorchromeexhausttiptotheexitofeachpipe.Sincetheexhaust
tipsarevisible,stainlesssteelispreferredbecauseofitscorrosionresistance.
Whenplanningtheexhaustpiperoutinglocations,alwaysbeawareofpipe-hangerrequirements.The
system must be properly supported to minimize movement and to avoid stresses. If the system is not
properlysupported,theweightofthemufflersandpipescaneasilyinduceexcessivestrainatanumberof
locations, including at the header collector and even all the way to the header flange attachment at the
cylinderheads.
Avoidsolidhangersforstreetuse.Allsupporthangersshouldprovidesomedegreeofcomplianceto
accommodate exhaust system vibration, engine-rocking movement during acceleration and deceleration,
and to allow the exhaust pipes to expand and contract under heat and cool-down. If you choose to use
attractivebillethangerstheyshouldatleastfeaturetemperature-resistantbushings.
ExhaustPipeSizing
Aswithcarburetorsorcamshafts,biggerisnotnecessarilybetter.Theexhaustpipediameterneedsto
bematchedtotheenginedisplacementandlevelofhorsepoweroutput.Althoughalargerdiameterpipe
maylookcool,ifthediameteristoolargefortheapplication,youloseexhaustvelocityandlow-tomidrangepowersuffersasaresult.Intheworldofpipe,publisheddiametersalwaysrefertothepipe’sOD.
Asaresult,theIDisslightlysmaller,theextentofwhichisdeterminedbythepipewallthickness/gauge.
PipeCFM
Youneedtodetermineengineintakevolumeandthenapproximatelymatchthisvolumefortheexhaust.
Theformula(whichisalsoinChapter7)forfindingthisis:
EngineIntakeVolume=RPM×.001×displacement÷2
Asyoumightimagine,exhaustpipestartsasstraightlengths.Thisisoneofthestockinventoryracksofstainlesssteelpipeatthe
Corsafactory.
For instance, if your target is to obtain optimum performance at, say, 4,000 rpm, and the engine
displacementis350ci,thevolumeis700cfm(4,000×.001×350÷2).Theexhaustpipes’totalCFM
should be in the same range. To determine the ideal exhaust pipe diameter, refer to “Exhaust Pipe
Diameter”onpage138.Todevelopanaccurateestimateofpipeflowcapacity,referto“PipeCFM” on
page139.
PipeBending
Three types of pipe bending approaches are available for automotive exhaust pipe: crush, wrinkle,
andmandrel.Thebendingprocessmaybehandledmanuallyorwithhydraulicassistance.
CrushBending
Alsocalledpress,ram,orcompressionbending,crushbendingreferstotubebendingforwhichonly
a radius die and backing shoes are employed, with no supporting mandrel placed inside the tube. The
result,althoughfunctional,isabendontheinsideoftheradius.Asthepipeisdrawnacrosstheradius
die, the outside of the tube bend (called the heel) is stretched, while the inside of the bend (called the
throat)iscontracted,leavingaslightdepressionalongtheinsideofthebend.
Crushbendingleavesatelltaledepressionalongthethroatofthebend.Thisdeformationresultsinanon-roundcrosssectioninthe
bendarea.Whilecertainlyfunctional,fromahigh-performancestandpoint,thisdoesnotprovideoptimalflow.
Exhaustpipesthatareintendedforspecificproductionvehicleinstallationsincludetheappropriatehangerpegsalreadyweldedin
position.
Exhaustpipemanufacturersgenerallyproducepipesforaspecificvehicleapplicationinlargequantitiesonagivenday,sothat
toolingandmachinecalibrationsdon’trequirechangingduringthatparticularday.
Althoughpipescanbebentusingawrinkle-orcrush-bendingprocessforOEMandOEM-replacementapplications,themajorityof
high-performancepipesaremandrel-benttomaximizeflowandtoobtainapleasingappearance.
Exhaustpipemakersoftenstoretheirprototypepipestokeepasareferenceifanyfuturechangesarerequired.Theseareafewof
thehundredsofdesignpatternsatStainlessWorks.
This is one of the most common, and equipment-wise, the most economical type of bending. The
typical exhaust shop likely uses this type of bender. While mandrel bending can result in a smooth,
consistentsurfaceonboththeoutsideandinsideofthebendradius,acrushbenderachieves,forlackofa
better term, a factory-original appearance. Crush bending results in a non-round, somewhat elliptical,
shapethatslightlyrestrictsflow.
WrinkleBending
Thisbendingprocess,whichisalsoreferredtoascrinklebending,doesnotuseamandrel.Heatis
appliedtothepipeattheareatobebentuntilthepipeisred-hot.Asthepipeisforcedalongthedie,the
inside throat of the bend collapses in a series of accordion-shaped wrinkles. This type of bend is
commonlyfoundinmanyOEManddirect-replacementaftermarketpipes.
Wrinklebendsslightlyreducetheinsidediameter,alongwithcreatingaseriesofhumpsthatcanaffect
both flow and sound. This type of bending that results in inside-radius wrinkles is typically achieved
during a high-production process. Wrinkle bending is not common for do-it-yourself bending or at any
customshop.
Wrinklebendingresultsinaseriesofwrinklesalongthethroatofthebend.Thepipeisheatedtosoftenthemetalandthenbent.As
thepipeisbent,theoutsideofthepipe’sbendstretchesandthethroatareacontracts,forcingtheinsideofthebendtofoldina
wrinklepattern.Thisiscommonlyfoundonhigh-productionOEMorOEM-replacementpipes.
MandrelBending
Both crush and wrinkle bending distort the pipe diameter at the bend area. Mandrel bending can
provide a smoother, virtually uninterrupted final product, free of kinks and creases. Mandrel bending
differsfromcrushbendingbecauseofthemandrelthat’splacedinsidethepipe.Themandrelsupportsthe
innerwallsofthepipeasthepipesweepsthroughthedieandbackingshoes,resultinginabendthatis
virtually free of diameter or profile changes. The mandrel is also lubricated, usually with a lithium
grease, to provide a smooth gliding action inside the pipe. However, just because a mandrel is placed
inside the pipe doesn’t automatically mean that you won’t have a slight deformation inside the bend
radius.
Mandrelbending,becauseoftheinsidesupportofthepipeduringthebendingprocess,maintainsaroundandconsistentIDandis
voidofexteriordeformation.
Here’sacomparisonbetweenamandrelbend(left)andacrushbend(right).Noticethedeformationintheinnerradius(arrow)of
thecrushbend.
Mandrels are comprised of a series of rounded radius-faced balls that are secured together by a
flexiblecable.Theseriesofballs(usuallymadeofbrass)isabletoflexandfollowthebendradiuswhile
supporting the inside walls of the pipe during the bending process. The width and the number of balls
have an effect on the final appearance. In addition, in order to achieve a super-smooth inside bend, a
wiperdieisusedonthebendingmachinethatsmoothsoutanyirregularitiesasthepipepassesthroughthe
dies,resultinginasmoothbendthatretainstheoriginaldiameterandenhancesappearanceatthesame
time.
Mandrelpipebendingusesamandrelinsertedintothepipeatthebendareainordertosupportthepipeandtopreventitfrom
collapsingordeforming.Themandrelconsistsofaseriesof“balls”thatareheldtogetherbyacable.Balldiameter,width,and
quantityaredictatedbypipediameter,bendlength,andbendanglestobeaccomplished.
Mandrelballsareabletopivotalongabandpath.Asthepipeisbentwithintheradiusdieanditsbackingshoes,theseriesofballs
followthebend,providingpipewallsupport.
Mandrel-bending equipment is expensive and is generally found only in high-performance exhaust
manufacturing facilities and high-level custom exhaust shops. If you purchase headers from any of the
leadingmanufacturers,thepipesaremandrelbent,asareexhaustpipesfromperformanceexhaustmakers,
suchasStainlessWorks,Corsa,Borla,Flowmaster,etc.Manyindependentcustomshopsoffermandrel
bendingifyouneedtohavecustomexhaustcomponentsmadetoordertofityourvehicleandoff-the-shelf
systemsarenotavailable.
Themandrelisinstalledbasedonthepipediametertobebent.Alubricantisappliedtothemandrelballstopermitasmoothglide
throughthepipe.
Awiperdieispositionedin-linewiththeradiusdie.Thisservestosmooth-outanyminorpipesurfacedeformationscreatedbythe
mandrelbend.
Comparedtocrushorwrinklebending,mandrelbendingisthepreferredmethodbecauseitproduces
smoothconsistentbends.Infact,itproducesnoabruptchangesinvolumethroughthebendarea:Optimum
engineperformancecanbeachieved.Pre-mademandrel-bentpipeorhavingcustompipemadewiththe
useofmandrelbendingismorecostly,butintermsofbothperformanceoptimizationandappearance,it’s
worthit.Yougetwhatyoupayfor.
Crush-bendingmachinesarethemostcommonatservicegaragesandlocalpipe-bendershops.Amanualorsemi-automatic
machinerequirestheoperatortopositionthepipeforeachbend.
ManualBendingMachines
A hand-operated kit that features a selection of dies and backing shoes can manually bend exhaust
pipe. Sheer brute force via a lever that pulls the pipe across the bending die does the bending. It’s the
cheapbutlabor-intensiveroute.Whilemanualexhaustpipebendersareadequateforaccomplishingthe
basic function of bending a pipe at a given angle, the results are less than ideal for optimum results.
Becausethereisnointernalsupportforthepipe,manualbendingresultsinkinksandcrushareasonthe
insideofthebend.
If you don’t care about pipe diameter uniformity or appearance, manual benders are certainly an
option.Themachinesrangewidelyinbothcostandquality.Inexpensiveimportedtoolscanbeobtained
foraslittleas$200to$700,butqualitycanvarywidely.
Be aware that there is a difference between tubing benders and pipe benders. Inexpensive tubing
bendersthatdoanadequatejobaregenerallysuitedtosmall-diametertubesuptoaroundthe1/2-inchdiameter size. Cheap pipe benders available at discount hardware outlets are commonly intended for
bendingthin-wallpipesupto2inchesindiameter.
Toaccommodateexhaustpipes,thebendermustbeabletohandleyourplannedpipediameter,which
islikelyinthe2.5-to3.5-inchrange.Theleast-expensivetoolstypicallyincludeabendingfixture(often
designedtobesecuredonabenchvise)andaselectionof“shoes”thataccommodatevarioustubeorpipe
diameters.
Usingacrush-typebendermachine,thepipeisplacedbetweentheradiusdieandapairofbackingshoes.Theradiusdiegenerally
featuresa180-degreethroat,whilethebackingshoesarestraight.
Asthepipeisbentusinghydraulicforce,thebackingshoes(placedonadjacentdiearms,spreadapart)createthedesiredangle.
Duringcrushbending,theoutsidewallofthepipeisstretched,whiletheinsidewall(orthroatofthepipe)compressesslightly.
Onamanual-bendingmachine,anindexpointerismanuallypositionedalongadegreescaleforthedesiredbend.Oncethepointer
reachesthedesiredangle,theoperatormanuallystopsthebendingprocess.
Manual benders are useful for the basic function of creating a bend, but you do not achieve a
distortion-freeinsideradius.Theinsideofthebendfeaturesacrushwheretheinsideofthebendtriesto
collapse slightly without being able to maintain a consistent diameter throughout the bend area. These
relatively inexpensive benders are available from a variety of retail sources, such as discount tool
suppliers.Qualityvariesgreatly,andmanyaremadeoverseas.
Othertypesofmanualbendingmachinesrequiremanualsettingofallcenterlineradiuspointsandpipe
angles.Theyprovidehydraulicforcetoactuallyperformthebends.Inthiscase,theterm“manual”simply
referstotheneedfortheoperatortoplacethepipeinthecorrectposition(lengthwiseandangle)forthe
intended bend. Depending on the size, number of features, and quality this type of bender can range in
pricefrom$800towellover$4,000.Again,withoutamandreltoprovideinternalpipesupport,youend
up with a slight deformation on the inside radius of the bend. Some owners can accept a slight
deformation or wrinkle to the pipes, but many high-performance engine builders and racers find this
unacceptable.
For manual bending, the radius die is the fixture that provides the bend angle. The backing shoes
(sometimesalsocalledwiperdies)provideacontactsurfaceattheoutsideofthebend,oppositeofthe
radius die. The pipe is captured between the radius die and the backing shoes. The backing shoes are
locatedonadjacentpivots,whichallowsthemtoturnwhilefollowingthebendprovidedbytheradius
die. While the operator must manually position the pipe for each bend and for each bend angle, a
hydraulicramprovidesthebendingforce.
Bendingmachinesarecapableofmorethansimplybendingthepipe.Dependingontheavailabledies,
you can perform a number of other tasks, including pipe flaring, expansion, reducing, ball flaring, and
more. When using dies and backing shoes without inside supportive mandrels to perform a bend, the
insideofthebenddistortssomewhat,slightlyreducingthediameteralongthesweepofthebend,witha
slightbulgeateachendofthebend.Thisiswheretheadvantageofamandrelbendreallyshines.Sincea
mandrel(locatedinsidethepipe)supportsthepipewallsduringthebend,youavoidtheslightcollapseof
the inside of the bend. If you want maximum performance and optimum appearance to maintain a
consistentpipediameterfreeofkinks,mandrelbendingistheonlywaytogo,andyou’resimplynotgoing
toachievethiswithamanualbender.Ifyourgoalistoobtainasmooth-flowingandvisuallyappealing
exhaustsystem,youhavethreechoices:Buyanexistingpre-madekitfromaleadingperformanceexhaust
maker that is already designed to fit your vehicle, take the vehicle to a custom pipe bender who has a
mandrelbendingmachine,orifyouinsistonfabricatingyourown,purchasepipesectionsinavarietyof
shapes (straight, 45-degree, 90-degree, etc.) so you can piece your system together by trimming and
welding.Theuseofball-typeflangeconnectorsattheheadercollectorexitshelptoadjustpipeangles.
CNCBendingMachines
A CNC pipe bender is highly prized for accuracy, repeatability, and production speed. Once a
softwareprogramhasbeenwrittenforagivenlengthofpipe,astraightsectionoftheselected-diameter
pipe is placed into the machine. Once the machine is set up, achieving the final pipe shape essentially
involvespressingabutton.Themachineautomaticallyfeedsthepipetoaspecifieddistance,performsa
bend,turnsthepipeintherequireddirection,makesthenextbend,etc.Themachinedoestheworkwhile
thetechnicianmonitorsthejobfromstarttofinish.
CNC(computernumericcontrol)bendingisaccomplishedbycreatingaprofileprogramforthelengthofpipe,whichthencontrols
thebendingmachine.Onceapatternpipehasbeendeveloped,thatpatternisdigitallymappedinordertocreatethesoftware
program.Here,atechnicianatCorsaplotsapatternpipeusingacoordinate-measuringmachine(CMM).Thepatterninformationis
thensenttotheCNCbendingmachine,wheremultiplepipescanbeproducedthatpreciselymatchtheoriginalpattern.
CNCbendingmachinesareexpensiveandareprimarilyusedbyexhaustsystemmanufacturers.The
nicefeatureofCNCbendingisitsrepeatability.Thelastpipeinaproductionrunthat’sformedaccording
toaspecificprogramisidenticaltothefirstpipethatwasformed.
Forhigh-productionpipedesigns(primarilyforOEMapplications),dedicateddiesaresometimesusedonCNCbendersinsteadof
selected-sizeradiusdies.ThisallowstheCNCoperatortofeedstraightpipeintothemachine,withallbendscreatedautomatically
usingcomputer-selecteddieformsasthepipetravelsthroughtheworkpath.(PhotoCourtesyAddison-McKee)
Theexhaustpipe’sdesignisprogrammedfortheCNCbender.ACNCtechnicianverifiesthatthecorrectprogramhasbeen
uploadedandmonitorsthebendingprocessbothatthediesandonthemonitorscreen.
Here,customapplication-specificdiesarebeingusedonaCNCbender.(PhotoCourtesyAddison-McKee)
OncethestraightsectionofpipeispositionedintheCNCbendermachine,theautomatedbenderissettorun.
TheCNCbenderfeedsthepipethroughthedies,automaticallyrotatingthepipeformultiplebendsaccordingtothesoftware
program.
Pipe-EndForming
D ependingonthespecificapplication,eachendofapipemaybeunmodified,featuringthesame
diameterastherestofthepipe.Or,oneorbothendsmayfeatureasmallerorlargerdiameterin
ordertoprovideaslip-togethermatingtoapipeormuffler.
Theendofapipecanbeundersized,usingahydraulicreducingdie,oritmaybeoversizedusing
a swedging oversize die and expander mandrel. A pipe end may also be formed to feature a 90degreeflangelipwhereaflatmatingflangeisneeded.
Aball-socketconnectioncanalsobeobtained,whereperhapsoneendfeaturesamaleradiused
ballshape,andthematingpiperequiresafemaleradiusedend.Aball-socketmating(securedbya
pair of flat bolt-together flanges) is often desired because it allows a degree of flexibility for
adjustingthesystemforpipeangles.
Pipebendershopsmaintainaselectionofpipeexpansion-andreduction-mandrelsforawiderangeofsizes.
Amanualorsemi-automaticpipe-bendingmachine,dependingonthemodel,mayalsofeaturepipe-flaringadapters.This
conicalmaledieisbeingusedtocreateafemaleflare.
Withthepipelockedinplace,ahydraulicramonthemachinepushesintothepipecreatingaflare.Sincethisisn’tafully
automatedmachine,theamountordepthoftheflaremustbevisuallymonitoredbytheoperator.
Ifasquarelipisrequired(forusewithbolt-togetherflanges),aflat-facedmandrelisusedtocontinuepressingagainstthe
initialflare,resultinginaflator“square”flarelip.
Amanualhydraulicorsemi-automaticbendingmachinemayalsofeatureapipeexpandercapability.Amulti-piecemandrelis
selectedforthedesiredexpansiondiameterandinstalledontothemachine’sarbor.
Withthemandrelcollapsed,itinsertseasilyintotheendofthepipe.
Thispipe’sendhasbeenreducedtoaspecificdiameterinareducerdie.
Dependingontherequirementsforaspecificpipe,oneorbothendsmayneedtobereducedorexpandedtoallowaslip-fit
joint.
This3-inchpipe’sendhasbeenexpandedtoallowaslip-fittoamufflerthathasa3-inchentry-pipestub.
Ball-socket-styleconnectionsallowslightangleadjustmentsforthepipes.
Afterexpandingthetip,thetechnicianverifiesfitwitha3-inchsamplepiece.
PipeShape
There’snorulethatsaysapipemustberound.Specialtydiesareavailablethatallowanovalprofile
tobecreated.
Whywouldyouwantanovalpipe?Thisshapeisfavoredprimarilywhenyouhaveclearanceissues.
While an oval pipe can be made with the same cross-section volume as a round pipe, an oval pipe is
narrowerononeplane,whichprovidesadditionalgroundclearanceforvehiclesthathaveextremelylow
groundclearance(suchasmanyNASCARapplications).Thisallowsyoutoflowthesamevolumewhile
providingamorecompactinstallation(relativetoundercarriageandground).Ovalpipesarealsousedon
streetrodswhereeithergroundclearanceorappearance(orboth)isanissue.
There’snolawthatsaysthatallexhaustpipemustberound.Ovalpipethatprovidesaddedgroundclearanceisreadilyavailable
fromraceexhaustpipemakers.(PhotoCourtesyBurnsStainless)
Ovalexhaustpipeprovidesaddedgroundclearancewhilemaintainingexhaustvolume.Oval-profilepipeisavailableinstraightand
angledsections.(PhotoCourtesyVibrantPerformance)
Exhaustpipematerialsincludemildsteelthatmayormaynotbecoatedandvariousgradesofstainlesssteel.SomeOEMsystems
thatareadvertisedasstainlessfeatureverylowlevelsofalloys,andcanrust,asseenherewitharemovedOEM“stainlesssteel”
system.Ifyouwantastainlesssteelsystem,youmustturntoaqualityaftermarketmanufacturer.
Intermsofappearance,ovalpipesprovideamoreuniquelooktotheexhaustsystem.Ifyouchoose
ovalpipe,youwon’tbeabletomakeyourownbends,butaslongasyoucanlaythesystemoutusinga
combinationofstraight,45-degree,and/or90-degreepipe,rawsectionsareavailableforyoutotrimto
lengthandweldtogether.
PipeMaterialandCoatings
Commonmaterialsincludemildsteelandvariousgradesofstainlesssteel.Mildsteelistheeasiest
material to bend, while stainless materials are a bit stiffer but still conform well when formed in a
precisionbendingmachineoperatedbyaskilledtechnician.Asyoumightsuspect,materialpricesvaryas
well;mildsteelistheleastexpensiveandhighergradesofstainlesssteelarethemostexpensive.
MildSteel
Mild steel pipe requires a coating to resist corrosion. This can include aluminized pipe (where a
molten spray of aluminum is applied to the outside of the pipe during manufacturing), galvanized pipe
treatedduringmanufacturing,orbytheapplicationofspecialtycoatings.Thesecoatingscanincludehightemperaturepaint,high-temperaturepowdercoatpaint,orspecializedceramiccoatings.
Of the many choices, ceramic coatings are the most durable. However, be aware that most ceramic
coatings(oranycoatingsforthatmatter)areappliedtotheoutersurfacesonly.Althoughmildsteelpipe
lastslongerwhencoated,bothintermsoffunctionandappearance,mildsteelcanstilldegradefromthe
inside. All exhaust systems are exposed to moisture on the inside walls, simply because of the thermal
changesthatresultfromenginecoldstartstowarmuptocool-down.
StainlessSteel
Stainlesssteel,whilestillexposedtothosesameconditions,ismuchmoreresistanttobothexternal
conditions as well as internal sweating. However, there are varying grades of stainless steel. Many
carmakersofferwhattheycallstainlesssteelexhaustsystems,wheninfactthismaybeaveryinexpensive
“mysterymix”steelformulathatfeaturesaverysmallpercentageofalloy.
As a case in point, in the late 1980s when my road race team ran Ford Mustang GTs in showroom
stockenduranceroadraces,thecarscamefromthefactoryequippedwithshortytubularexhaustheaders
thatwereadvertisedasbeingmadefromstainlesssteel.Afteronerace,theheadersturnedbrownandhad
quiteabitofsurfacerust.Althoughlikelymoredurablethanmildsteel,thestainlessmaterialcertainly
didn’tliveuptoitsname.
High-qualitystainlesssteelprovidesexcellentheatmanagementandresistancefromrust/corrosion.Inaddition,ahigh-grade
stainlesssteelcanbefullypolishedtoachrome-likeappearance.
Toavoidappearanceissuesandtoincreasesystemlongevity,eitherceramic-coatedsystemcomponentsora100-percentstainless
steelsystemisrequired.Theonlyrealdifferencetoconsideristhataceramiccoatingprotectsonlytheexteriorofthepipessolongtermcorrosionontheinsideofthepipescantakeplace.Ahigh-gradestainlesssteelpipeprovidesanti-corrosionprotectiononboth
insideandoutsidewalls.
Aftermarket performance sources tend to use much higher grades of stainless, such as 304 or 321.
These have much higher alloy content and do not rust. Some exhaust makers offer a choice of stainless
grades, priced accordingly. A higher grade, 321 for example, provides higher fatigue resistance and is
bettersuitedforextremetemperatureandvibrationconditionssuchasturbocharging.
In terms of function, even stainless steel can benefit from aftermarket ceramic coatings. Although
stainlesssteelcapturesheatwithlessoutwardheatradiationthanmildsteel,applyingaceramicthermal
barriercoatingimprovesthisevenfurther.Thelessheatradiation,themoreefficienttheexhaustsystem
becomes,intermsofthermalmanagementand(atleastintheory)engineperformance.
Naturally,thehighestlevelsofheatexistattheexhaustheaders,withexhausttemperaturediminishing
asexhausttravelsthroughtherestofthesystem.Formaximumperformance,ceramiccoatingtheheaders
ismorebeneficialthancoatingtheremainderofthesystem.Ifyou’reusingahighgradeofstainlesssteel
intheentiresystem,youprobablydon’tneedtherestofthesystem(aftertheheaders)tobecoated.
Ifyourapplicationisastreettoyandyou’reusingstainlesssteelheaders,pipes,andmufflerstheonly
reasontojustifyanadditionalceramiccoatingisforthesakeofcolorappearance.Iftheheadersandthe
remaining system is mild steel, then coating (of any type) is definitely needed. At the very least, a
combinationofceramic-coatedheaders,aluminizedpipe,andaluminizedorpowdercoatedmufflersmake
sense.
Fromthestandpointoffunction,eithermildsteelorstainlesssteelareviablechoicesforjustabout
any application, including street, drag racing, road racing, and off-road racing. The advantages of
stainlesssteelincludeincreaseddurability,reducedradiantheat,slightlylighterweight,andappearance.
Iconel
Forastreetapplicationwherelong-termrustpreventionisaconcern,stainlessisabetterchoice.For
very extreme applications where exhaust heat and exhaust pipe stress is incredibly high, such as in
Formula1,Inconeliswidelyusedbecauseofitshigh-temperaturefatigueresistanceandresultingability
to withstand extreme heat levels. However, Inconel is very difficult to work with and generally costs
aboutfourorfivetimesthatofcomparable-sizestainlesssteel.
Titanium
Anotherexoticmetalusedinextremeapplicationsistitanium,favoredforitslightweightandhigher
corrosionresistancecomparedtostainlesssteel.Again,costisafactor;itisusuallyasmuchastentimes
thecostofstainlesssteel.
CrossoverPipes
AcrossoverexhaustsystemisusedwithaV-typeenginethathastwoopposingcylinderbanksanda
dual-exhaustsystem.Anumberofmethodscanbeemployedtoimproveengineexhaustefficiency(andto
gainpower)thatinvolveconnectingeachbankofadual-exhaustsystem.
Anexhaustpipecrossoverconnectsthedriver-sideandpassenger-sidepipesatanintersectionpoint
betweentheheadersandthemufflers.Thisconnectionoftheright-andleft-bankexhaustcanhelptoeven
outtheengine’sexhaustpulsesandcanimproveexhaustscavenging.Duringscavenging,theflowofeach
pipehelpstopulltheexhaustthroughtheadjacentpipe,whichismuchliketwobranchesofastreamthat
jointoformariver,andinthiscasethecurrentvelocityincreases.
ThissystemfeaturesanX-patterncrossover.Itaidsinbalancingenginecombustionpulsesandimprovesexhaustflowscavenging.
Asthedualpipesmerge,eachenginebank’spulseshelptocollectivelypullexhaustthrougheachpipe.
Bendingthetwopipestogethercanformacrossoverandanellipticalcutoutoneachpipewherethey
mate,merge,andareweldedtogether.Thiscanalsobeachievedbyconnectingthetwopipestogetherby
meansofadditionalpipesinanHorXpattern.AnH-pipedesignsimplyfeaturesahorizontalpipethat
runslefttorightandconnectsthetwopipes.AnX-pipedesignconnectsthetwosidestogetherwithan
addedX-shapedpipearrangement.Amerged,orX-pipe,designprovidesasmootherflowthananH-pipe
designbecauseithasahigherdegreeofexhaustflowscavenging.
Insomecases,thevehicle’schassisandclearanceissuesmaydictatethechoiceofcrossoverdesign.
Generally speaking, any type of crossover merge (rather than two separate engine bank pipes) should
improve horsepower and torque. Depending on the specific engine and the chosen exhaust design, a
crossoverconnectioncanalsohaveaneffectontheexhausttone.
Here’sanexampleofanH-pipecrossover.Althoughithelpsbalancetheenginepulsesbetweenthetwoenginebanks,itdoeslittleto
improveexhaustscavenging.
WhendesigningadualsystemtoacceptanH-pipe,oranycrossoverstyle,makesurethatthecrossoverlocationdoesn’tinterfere
withtheverticalmovementofthedriveshaft.NoticethatthisH-pipeislocatedforwardofthetransmissionoutput.
ThisX-pipeassemblywasmadefromfourseparatepiecestoachievestraightparallelinletsandoutlets.
ThisX-pipewasdesignedwithopposingentryandoutletangles.(PhotoCourtesyBurnsStainless)
ConvertingtoDualExhaust
Convertingasingle-exhaustsystemtoadualsystemonlymakessensewhendealingwithanengine
that features two cylinder banks (a V-style engine). Converting to a dual system provides increased
breathingratherthanchokingbothbanksintoasingleexhaustpath.Thisreducesexhaustrestrictionand
greatlyenhancesbank-to-bankexhaust-pulsebalancing.Beyondtheperformanceadvantage,switchingto
adualsystemjustplainmakestheenginesoundbetter.Therealityisthat,forastreetmachine,theexhaust
soundisanimportantfactor.
Switching from a single to a dual can provide a performance advantage, depending on a host of
variables including engine displacement and power output. In some cases, let’s say with engines under
300hp,youmaynotgainanyadditionalpower,butyouenhanceappearancebyhavingtwoexhausttips
insteadofonlyone.Thataside,youneedtoconsiderthefollowingareasbeforejumpingin.
Mergedcrossoversareprecision-cutforflow,andthencarefullyTIGweldedtosealallseams.Thisresultsinasmoothflowtransition
withminimalexhaustflowturbulence,asopposedtoastraightbalancepipethat’sweldedat90degreestothemainpipes.
ManifoldCap
Theoriginalsingle-exhaustsystemlikelyfeaturescast-ironexhaustmanifoldsthatcrossovertoeach
other via a downpipe, in which case the crossover from one of the manifolds must be capped off.
Dependingonyourmake,model,andyear,thesecapsmaybeavailableintheaftermarket,oryouneedto
fabricateone.Inaddition,onemanifold’soutletmaybeofadifferentdiameterthantheothermanifold’s
outlet.Thiscanbeaddressedbyeitherhavingyourexhaustshopreduceorenlargethepipediameterof
either side pipe where it mates to the manifold. On the other hand, you can purchase a pair of exhaust
manifoldsthatfeaturethesameoutletdiameter,oryoucanmovetoapairoftubularheaders.
Material
In the majority of cases where someone is converting from a single- to a dual-exhaust system, an
upgradefromcastexhaustmanifoldstotubularheadersiscommon.Afterall,thereasontoswitchfrom
singletodualexhaustistoincreaseperformance,sothedecisiontoswitchtoheadersisprettymucha
foregoneconclusion.
PipeHangers
Dependingontheparticularvehicleandthefactoryoptionsthatwereavailableforthatmake,model,
andyear,theinstallationofadualsystemmayormaynothavebeenplannedbytheautomaker.Thismeans
thatyouneedtodetermineyourownlocationsforpipehangersandyouneedtopaycloseattentiontothe
space available for muffler installation. This can dictate the shape and size of mufflers that fit without
interferingwiththedriveshaft,fuellines,fueltank,rearsuspension,andrearaxle,inadditiontoground
clearanceconsiderations.
Aluminumandstainlesssteelaftermarketpipehangersareavailableinawidevarietyofconfigurations.Thisexamplefrom
StainlessWorksisattachedtothepipewithaT-boltclamp.
Fitment
In terms of muffler shape, an oval profile generally provides more clearance than a round profile.
Again,thisvariesdependingonthediameterofpipethatyou’rerunningandthetypeofsoundthatyou’re
after.
Another area of possible concern is the transmission crossmember. If the vehicle was originally
equipped with a single exhaust, the transmission crossmember may feature only one hump for pipe
clearance, in which case you need to install a double-hump crossmember to accommodate the dual
system.
Makesurethatyouhaveadequateroomfordualpipes;theymustproperlyclearthefueltank.Ifthe
fuel tank is slightly offset, you may not have enough pipe clearance on one side. This might require
replacing the fuel tank with one that allows left-to-right centered mounting. In general, exhaust pipes
should be at least 1.5 inches away from the fuel tank and additional clearance is preferable. If your
vehiclemodelwasneverofferedwithadual-exhaustsystem,youneedtohavethepipescustombentata
competentexhaustshop.
Alsopayattentiontotherearvalance.Iftherearvalancepanelfeaturesasinglecutoutfortheexhaust
tip,convertingtoadualsystemlikelyrequiresnotchingtheoppositesideinordertoachieveaproperfit
andabalancedappearance.
Iftheoriginalsingle-exhaustsystemfeaturedatransversemuffler(positionedlaterallyunderthecar),
youneedtocarefullyselectyourmufflerstosuitthenewlongitudinalpositioning.Ifyousimplydonot
haveenoughgroundclearancetoaccommodatetwomufflersmountedin-linewiththepipes,analternative
mightbetoconsiderbafflesinstalledinsidestraightsectionsofthetwinpipes.
The starting point begins with the headers because the termination of the headers at the collectors
dictates where the pipes begin. The collector typically features a mounting flange that provides the
attachmentpointforthepipe.Althoughafixedmatingflangeatthefrontofthepipemayworkinmany
cases, the angle provided by the installed header collector may shoot straight back, or it may angle
slightlyupwardordownward,orleftorright.
Connectors
Toprovideadegreeofangleadjustment,considertheuseofaball-styleconnector.Thisprovidesa
pivotpoint,allowingyoutoadjusttheinitialangleofyourpipetobestfityourapplication.Tightenthe
boltsattheball-styleconnectorlightly;snugenoughtoholdpositionbutlooseenoughtoallowmovement
whileyoucontinueroutingyourpipesrearward.
Again, depending on the specific vehicle, you should be able to run straight pipe sections from the
collector rearward, keeping them somewhat parallel to the vehicle floor. Locate the best locations for
yourmufflers,intermsoffitandclearance,betweenthebellyanddriveshaft.
Astainlesssteelbraidedflexcouplerwithinternalsleeveallowsapipe-to-pipeconnectionthatreducesoreliminatesstressbetween
thetwopipeends.Aflexcouplerisusefulinhigh-movementorhigh-vibrationareaswhereadegreeof“give”isrequired.(Photo
CourtesyVibrantPerformance)
AV-bandclampassembly,withcouplerflanges,isathree-piecearrangement.Thecouplerflangesareweldedtoeachpipeend.With
thetwopipeflangesbuttedagainsteachother,thegrooveinsidetheV-bandclampcapturesthepairofflanges.Whentightened,the
clampprovidesaleak-proofandsolidconnection.(PhotoCourtesyVibrantPerformance)
In most cases, you locate the mufflers forward of the rear axle and fuel tank. Make sure that the
placementprovidesampleroombetweentherearofthemufflersandanyroutingobstacles,suchasthe
rearaxleandthefueltank,tocontinuemostconvenientlywithanyrequiredpipebends.Ifatallpossible,
temporarily suspend the mufflers in the desired locations. This provides an easy target for routing the
frontpipesfromthecollectorstothemufflers,soyoucandeterminepipelengthandanyrequiredbends.
At the very least, position each muffler in the desired location and place chalk marks on the belly to
creatematchmarksformufflerlengthreference.
Examinetherequiredpathfromthecollectortothemufflerinlet.Ifyougetlucky,you’llbeabletorun
straightpipebetweenthecollectorsandmufflers.Ifthepathrequiresthatthepipesangleinboardtoward
the center of the belly, the front pipes may require a slight bend inboard and a subsequent bend that
regainsaparallelrunbacktothemufflers.Thisiswheretheuseofmufflersthatfeatureoffsetinletand
outlet necks come in handy, as this can minimize or even eliminate the need to create any bends in the
frontpipes.
Ifyou’repiecingthesystemtogetheronyourown,keepinmindthatball-styleconnectorscanoften
aidinachievingdesiredangles,althoughtheyaddabitofbulkalongthepath.
Asyouputthesectionstogetherconsidertheuseofband-stylepipeclampsratherthanU-bolt-style
clamps. The band clamps don’t deform the pipe and provide added wiggle room while adjusting your
system.Inaddition,band-styleclampsprovidesuperiorjointsealingandmakeiteasiertodisassemble
thesystemlater.
Connecting each pipe to a muffler requires one of two approaches: welding or clamping. The pipe
mayslipintothemufflerneck,requiringapipeODthatmatchesthemufflerneckID.Thisisknownasa
slipjoint.
For instance, if the muffler features a 3-inch ID, the pipe must feature a 3-inch OD. The firm
connection can then be made by either welding or with the use of a pipe clamp (again, I prefer band
clamps).IfthepipeODandmufflerneckODareequal,aslipjointisn’tpracticalunlessyouuseapipeexpandertooltoslightlyenlargethemufflerneckdiameter.
However,thereareotherwaystoaccomplishthetask.Withthetwoendsbuttedupagainsteachother,
awidebandclampcanbeusedtosecuretheconnection,orifyouprefer,theendscanbeweldedtogether.
Rubberhangerisolatorsandashortflexcouplerarebeingusedonthis2008PontiacG8GT.Useofaflexcouplereliminates
suspectedexhaustpipestresspoints.
Thisstyleofclampcanbeweldedtothepipe.Ifyoucanimaginethestyleandshapeofhangerdesired,chancesaresomeoneinthe
aftermarketoffersit.
Ifyou’refacedwithtwodifferentoutsidediameters,asteppedbandclampcanbeusedthatfeatures
differentdiametersateachend.
ExhaustPipeBendingandInstallation
Startingwithalengthofstraightpipe,theinstallermanuallyfeedsitintothebandingmachine’sbackingshoes.Amanually
controlledbenderthatfeatureshydraulicassististhemostcommontypeofmanualpipebender.
Aftermeasuringtheangleneededforthefrontpipe,thetechniciancreatesa45-degreebend.
Oncethepipeisreleasedfromthebendingmachine,itistest-fittedtothevehicle.Thisallowsthetechniciantotrimexcess
lengthatthefrontbendareatoaccommodatethedistanceofthepipedropfromtheheader,relativetothefrontcrossmember.
Thebentandlength-trimmedfrontpipeischeckedforfitattheheadercollectorbeforewelding.
Ahelperholdsthefrontpipeinpositionwhilethewelderappliesafewtackweldstosecurethepipetotheheadercollector
outlet.
Theweldorcompletesthefull-seamweldatthefrontpipetotheheadercollector.
Aconnectorpipeisfullyseamweldedtothemufflerwhileaccessible,beforeconnectingittotheendoftheforwardpipe.
HeatShielding
If your system requires the use of catalytic converters, keep in mind that the converters must be
locatedearlyintheexhauststreamaheadofthemufflers.Catalyticconvertersposehigh-heatconcerns,so
makesurethattheyhaveadequateclearancewithregardtothefloor,fuellines,brakeslines,transmission
coolinglines,etc.Essentially,youwanttoprovideasmuchclearancearoundtheconvertersaspossible.
Adding heat-shielding material is always a good idea; for instance, on the underside of the floor
adjacenttotheconverterlocations.Qualityheat-shieldingmaterialisavailablefromfirmssuchasDEI
and can be trimmed to fit a desired location. Don’t attach the heat shield directly to the converter; you
wanttoallowheattoradiateawayfromtheconvertershell.Dependingonlocation,heatshieldingcanbe
mountedusinglarge-headedrivetsorscrews.
Iftheconverterrunsclosetoanyplumbingorwireharness,tubularorwrap-typeheatshieldingcanbe
secureddirectlyontothelinesorwiresthatareincloseproximitytotheconverter.Again,sourcessuchas
DEIofferawiderangeofthermalshieldingproductsspecificallydesignedforperformanceandracing
exhaust-systemapplications.
ExhaustSystemSupports
The pipes, mufflers, and/or catalytic converters must be supported in any application in which the
exhaust system extends beyond the headers. Properly supporting the system removes strain from the
headers, avoids ground contact, and eliminates or reduces system vibrations and harmonics. With any
productionvehicle,younoticethatexhausthangersinvolvesometypeofisolatorordampeningdevice.
Thismightinvolveareinforcedrubberstraporarodthat’sweldedtothepipeormufflerandinserted
intoarubbermountatthechassis.
Theuseofsuchdampeningdevicesisdoneforseveralreasons.Acompliantconnection,asfoundin
rubber or soft-durometer urethane, dampens vibrations that otherwise are transmitted from the exhaust
systemtothechassis.Thiscompliancealsoallowstheexhaustsystemtomoveinrelationtotheenginein
terms of vibration and torque loading, without straining the exhaust system. Isolators also allow the
exhaustsystemtochangeitslengthbecauseofthermalconditions.
Consider that the exhaust pipes are made of metal, which tends to contract when cold and expand
whenhot.Athermalchangeinpipelengthmaybesmall,butbyprovidingcompliantsupportconnections,
any thermal-caused dimension changes are accommodated by the compliance of the supports. The last
thing you want to do is rigidly nail down the entire exhaust system. If you use solid pipe and muffler
supports,stressesbuildupastheenginerocksonitsmounts,asthesystemexperiencesthermalgrowth,
etc.Inaddition,byrigidlymountingtheexhaustsystem,engineexhaustresonanceisdirectlytransmittedto
thechassis,resultinginarangeofbothersomenoises.
Aftermarket exhaust system hangers constructed of chrome-plated steel or polished stainless steel
have been available to the custom market for decades. Many of these hangers look great and serve to
finish-offacustomappearance.However,justmakesurethatthereissometypeofcomplianceintheform
ofrubberorurethane.
Customexhausthangersshouldalwaysfeaturesometypeofcompliancebushing.Thesepolishedstainlesshangershaveurethane
isolatorbushings.
Variousstylesofcustomaftermarketexhausthangersareavailable.Thesearedesignedtobefastenedtoastructuralsurface(such
asaframerail,crossmember,orbracket)withapairofsocket-headcapscrews.Aweldedbracketonthepipeisthenfastenedtothe
hangerwithasingleboltandnutthatpassesthroughaurethanebushing.Thebushingprovidesadegreeofcomplianceforpipe
movementandservestohelpisolatepipevibrations.Thetypeshownhereprovidesthreeoptionsforbushinglocation,allowingyouto
trimofftheexcessunusedportionofthealuminumhangerifsodesired.
Evenalowlysteel-strap-stylepipehangershouldhaveacompliantreinforcedrubberisolator.Thisstyleallowstheuseofa
conventionalU-bolttocapturethepipe.
Arock-solidsupportthatdoesn’tallowthesystemtomoveatallsimplyisn’tagoodidea,atleastfor
astreetvehicle.Iftheexhaustpipesandmufflersaresupportedatthechassisbysolidmountsthatofferno
compliance,youruntheriskofimposingstressesthatcanleadtoweldfracturesattheheadertubes.Itcan
even damage the header flange-to-cylinder head mounts in terms of broken flange bolts or cracks and
leaksattheprimarytube-to-flangepoints.
Inadditiontostressesthattheexhaustsystemcanexperiencebecauseofvibrationandenginepivoting
movementduringaccelerationanddeceleration,astheexhaustsystemtemperaturesriseduringoperation,
thepipesmayslightlygrowinlengththankstothermalexpansion,whichcanimposestressesateachjoint.
Evenifnothingcracksorbreaks,asolid-mountedexhaustsystemresultsinannoyingresonanceswithin
thepassengercompartment.Exhausthangermountingpointsshouldprotectthesystemfromunduestress
andprovideabitofcompliancetominimizeannoyingbuzzesandunwantedresonancewithinthecabin.
I’m not saying that your exhaust system needs to be able to wag around under the car like a wet
noodle,butitneedsasmalldegreeofisolationbetweentheexhaustsystemandthechassis.
In order to further address the allowance for pipe movement during thermal changes and vibration,
someOEMandevencustomdesignersmayemployaflexiblecoupling,whichconsistsofashortsection
offlexpipethatfeaturesastainlesssteelbraidedshield.Thisprovidesasmalldegreeofpipeposition
angleand,insomecases,canreducevibration.
Asupportstandservestomaintainthepositionandangleofthepipeastheinstallercontinuestoweldthefrontofthepipetothe
forwardpipe.
PipeConnections
When connecting pipes to pipes or pipes to mufflers, several attachment methods are available. A
commonpracticeistousealapjoint(alsocalledaslipjoint),whereonepipeslipsintoanotherpipe(or
pipetomuffler),orabuttjoint,wherethetwopipeshavethesameOD.
LapJoint
Withalapjoint,theODofonesideslipsintotheIDofthematingside.Withthistypeoffitment,a
saddleclamp,welding,orabandclampsecurestheconnection.Asaddleclamp,oftencalledaU-clamp,
hasaradiusedinsidesaddlematedtoaU-bolt.Thistypeofclamp,whentightened,crimpsthetwopipes
together. This is a common-style clamp and has been used for decades. The drawback is the crimping
action.Thiscanmakeitdifficulttoseparatethetwopipesinthefuture.
Welding the joint together with a continuous bead provides an excellent seal but it has drawbacks.
Welding is not always a practical option for the do-it-yourselfer. In addition, because a welded
connection is permanent, future disassembly requires cutting the pipe(s). In addition, the process of
weldingdiscolorsthepipesandruinsanyprotectivecoatingthatwasappliedtothepipes.
Astepped-bandclampprovidesanalternative.Thisstainlesssteeltubularclampfeaturesadifferent
diameterateachend,witheachendsizedtoaccommodatethetwodifferentpipediameters.Abandclamp
featuresasplitseamthathasastopblock.Oncethebandclampisplacedinposition,apairofboltsis
tightened to squeeze the band tightly onto the pipes. Because of the broad contact area of the band, the
pipes are not crimped. This provides an easy method of securing the pipes, provides an enhanced
appearance,andmakesfuturedisassemblyeasy.
ButtJoint
Whenusingabuttjoint,thetwomatingpipeshavethesameOD,soyouhavethreechoicesformaking
the connection. You can weld the pipes together. You can use a band clamp for the particular pipe
diameter.(Thisstyleofbandclampfeaturesthesamediameterfromendtoend.)Ontheotherhand,you
canuseapipeexpandertoslightlyenlargeoneofthepipestocreateyourownlapjoint.
Unlessacontinuousbeadofweldisappliedtothejoint,anystyleofclamphasthepotentialforavery
slight exhaust leak. I rarely use saddle clamps unless I’m performing a restoration in which I need to
replicate the OEM design. Whenever I use a saddle or band clamp, I first apply a thin bead of hightemperatureRTV,suchasPermatexUltraCopper,totheoutersurfaceofthepipeswherecontactoccurs.
This acts as a lubricant when fitting the pipes together and serves to fill any small voids that might
otherwiseprovidealeakpath.
Ultra-wide band clamps are extremely handy for pipe-to-pipe and pipe-to-muffler connections.
Becauseofthenon-crimpingdesign,bandclampsmayalsoallowasmalldegreeofthermalexpansionin
termsofpipelength.Yes,bandclampsaremorecostlythansaddleclamps,butthebenefitsfaroutweigh
thecostdifference.
AwidestainlesssteelbandclampisabetterchoiceforpipeconnectionsthanacommonU-bolt,sincethebroadfootprintoftheband
doesn’tcrimpthepipes.
Bandclampsareavailableas“straight,”withacommondiameterallthewaythrough,designedtoconnecttwopipesthatareofthe
samediameter.Steppeddesigns,suchastheoneshownhere,allowaneasyconnectionofdifferent-diameterpipes.
Bandclampsfeaturestoppingblocksatthematingarea.Theboltpassesthroughoneblockandthreadsintotheotherblock.
MatingStyles
Pipe-to-pipeorheadercollector-to-pipematingcaninvolveoneofanumberofmatingstyles.Asliponconnectioninvolvesonepipefittingintoanotherpipe,securedasdescribedearlierbyweldingora
selected style of clamp. A flared or ball-socket style of mating involves a pair of flanges that bolt
together,capturingthetwoends.
AV-bandclampassembly,withcouplerflanges,isathree-piecearrangement.Thecouplerflangesareweldedtoeachpipeend.With
thetwopipeflangesbuttedagainsteachother,thegrooveinsidetheV-bandclampcapturesthepairofflanges.Whentightened,the
clampprovidesaleak-proofandsolidconnection.(PhotoCourtesyVibrantPerformance)
OnthisV-bandclamptheinternalfemalegroovecapturesthemaleflangesofthetwoopposing-pipeweld-onflanges.(Photo
CourtesyVibrantPerformance)
Aflat-flaredpipefeaturesanexpandedfemaleflarewithaflatfaceoneachpipe.Eachmatingpipe
features a two-bolt or three-bolt flange captured on each pipe. The seal is created by a flat hightemperaturegasketplacedbetweenthetwoflanges.
Aball-socketflare,oftenfoundonsomeheadercollectorconnectionsandsomecatalyticconverters,
featuresamaleradiused/ball-styleflareonthecollector,matedtoafemaleradiusflareontheattaching
pipe. Flanges on each side of the connection secure the collector to the pipe using two or three bolts,
dependingonthestyle.
AT-boltclampgetsitsnamefromtheshapeoftheadjustmentbolt.IthasaTatthecapturedendandamalethreadattheopposite
end.Thiscanbeagoodchoicewhenmatingtwopipesofdifferentdiameters,wherethelarger-diameterpipeendfeaturesslitsthat
allowittocompressaroundthesmallerpipe.
The advantage of a ball-socket attachment is that it allows easy angle adjustment of the pipe. The
femalesocketflareonthepipecanpivotonthemaleballflare,allowingaddedfine-tuningintermsof
pipeangle.Thisstyledoesnotrequireaflatgaskettobeplacedbetweentheflanges.
A V-band clamp is often favored for applications where the exhaust system is subject to frequent
disassembly/reassembly.
AV-bandclampissimilartoaT-boltclamp,inthataflatbandistightenedviaaboltthatengagesinto
afemalethread.UnlikeaT-boltclampwithasmooth,flatinnersurface,aV-bandclampfeaturesagroove
on the inside surface. With each mating pipe end featuring a flat flare, the pipe flare edges fit into the
clamp’s groove and provide a butt-to-butt mating of the two pipes. The adjustment bolt is tightened to
holdthepipestogether.Withthistypeofclamp,it’seasytoconnectanddisconnectonepipefromanother.
AvariantoftheV-bandclampisintendedforstraight-endpipesthatdonothaveflares.Thisstyleof
clamp includes a pair of flares that slip onto the pipes and are welded to the pipes. The flares are
machinedpiecesthatweldtothepipes,whicheliminatestheneedtoflarethepipeends.Thewelded-on
flaresalsoprovideaddedmassandstrengthforamoredurableconnection.
AstainlesssteelT-boltbandclampprovidesasecureattachmentandapleasingappearanceformountinganexhausttipthat
featuresanotched/splitentry.Thebandclampcompressesthetipenoughtoprovideasecureattachmentwithoutcrimpingthetipor
pipe.
Wide-bandclampsareavailablepre-formed,butalsoasaflatbandthatiseasilybentandshapedtothepipe,featuringalow-temper
stainlesssteelbandthatbendseasily.
Aflatbandclampishand-formedaroundthepipe.Thenitisfastenedtogetherwithapairofboltsandnuts.Anunformedstyleof
bandclampislessexpensivethanapre-formedstyle,butdoesn’tfeatureastopper-blockbetweentheclampingends.Italsodoesnot
sealaswellasapre-formedbandclamp.
Widestainlesssteelbandclampscomeinavarietyofpipediameters,forbothbutt-matingpipeofthesamediameterandholding
differentpipediameterstogether.Thismakesquickworkofanypipe-to-pipeorpipe-to-mufflerconnection.
Pre-formedbandclamps(sizedforaspecificpipediameter)areavailableinallcommonpipediametersizes.Astraight-through
bandclamphasthesamediameterfromendtoendandisusedwhenbuttingtwopipesofthesamediameter.
InstallationTips
Hereareafewhintstomaketheprocessofinstallingyournewsystemeasierandmoreefficient.
Thesealsosaveyoufrommakingcostlyorannoyingmistakesthatadverselyaffectperformance
and/orappearanceofyourexhaustsystem.
• Ifyouhaveabrand-newenginethathasneverbeenfired,beawareoftheexhaustheatgenerated
duringhigh-enginerevswhenbreakinginaflat-tappetcamorwhenrunningthenewengineona
dyno.Itisstronglyrecommendedtoinitiallybreakintheengineusingspareheadersorexhaust
manifoldsinsteadofyournewheaders.Ifyournewheadersareceramiccoatedorpainted,they
candiscolorundertheseconditions.
• Keeptheheaders,pipes,andmufflerscleanofgrease,fingerprints,etc.Wheninstallationis
complete,wipedownallsurfacestoremovecontaminantsthatcandiscolorthesurfaceswhen
hot.
• Closelyinspectallconnectionsandmakesurethatallpipesandmufflersdonotcontactany
frame,body,driveline,orsuspensionarea.
• Ifanyconnectionseemssuspecttoyouintermsofsealing,consideraddinganarrowbeadofa
high-temperatureRTVtomatingsurfacesbeforefinalassembly.
• Checkforgroundclearance.Makesurethatnopartofthesystemispositionedlowenoughto
contacttheroadsurface.
• Makeabsolutelycertainthattheexhausttipsfullyexitthebody,whetheryou’replanningarearor
sideexit.Exhaustthatexitsunderneaththevehiclecaneasilyresultincarbonmonoxideentering
thepassengercompartment.
Both types of V-band clamp operate the same. The only difference is that one requires flared pipe
ends,whiletheotherismoresubstantialandrequiresthatthemaleringsbeweldedtothepipes.Ineach
variant,agrooveintheinsidewalloftheclampcapturesthetwomaleribbedflarestogether.
SampleExhaustSystemInstallations
Ihavecoveredtheinstallationofeachessentialcomponentoftheexhaust.Now,Iamgoingtoshow
youtheinstallationofacompletesystemandanX-pipeinastep-by-stepformatsoyoucancompletethe
sametaskwithyourparticularprojectvehicle.
2014Chevy/GMC1500Cat-BackInstallation
This project is a complete installation of a Flowmaster performance exhaust system on a 2014
Chevrolet/GMC1500trucksoyoucanseehowanentiresystemisinstalled.Youcanfollowthissame
basicprocessforinstallinganyexhaustsystemonanAmericanrear-wheel-driveV-8–poweredvehicle.
Raisethevehicleonahoistorracktoworkingheight.Ifyoudon’thaveaccesstoahoist,raisethe
vehicleandsupportitsecurelywithsturdyjack-stands.Supporttheoriginalmufflerwithastand.Usinga
hacksaworreciprocatingsaw,cutoffthetailpipejustbehindthemuffler.Separatethewirehangersonthe
tailpipefromtherubbermountsonthevehicleandremovethetailpipe.Thisstepmaynotbemandatory,
butitmakesremovaleasier.
Usinga15-mmwrench,loosentheboltontheclampedballconnectionbehindthecatalyticconverter.Separatethewire
hangersfromtherubbermountsandlowertheinletpipeandmufflerfromthevehicle.Alubricant,suchasWD-40,makes
removaleasier.Youneedtoremovetheballclampfromthestocksystemandsaveit,asitwillbeusedduringinstallationofthenew
system.(PhotoCourtesyFlowmaster)
Placetheinletpipeassemblyintopositionontotheballconnectionandtightenitenoughtohold,butstillallowforasmall
amountofadjustment.Connectthehangeronthepipetotherubbermountonthevehicle.(PhotoCourtesyFlowmaster)
IfyouareinstallingthesystemonaCrewCabtruckwitha6.5-footbed,placetheextensionpipe(PNTB700S)ontotheback
oftheinletpipe(PN26401S).Placeaclampfromthekitontotheslip-fitandtightenenoughtoholdinposition.Thisextensionpipe
isnotusedonDoubleCaborCrewCabmodelswitha5.5-footbed.(PhotoCourtesyFlowmaster)
Placeasupplied3-inchclampovertheinletofthenewmuffler,thenslidetheinletontotherearoftheinletpipe.Besureto
useastandtosupportthemuffler.Tightentheclampjustenoughtoholdthemufflerinposition.(PhotoCourtesyFlowmaster)
Slidetherubberhanger(PNHA168)ontotheendofthedriver-sideframehanger(PN226HA).Usingthesuppliedbolts,
washers,nuts,andbackingplates(PNHA566),mounttheassemblytotheframeonthedriver’ssideofthevehicle,usingthe
existingangledoblongholeintheframerail.Note:Thebackingplatesareinstalledonbothsidesoftheframe,betweenthehanger,
washers,andboltstocovertheoblongholes.Thispreventstheframehangersfrombendingintotheoblongholes.Placetwoofthe
supplied2.5-inchclampsoverthepassenger-sideanddriver-sideoutletsofthemuffler.Placethepassenger-sidetailpipe(PN
86095S)intopositionovertherearaxle,andslidethistailpipeintothepassenger-sidemuffleroutlet.Connectthetwohangersonthe
pipetothevehicle’srubbermounts.Thepipeshouldbeapproximately3/4inchawayfromtheshockabsorberwhenthepipeis
adjusted.(PhotoCourtesyFlowmaster)
Placethedriver-sidefronttailpipesection(PN86094S)intopositionovertherearaxle,andslidethistailpipeintothedriversidemuffleroutlet.Placeasupplied2.5-inchclampontotherearofthepipe.Thedriver-sidereartailpipesection(PN
86096S)attachestothefrontsection.Connectthehangerthat’sweldedtotherearofthepipetotherubberhangerthatwasinstalled
inStep5.Thepipeshouldbecenteredbetweentheshockabsorberandthesparetirewhenadjusted.Tightentheclampsjustenough
toholdposition.(PhotoCourtesyFlowmaster)
Slidethesideexitpipes(PN86097S)ortherearexitpipes(PN86098S)ontotheendsofbothover-axlepipesandplacea
supplied2.5-inchclampontoeachoftheseslip-fitconnections.Tightentheclampsjustenoughtoholdposition.Placethetwo
stainlesssteeltips(PNST461)ontotheexitpipesandtightenthemenoughtoholdposition.Rotatetheexitpipesandtipstothe
desireddistancefromthebodyorbumper,sothattheangle-cutonthetipsareinthedesiredlocations.(PhotoCourtesyFlowmaster)
Adjustthepositionofallpipesandmufflertoprovideasatisfactoryfit.Aminimum3/4-inchclearancearoundallpartsmust
bemaintained;remembertokeepsuspensiontravelinmind.Tightenallclampedconnectionssecurely.Placethesupplied1/2-inch
hangerkeepersontotheendofeachinletpipeandtailpipehangers.Slidethe7/16-inchhangerkeeperontothedriver-siderear
framehangeruptotherubbermounts,tofirmlyholdthesysteminplace.
Topreventtherearaxlebreathertubefromcontactingthedriver-sidetailpipeandpossiblybeingdamaged,useasuppliedziptie
tofastenthebreathertubetothebrakelinebracketlocatedontherearaxlehousing.Besuretoallowenoughslackinthehoseforthe
suspensiontofullycompressandextend.Foramoresecureinstallation,Flowmasterrecommendsweldingallslip-fitjoints.(Photo
CourtesyFlowmaster)
2011FordMustangGTXO-PipeInstallation
ThisprojectshowsyouhowtoinstallanX-pipeona2011FordMustangGT.
An X-pipe balances exhaust gas pressure and pulses and, therefore, is a common exhaust system
component to install for high-performance V-8 muscle cars. The X-pipe installation is common for a
completeexhaustsystemonaparticularcar,whichtypicallyincludesheaders,cat-backmufflers,anda
stainlesssteelexhaustpipe.
Usinga13-mmsocketwrench,loosenthesphericalclampsattheinletoftheH-pipeonthepassenger’ssideanddriver’s
side.NotethatsomeMustangswereshippedwiththeboltheadfacingawayfromtheground.Useanopen-end13-mm
wrenchtoloosenthethreadedendandthenturnitbyhandtoremovetheclamp.(PhotoCourtesyCorsaPerformance)
Usinga15-mmwrench,loosentheclampsaftertheH-pipeassembly.(PhotoCourtesyCorsaPerformance)
Onthedriver’sside,useaflat-bladescrewdrivertoprytheclampspringclipfromtheretainerpinontheH-pipe.Repeatthis
onthepassenger’sside.(PhotoCourtesyCorsaPerformance)
SlidebothclampstowardtherearofthevehicletothefreeendoftheH-pipes.Slidebothclampsrearwardtoremovethe
stockH-pipe.(PhotoCourtesyCorsaPerformance)
PullthefrontoftheH-pipefromtheclampjointonbothsidesandslidetheoutletendsfromtheclampstocompleteremoval.
(PhotoCourtesyCorsaPerformance)
Theclampsontheoutletendswillbereused.RemovethemfromtheH-pipeassemblyandinstallthemontotheaxlepipes.
(PhotoCourtesyCorsaPerformance)
TheCorsaXO-pipekitincludestwo70-mmsphericalclampsandone2.75-inchclamp.Applythesuppliedanti-seize
lubricanttothethreadsofallclamps.(Thisisnecessarytoavoidgallingofthenuts.)Alignallclampssothatthecenterof
eachclampboltis90degreesfromthenotchinthepipe.Allclampsshouldbetightenedusingaqualitytorquewrench.(Usinganair
impactgundamagestheclampandcouldcausethejointtoseparate.)Pre-assembletheXO-pipeassemblywiththeseparatepipeon
thedriver’sside,theflat2.75-inchclampovertheexpansion,andone70-mmsphericalclamponeachend.Aligntheclampssothat
theyareaccessiblewheninstalled.(PhotoCourtesyCorsaPerformance)
SlidetheoutletoftheXO-pipeintotheclampsattheinletoftheaxlepipes.(PhotoCourtesyCorsaPerformance)
Withclampsinplace,holdtheXO-pipeassemblyparalleltothegroundandtightenthefrontsphericalclampsto21ft-lbs.
(PhotoCourtesyCorsaPerformance)
SeatthefrontflareoftheXO-pipeonthepassenger’ssidetothestockmatingpipeandrepeatthisonthedriver’sside.Hold
theXO-pipeparalleltothegroundwhiletighteningthefrontsphericalclamps.(PhotoCourtesyCorsaPerformance)
Slidetherearclampsforwardsothattherearedgeisapproximatelycenteredbetweenthetwoholesintheaxlepipebracket.
Usinga15-mmwrench,tightenthefourboltsontherearclampsandthe2.75-inchclamponthedriver’ssideoftheXO-pipeto45
ft-lbs.(PhotoCourtesyCorsaPerformance)
Itisstronglyrecommendedthatallclampsarecheckedandretightenedtotherecommendedtorqueafterinitialroadtesting
ofthevehicle,asthermalcyclingmaycauseslightloosening.Besuretowaituntiltheexhaustsystemhasfullycooledbefore
checkingfortightness.Tightenallclampboltsto45ft-lbs.(PhotoCourtesyCorsaPerformance)
CHAPTER4
HEADERDESIGNANDFUNCTION
Exhaustheadersdifferfromexhaustmanifolds.Aheaderprovidesadedicatedtubeforeachcylinder
head exhaust port, while a manifold collects and routes all exhaust ports to a single outlet, which is in
closeproximitytothecylinderhead.Individualpipesthatarededicatedtoacylinderarereferredtoas
primarypipes.Onheaderdesignsthatfeaturethebankofprimarytubesgroupedtogetherintoacommon
outlet, this common outlet is referred to as the collector. It’s where all of the cylinder head’s primary
tubes are “collected” together as a group. (Chapter 7 discusses exhaust system–related math to aid in
determiningtheappropriateheaderprimary-tubediameterandlengthforagivenapplication.)
Although an exhaust manifold does allow exhaust gases and pressure to exit the cylinder head,
transferring to exhaust piping, an exhaust manifold is generally considered a compromise in terms of
performance. A manifold provides an initial path for exhaust exit. The exhaust pressure differs from
cylinder to cylinder. An exhaust header provides an individual primary tube for each cylinder, which
offers a more efficient and more evenly balanced exhaust route for each cylinder, with far less
backpressure.
Inadditiontosolidcolors,customgraphicsareavailablefromsomecoatingservices.Thisisanexampleofaflamescheme:orange
flamescoupledwithachromelusterceramic.(PhotoCourtesyJetHot)
Inaperfectworld,theexhaustheadertubesforeachcylinderheadhaveequallengths,allowingeach
cylinder’sexhaustflowandpressuretobesomewhatequal.Inreality,dependingonthespecificengine
andvehicle,equal-lengthheadersmayormaynotbepractical,simplyintermsoffitandavailablespace.
Regardless, the use of headers (rather than exhaust manifolds) should, in most cases, provide a
performance benefit and allow the engine to breathe more freely with less restriction. In addition, a
tubularheaderprovidesaweightreductionascomparedtomostcast-ironexhaustmanifolds.
SelectionConsiderations
Selecting the appropriate exhaust header for a given vehicle involves a number of considerations.
First of all, establish your priorities, which differ depending upon the individual application. While
upgrading from cast OEM exhaust manifolds to tubular headers generally provides a degree of
performance improvement in any application, some enthusiasts are more concerned with maximizing
enginepowerandothersmaybemoreconcernedwitheaseofinstallation.
Performance
Ifperformanceiskey,youneedtoselectheadersbasedupontheengine’sbreathingrequirementsin
termsofmatchingtubediameterandlengthtomaximizelow-endtorqueorhigh-endhorsepower.Longtubeheadersareidealforenhancingtorqueatlowerenginespeeds,butdependingonthevehicle,longtube header installation may prove to be more difficult. In addition, you may run into ground clearance
issueswithaspecificbrandandmodelofheader.
EaseofInstallation
Ifyourbiggestconcerniseaseofinstallation,headerswithshortertubesand/orheaderswithunequallengthprimarytubesmayprovideabetterfit.Forastreet-drivenvehicle,easeofinstallationandheaders
that provide better clearance both underhood and at the belly of the car is a better choice, even if that
means not being able to absolutely maximize the engine’s performance potential. This might involve
choosingheaderswithslightlysmallertubediameters,andsharperbends,andasaconsequenceit’smore
restrictive.
If,forexample,yourparticularengineideallywantsaheaderthatfeatures1.75-inchprimarytubes,
butaheaderwith1.5-inchprimarytubesfitsmoreeasilywithouttheneedtomodifyinnerfenders,power
steeringlines,wireharnesses,etc.,youmaybebetterofflivingwiththisslightcompromise.Whenarace
carisbuiltfromscratch,youhavetheluxuryofplanninganddesigningforanidealexhaustsystem.Witha
production vehicle, unless you’re willing to make necessary modifications, the reality is that you may
havetomakeafewcompromisesintermsofsacrificingafewpotentialfoot-poundsoftorqueorabitof
horsepower.
Cost
Consideryourbudgetaswell.Ifyou’renotplanningtorunthevehicleincompetition,doyoureally
needtobuythehighest-dollartuned-lengthproracingheaders?Theanswerisno.
Also consider header material from a cost standpoint. All things being equal, mild steel uncoated
headersoffertheleastexpensiveinitialcost.Ceramic-coatedstainlesssteelheadersarelikelythemost
expensiveoption.Thinkabouthowyourheaderswillbeused.Mildsteeleventuallyrusts,eitherfromthe
outside in or from the inside out. Adding a ceramic coating or painting with high-heat paint can extend
steelheaderlife.
Aceramiccoatingprotectstheexteriorsurfaces,butmoistureinsidethetubescaneventuallytakea
toll.Ifthevehiclewillbedrivenyear-roundinvaryingclimates,thebestchoiceisstainlesssteel.Evenif
theycostmoreinitially,stainlesssteelheaderspayforthemselvesintermsoflongerlife.Ifyouplanto
drivethevehicleonalimitedbasis,forexampleduringwarmweathermonthsonly,ceramiccoated-steel
headerslikelyprovideasatisfactorylifespan.
Exhaustheadersareavailableinanalmostinfinitearrayoftubediameters,lengths,andshapestosuitdirectreplacementin
productionvehiclesandcustomapplications.
Most of today’s header makers offer their products already ceramic coated. If you have mild steel
headers custom fabricated, or if you’ve purchased a set of new or used steel headers that are bare or
painted,myadviceistoshipthemtoaqualifiedcoatingshop(suchasJetHot)forblastingandceramic
coating.I’vehadavarietyofheaders,newandold,treatedtothesecoatings,andIhaveyettoseeonethat
hasexperiencedanycorrosionissues.However,Ihaveseenproblemswithheaderscoatedbyless-thanskilledshops.
Asacaseinpoint,Ioncehadasystemcoatedbytwodifferentshops.IsenttheheaderstoJetHotand
thepipestoanothershopthatclaimedtousethesameprocess.Onceeverythingarrived,piecesfromboth
shopslookedidentical.Afterbeingstoredinthesamestorageareainmyshopforabouttwomonths,I
wasreadytoinstallthesystem.
At that point, the Jet Hot pieces were in the same condition as new, while the other items showed
serioussignsofsurfacerust.IendedupshippingtheflawedpiecestoJetHotforstrippingandre-coating,
afterwhichIhadnoproblems.Thesystemwasinstalledontoa1933Fordthree-windowstreetrodthatI
hadbuiltforacustomer.Isawthevehiclefouryearslateratacarshow,andtheentireexhaustsystem
lookedthesameasthedayIinstalledit.
GeorgeRumoreofStainlessWorksnotedthateveryheaderdesignforeachspecificapplicationinvolvesaninitialpatternthathas
beendevelopedforspecificengineandvehiclefitments.Eachprototypepatternisstoredforreference.
StainlessWorksstoresthousandsofinitialheaderpatterns,categorizedbyengineand/orvehicleapplication.Eachsetisrecorded
withallfitmentdata,suchasvehiclemake,model,year,enginesize,andaccommodationforaccessoriessuchasA/C,power
steering,etc.Ineachcase,theinitialpatternwasdevelopedforfinalizedfitment.Ifanissuearisesinthefuturewithacustomer’s
application,theshopcanrefertotheinitialpatterntodetermineifthecustomer’svehiclehasanirregularitythatposesafitor
clearanceissue.
Mypoint?Don’tchooseacoatingshopwithoutcheckingoutitsreputationfirst.
Header installation can be quite labor-intensive for the hobbyist or anyone who doesn’t install
headersonaregularbasis.Someareeasyandsomeareanightmare.ThereasonI’vedevotedsomuch
attentiontothesubjectofmaterialsandcoatingsistospareyoufromtheneedtoremoveandrefinishor
replaceyourfirstsetofheaders.It’sbettertodothejobonceandbedonewithit.
HeaderStyles
Apartfromfullycustom-designedsetups,commonheaderstylesincludefull-length(oftencalledlongtube),shorty(oftencalledblock-hugger),lakester,andtri-Y.
Long-tubeheadersareusuallydesignedwith(inthecaseofan8-cylinderengine)fourprimarytubes
thatmergeintoasinglecollector.Long-tubeheadersaregenerallydesignedtofavorlow-endandmidrangetorque.
Shortyheaders,asthenameimplies,featurerelativelyshortprimarytubes.Shortiesarepopularfor
manyhard-to-fitapplicationswithregardtoavailableunderhoodspaceandoftenhelpeaseinstallation.
ShortprimarytubestendtofavortheupperRPMrange,butasacompromiseforastreetvehiclewhere
spaceisanissuetheyremainpopular.
Lakesterstylesareso-namedbecauseoftheiruseinearlydrylakeracing.Thisstylefeaturesshort
primarytubesthatimmediatelyenteralongstraightmegaphone(tapered)stylesecondarypipe.Lakesters
usuallyfeatureacutoutplate,allowingyoutorunthroughapipe/mufflersystemoropen,bypassingthe
restofthesystem.Thisstyleremainspopularfornostalgia/old-schoolhotrodbuffs.
Tri-Yheadersareregularheadersthathavetwooftheprimarypipesmergingintoadjacentprimary
pipes(creatingaYintersection).Thebankoffourprimarypipesisrelativelyshort,connectingtoapair
of larger-diameter primary pipes, ending with the two larger primary pipes merging into a common
collector.TheideabehindaTri-Ydesignistocreateaportseparationbypairingthecylindersthatare
firing 180 degrees apart. Merging the primaries and transitioning into a larger-diameter pipe helps to
increase exhaust scavenging. Pulling exhaust gases out more quickly promotes increased air intake,
speedingupthecombustionprocess,andtheoreticallyproducingmorepower.
Shortheadersareoftenagoodchoiceforuniversalvehicleapplications,fromthestandpointoffitmenteaseandgreatlyreduced
clearanceissuestocomponents,suchassteeringshafts.Shortyheadersareoftenfavoredforapplicationssuchasstreetrodswhere
spaceisatapremium.Whileequal-lengthprimarieshelptopromoteengineefficiency,forthestreetit’snotaconcern.
Materials
Exhaust headers are made using either mild steel, specific grades of stainless steel (usually 304
stainless),orinrarecases,morecostlymaterials,suchasInconelortitanium.Sometimesa“mysterymix”
ofmaterials,withaverydiminishedlevelofalloys,isused.
MildSteel
Steel tube headers normally feature 16-gauge or thicker mild steel tubing, while stainless primary
tubescanbemadewithaslightas.040-inchwallthickness.However,typicalwallthicknessesare.049
inch/18gaugeand.065inch/16gauge.Flangesontheprimarytubesthatmounttothecylinderheadsare
traditionallymadeofeitherflat-stocksteelorstainlesssteel,ranginginthicknessfromabout1/4inchtoa
more-common3/8inch.
StainlessSteel
Headers that are designed specifically for turbocharger applications usually have a heavier wall
thickness to better handle the high exhaust gas temperature (EGT) generated during turbo operation, in
order to provide added durability under extreme temperature. As an example, Stainless Works makes
OEM-replacementandcustomstainlesssteelexhaustsystemsthatuse16-gauge304Lstainlesssteelforits
turboapplications.
Theuseof18-gaugestainless304isoftenfoundintheconstructionofstainlessheadersbecauseit’s
about 15 percent less expensive and about 24 percent lighter (with cost a consideration for most
consumers and weight a consideration for racers). The heavier-wall 16-gauge stainless material is a
popular choice for turbocharged applications where the exhaust tubes encounter higher temperature
extremesandstresses.
Performance aftermarket exhaust system manufacturers most commonly use two grades of stainless
steel:304and321.Grade304stainlesssteelishighqualitystainless,highlyresistanttocorrosion(both
fromexhaustdepositsandfromroadconditions),andnon-magnetic.
Grade304stainlesssteelisusedasanindustrystandardforhigh-qualityexhaustcomponents.Grade
321stainlesssteelisoftenreferredtoasaircraftgrade.Itoffersevenhigherresistancetoheat,fatigue,
andcrackingatextremeheatlevels.Thisgradeisagoodchoiceforextremeheatlevels,suchasthose
encounteredinturboexhaustsystems.
MaterialMeltingPoints
Material
Low-carbonmildsteel
High-carbonsteel
Stainlesssteel
Aluminum
Titanium
Inconel
DegreesF
2,500
2,600
2,750
1,220
3,040
2,600
In essence, a high-quality stainless steel exhaust component, such as headers, pipe, muffler, etc.,
shouldbemadeofeitherGrade304orGrade321stainlesssteel.
Iconel
Inconel,availableingrades625and718,isahigh-strengthnickel-chromium-iron-molybdenumalloy,
whichisoftenreferredtoasasuperstainlesssteelmaterial.Thehighlevelofnickelmakesthismaterial
useful for extreme applications, including Formula One, NASCAR Cup, submarine propulsion motors,
navalmilitaryexhaustsystems,etc.
Inconel’s benefits primarily include its high-rupture strength and ability to withstand extreme
temperaturesforprolongedperiods.
Titanium
Comparedtosteel,titaniumoffersahigherstrength-to-weightratio,anditsmeltingpointishigherthan
othermaterials.However,theaverageenthusiastdoesn’tencounterhigh-enoughtemperaturestojustifyits
use.Theprimaryreasontoruntitaniumexhausttubingistosaveweight,period.Ifyouwanttospendthe
extradoughtolightenthevehicle,haveatit.
By the way, while Chevy touts the late-model Corvette as having a titanium exhaust system; it only
referstotherearmuffler/tipsection,nottheentiresystem.Thecarmakerusedtitaniuminthereartoshave
offafewpounds.
Turboinstallationsnaturallygeneratelotsofheat.Theuseofcoatedtubingorhigh-grade321stainlessisvital.Exhausttubingthatis
highlyresistanttoheatiscriticaltohelpreduceunderhoodtemperatures.Theexhausttubingmustbeabletocapturetheheatand
promoteexhaustheatflowwithminimalheatradiation.
MysteryMix
InexpensivestainlesssteelgradesarecommonlyusedbymanyOEMsthatcontaina“mysterymix”of
materials, with a very diminished level of alloys. That’s why you often see an OEM stainless steel
exhaustsystemthatisrusty.Inmanycases,acarmakerusesjustenoughalloytojustifycallingthematerial
stainless,wheninfactthemetalformulamaycontainapotmetalmixofmagneticparticles(ferrous)to
reduce manufacturing cost. Although this hybrid stainless may last longer than a mild steel component,
referringtothislevelofcheapmetalmixasstainlessisajokethat’snotfunny.
In addition to the long-term appearance and durability factors, cheap OEM “stainless” components
alsodonotprovidethesamelevelofheatretentionasahigh-gradestainlesssteel.Essentially,OEMsuse
low-gradestainlessmaterialtoreducecostandhelpmakethesystemlastjustlongenoughtosurvivethe
vehiclewarrantyperiod.
Ownersofnewlypurchasedvehiclesthatfeatureastainlessexhaustsystemareoftenfrustratedand
angeredwhentheynoticesurfacerustafteronlyafewmonths.Ifyouwantrealstainlesssteel,youmust
relyontheperformanceaftermarket,notthecarmaker.Whenitcomestotheexhaustsystem,carmakersare
moreconcernedaboutshavingcostfromtheirproductionruns,wherequalityaftermarketstainlesssteel
headermanufacturersknowthatenthusiastsarewillingtoupgradetoahigh-qualityproduct.
HeatRetention
Thermal properties are always a consideration for exhaust headers. The heat generated by the
combustionprocessshouldideallybecapturedandretainedinsidetheexhaustsystemratherthanbeing
allowedtoradiatefromthepipes.Containingtheheatinsidethepipesleadstoincreasedpower.Steel
tubing tends to absorb heat, making the tube surface hotter as heat is released. This is detrimental to
powerandcausesexhaustheattoincreaseunderhoodtemperatures.
Fullypolishedstainlesssteelheadersprovideabrightappearanceandareeasytoclean.Ifmadefrom304or321stainlesssteel,
theyprovidebetterthermalefficiencythansteelanddon’trust.
High-quality 304 or 321 stainless steel is better at retaining heat, promoting a more efficient
combustioncontrol(morepower),andreducingunderhoodradiatedheat.Ofcourse,steelheadersmaybe
treated to reduce radiant heat loss by either installing thermal-wrap fabric or by the application of
ceramiccoatings.
Theseheaderswithweldedflangesarereadyforcollectorinstallation.
Whenrunningafreshengineonadyno,avoidusingtheheadersthatwillbeinstalledinthevehicle.Dynosessions,especiallyona
newengine,resultinhighexhausttemperatures,whichcaneasilydiscolornewheaders.Agooddynoshophasheadersonhand
specificallyfordynoruns.Don’tsacrificeyournewheadersforadynosession.Ifthedynoshopdoesn’thaveheadersonhandfor
yourengine,buyorborrowausedset.
Inshort,ifyou’reusingsteelheaders,it’swisetohavethemcoated(orbuythemalreadycoated)orto
wrap the tubes with specialized heat-wrap. If you’re running stainless steel headers, the heat passes
through more quickly and the surface temperature of the tubes is lower. If you want to go even further,
stainlessheaderscanbeceramiccoatedtoachieveanevenhigherlevelofthermalmanagement.
HeaderCoatings
Tubularheadersaretypicallymadefromeithermildsteelorstainlesssteel.Stainlesstype304or321
is the most common. Materials and the need/advantages of specific coatings must be compatible. Mild
steelabsorbsheatmuchmoresothanstainlesssteel.Therefore,thissteelislessefficient;itallowsheat
toabsorbintothetubeandradiateawayfromthetube.Asaresult,thisraisesunderhoodtemperatures,
andthisallowsheatloss,whereideallyyouwanttheheatcapturedinsidethetubetobeevacuatedoutof
the tubes for superior exhaust travel and better performance. High-quality stainless steel tends to do a
much better job at capturing exhaust heat with less radiation (lowering underhood temperature in the
process).
With that said, it’s more important to apply a thermal barrier coating to steel headers (typically a
ceramic-based coating). The other factor at play involves preventing tube corrosion. Steel can rust,
resultingindegradedappearanceandashortenedlifespan.
Stainlesssteel,asmentionedearlierinthischapter,isavailableinvariouslevelsofalloycontent.An
OEMstainlesssteelexhaustheaderormanifoldmayactuallyconsistofamystery-mixofvariousmetals,
whichmaybepronetosurfacerustandexcessiveheatradiation.Whetheryou’reusingsteeloranOEM
“stainless”component,theyarebothcandidatesforaprotective/thermalbarriercoating.
Here’sagoodapplicationforstainlesssteel.Thissmall-blockFordisdestinedforaGT40installationandwillrequireanoverhead
design.Ifsteeltubingisused,athermalbarriercoatingismandatory.TheoverheadplacementisrequiredfortheGT40duetospace
constraintsinthisparticularvehicle.
Even though a high-quality 304 or 321 stainless steel already offers appearance, longevity, and
thermalbenefitswithoutrequiringathermalbarriercoating,theadditionofthistypeofsurfacecoating
furtherfacilitatestheheader’sheat-captureandthermalefficiency.
Sprayinghigh-heatheaderpaintonsteelheadersisanold-schooltechnique,andit’scertainlybetter
thannothing.Whileitprovidesalevelofcorrosionprotectionontheoutersurface,thetubescanstillrust
ontheinsidewalls.Idon’tthinkthateventhebestheatpaintappliedfromaspraycanadequatelyreduce
heat absorption/radiation. In my opinion heat paint offers only temporary protection and appearance
longevity.Ifyou’rerunningsteelheaders,theyshouldbeceramiccoated.Youcanobtainthiscoatingby
eitherpurchasingheadersthatarealreadycoatedbythemanufacturer,oryoucansendbare-steelheaders
toaspecialtycoatingshop.
Withsteelheaders,athermalbarriertreatmentisalwaysawisemove.Steelabsorbsheatquickly,whichresultsinhigherunderhood
temperaturesandaless-efficientheattransferfromtheenginetotherestoftheexhaustsystem.Ceramiccoatingsarepopularand
effective.Shownhereisasteelmanifoldandaturbohousingthathavebeentreatedwithaspecialextreme-temperaturecoating.
(PhotoCourtesySwainTechCoatings)
Here’sanexampleofasatin-blackceramiccoatingonaHookerheader.Notetheflatmodificationononetube,whichwasdoneto
accommodateaclearanceissue.ThisheaderisdesignedtoswapaGMLSengineintoa1955–1957Chevy.
Manyceramicfinishescanbeeasilyappliedtooff-the-shelfheaders.Asaresult,headerscaneasilycoordinatewithcustomengine
colorsandfinishes.
Someextremelyhigh-temperaturethermalbarriercoatingscanprovideathickfinish.Inordertoproperlyfitaslip-onexhaustpipe,
thematingendmayrequiremaskingtopreventcoatingapplication.Bearinmindthatcoatingthicknesscanvarydependingonthe
specificdesignapplication.(PhotoCourtesySwainTechCoatings)
Ceramiccoatingsarenowavailableinavarietyofcolorsandsurfacefinishes.Here’sanexampleofasilverhigh-lusterfinish.
It’seasytohaveyourexhaustheadersceramiccoated.Whetherneworused,bare,painted,oralready
coated,aqualityshopcanprepandapplyheatbarrierceramiccoatingstoyourheaders.
Iftheheadersareusedandhaveafewrustissues,thecoatercanstripthemtobaremetalandthen
decide if repairs are necessary before coating. Typically, coating shops don’t like to perform repairs,
suchasweldingpinholes,etc.Incasesofusedheadersthathavedamage,thecoaterlikelywilldiscuss
thiswithyou,likelyadvisingthathereturnstheheaderstoyoutoberepairedpriortocoating.
If the headers have no damage, the coater strips to bare metal and prepares the surface. This may
involvealightmediablast,dependingontheconditionofthesurfaces.Thenthecoatingisapplied.The
applicationprocessofthecoatingandtheformulasusedaregenerallytightlyheldsecrets.
Sources
Specialtycoatingcompaniescanapplycoatingsthataddressbothheat management and appearance.
SourcesincludeJetHot,SwainTechCoatings,andPolydyn(alsoknownasPolymerDynamics).
WhenheadersaresenttoJetHotforcoating,thefirststepistoheattheheaderfor“pre-blast.”Thiscooksoffanyoils,carbon,or
othercontaminants.(PhotoCourtesyJetHot)
Jet Hot has many products. In addition to anti-friction coatings for various engine and driveline
applications,theyoffertwolevelsofceramicheadercoatingsthatareappropriateforstreetandraceuse.
The Extreme 1300 series withstands up to 1,300 degrees F and comes in a high-luster (almost-gloss)
finish, which is very smooth and very easy to clean. The Extreme 2000, which is designed for up to
around2,000degreesF,isidealforturboapplications.Thefinishonthisversionprovidesasomewhat
roughfinish.
Inall,JetHotoffersaround35colorsfromwhichtochoose.Thesecoatingsareidealforstreetuse,in
termsofcorrosionprotection,appearance,andeaseofmaintenance.
Aftertheheaderis“cooked,”itrunsthroughavibratorymediacleansing,whichremovesallsurfacerustandprovidesaready-tocoatbare-metalsurface.(PhotoCourtesyJetHot)
Swain Tech Coatings offers many specialty coatings for all aspects of engine, driveline, and brake
systems. These include thermal barriers, anti-friction coatings, oil drainback coatings, etc. Its exhaust
coatings focus on heat management for engine efficiency and reduction of radiated heat, and as a result
corrosionprotectionisanadditionalbenefit.Swain’sTBCcoatingsarespecificallydesignedforextreme
insulatingandnotintendedforshowcarappearance.Theseareformulatedforcompetitionapplications,
but work for other applications as well. The finish is white, with a slightly rough texture, applied at a
thicknessofabout.002inch.
Polydynproducesavarietyofspecialtyperformancecoatingsformanyengine,driveline,andbrake
components.Itsceramicheader/exhaustsystemcoatingsarepolymercompositeswithinsulatingceramics,
designed to reduce heat transfer and protect against damaging heat saturation. The exhaust coating is
typically a bronze color with a semi-matte finish. According to Polydyn, when applied to the interior
walls of a chromed pipe or chromed headers, the thermal insulating properties eliminate discoloration
andbluing.
Inadditiontosolidcolors,customgraphicsarenowavailablefromselectcoatingservices.ShownhereisanexampleofJet-Hot’s
Dragongraphic.(PhotoCourtesyJetHot)
Ihaveusedallofthesefirmsforvariousheaderandpipecoatings,andI’vebeenthrilledwitheach
result.Timeframesvarydependingontheirworkloads,butthetypicalturnaroundtimehasbeenaround1
to2weeks(sometimesmuchless).
Cerakoteisanotherservicethatspecializesinexhaustcoatings.Theyofferawiderangeofcolorsand
finishes.
Flowtech is a performance header and exhaust system brand, which is part of the Holley product
family.
Intheearlydaysofceramiccoating,alimitednumberofcolorsandfinishlusterswereoffered.Today,coatingbusinessesoffera
widerangeofcolorsandsurfacesheens,frommattetohigh-luster.ShownhereisanexampleofCerakote’sTitaniumfinish.(Photo
CourtesyCerakote)
Cerakote’sCobaltfinishhasbeenappliedtotheseheaders.(PhotoCourtesyCerakote)
ThisisanexampleofCerakote’sTungstenfinish,whichoffersaslightsatinsheen.(PhotoCourtesyCerakote)
ThisisanexampleoftheCerakoteBlackVelvetfinish,withablackcolorationaccompaniedbyanattractivesatinsheen.(Photo
CourtesyCerakote)
ThisheaderiscoatedinCerakote’sSatinNickelfinish.Othercoatersalsoofferadditionalcolors,suchasblue,red,orange,green,
yellow,etc.Today’sofferingsprovidethebuilderwithaslewofappearanceoptions.(PhotoCourtesyCerakote)
Ahigh-lusterfinishisextremelypopularandmostheadermanufacturersofferitaswellascoatingservices.Shownhereisan
exampleofFlowtech’sBrightLusterceramicfinish.
PrimaryTubes
As with the selection of carburetors, fuel injectors, tires, wheels, etc., bigger is not always better.
When you consider header primary tube diameter, many variables come into play, including engine
displacement,camshaftprofile,andtheintendedvehicleuse.Doesthedriverdemandmorebottom-end
torqueorpowerathigherenginespeed,etc.?
In a way, you can relate exhaust tube diameter to liquid plumbing pipe diameter. At the same input
pressure, a smaller-diameter tube creates higher water pressure, while a larger tube results in lower
waterpressureoutput.
Inoverallterms,smaller-diameterprimarytubespromotelow-endtorque,butiftubediameteristoo
small,backpressurecanincreasetoadetrimentallevel(lossofpowerandburntexhaustvalves).Largerdiameterprimarieslendthemselvestotop-endpower,butiftoolarge,insufficientbackpressureresultsin
a loss of power, especially at bottom-end and mid-range operation. As far as primary tube length is
concerned, longer tubes provide increased torque, while shorter tubes may aid in quicker exhaust gas
scavenging.
Ratherthanbuyingthelargest-diameterexhaustheadersavailableforyourengine,youneedtomatch
thetubediametertotheneedsoftheengine.Inotherwords,youneedtotailortheengine’spowerband
withinthetargetoperatingRPMband.
If you want peak torque at, say, 4,000 rpm for a street car, the header tube diameter needs to
accommodatethis.Ifyouhavearacingenginewherepeaktorqueneedstobeinthe6,000-rpmrangeand
peakhorsepowerisinthe8,000-rpmrange,youneedtomatchtheexhaustsystemtoworkbestatthose
enginespeeds.
PipesandOutput
PrimaryPipeSizeandLength
Primarytubestoosmallindiameter
Primarytubestoolargeindiameter
Shorterprimarytubes
Longerprimarytubes
Longandsmalldiametertubes
Shorterandlargerdiametertubes
Output
Canchoketheenginebycreatingexcessive
backpressure
Mayimprovetopendpower,butlow-endtorque
andmid-rangepowerarereduced
SuitedforpowerbandathigherRPM
SuitedforpowerbandatlowerRPM
MoretorqueatlowerRPM(theoretically)
MorehorsepowerwithtorqueathigherRPM
(theoretically)
Shortyheadersrepresentacompromisebetweenfitandperformance,withfitmenttakingpriority.Theyareprimarilyintendedfor
streetapplicationswhereclearanceissuestakeprecedence.
Long-tubeheadersprovideincreasedtorqueatlowerRPM.Performance-wise,thisisthepreferredapproachwherebottom-end
powerisdesired.Long-tubeheadersprovidebetterscavengingfortheexhauststream,whichpromotestorque.
Astreetenginedemandsmoretorqueatbottomandmid-range,whilearaceengine(dependingonthe
application)cansacrificebottom-endtorqueinfavorofgeneratingmaximumhorsepower.Headerdesign
isallaboutexhaustgasvelocity.
Shorty headers provide easy installation because they reduce fitment issues and have fewer
installationchallenges.Whilenotappropriateforaraceengineforwhichyouneedtoextractmaximum
power,shorties(evenwithoutequal-lengthprimarytubes)stillprovidesuperiorperformancetoastock
exhaust manifold. Unless you’re building a dedicated race car, it’s not worth obsessing over extracting
everyounceofpower.
Step-tube headers have changes in primary tube diameter from the cylinder head to the collector.
Thesechangesintubediameteralongtheexhaustpathcanaffectthespeedofgasflow.Foranaturally
aspiratedengine,astepheaderfeaturessmaller-to-largertubediameter.Thismeansthattubediameterat
the entrance (from the cylinder head flange) is smaller, with tube diameter progressively changing to
larger diameter(s) until terminating at the collector. This aids in pulling exhaust gas out of the cylinder
headandspeedinguptheexhaustpulse(improvedscavenging).
ThisAAdragcarposesnofitmentissues,soequal-lengthprimarytubescanbeusedtomaximizeengineoutput.(PhotoCourtesy
BurnsStainless)
Theenginedoesn’tcarewhichwaythecollectoraims.Aseeminglyunorthodox“backward”installationonthisstreetrodsolved
clearanceissuesrelatingtothesteeringshaftandbody.
Astepheaderfeaturestubingdiameterchangesfromthemountingflangetothecollector.Fornaturallyaspiratedengines,the
primarytubediameterchangesfromsmaller(attheentrance)tolargerasthetubelengthprogresses.Thisimprovesscavenging.For
aturbochargedapplication,thediameterchangemovesfromlargertosmaller.ShownhereisaracingstepheaderfromBurns
Stainless.(PhotoCourtesyBurnsStainless)
For a turbocharged application, a step header features primary tubes that transition from larger to
smallerdiameters,creatingasqueezevelocitythathelpstospooltheturboquicker.
PrimaryTubeProfile
In a perfect world, the first 2 to 3 inches of the primary tubes should be relatively straight. This
extendsthecylinderheadexhaustports,effectivelymakingthemlongerandhavinggreatervolume.Unless
you’re dealing with a race car application where this is possible (with no obstructions that allow a
longer,straightershotoftheprimarytubesformthecylinderheads),youmaybeforcedtobeginbands
closertothecylinderheadexhaustportsinordertomaketheheadersfittheconfinesofthevehicles.
Primary tube bends are all about making headers fit the engine and also fit with the engine in the
vehicle.Bendradiusisalimitingfactor.It’snotpracticaltobendapipeinaradiusthat’stighterthanthe
tube OD; it’s just too tight. This forces the fabricator to extend a tube farther before a bend. However,
there’sawayaroundthis.
An example is Ultimate Headers’ approach when an immediate tight turn is required. They
investment-casthigh-gradestainlesssteelinatightelbow,thenTIGweldthecastelbowtothestainless
steel tube (all of Ultimate’s headers are 100-percent stainless steel). This allows the tube to begin its
bendmuchclosertotheheaderflangewithoutlosingcross-sectionarea.Tightbendssuchasthislimitthe
effort of extracting maximum power, but because of fitment issues, tighter-than-desired bends are
sometimesanecessaryevil.
TubeDiameterandArea
Thischartillustrateshowprimarytubediameterrelatestopercentageincreasesinareaastube
diameterchanges.
UltimateHeadersoffersstainlesssteelheaderswithinvestment-caststainlessflanges,whichprovideacustomCNClooktothe
package.
FlangeWelding
Flanges are welded to the primary tubes on a jig, which simulates the cylinder head. The flange is
boltedtothejigtoholditflat.Thejigholdstheindividualflangesintheproperspacing,suchaswhenthe
primarieshavemultipleflanges.Oneflangepertubeisusualorthefrontandreartubeshavetheirown
flangesandthecentertwoprimarytubesshareacommonflange,suchaswithGenIsmall-blockChevy.
Thejigsecurestheprimarytubestothecylinderheadflangeandholdsalltubesintheproperlocations
forspecificvehicleapplications.
Headermanufacturersmakepatternsforeverymake,model,andyearvehicle,inadditiontovariants
for specific engines and engine accessories, such as A/C, power steering, etc. This represents an
enormouslytime-consumingandcostlyinvestment.Pricescouldbemanytimeshigher.
Productionheadermanufacturerscommonlytakeadvantageofadvancedlaser-cuttingmachinestoproduceexhaustflanges.This
high-capacitylaseratStainlessWorksproduceshundredsofexhaustflangesdaily,providingahighdegreeofprecision.
Theseareexamplesofstainlesssteelflangesproducedbyalasercutter.Precision-cutflangesareproducedbasedontheexact
dimensionsrequiredforaspecificcylinderhead,ensuringproperportandboltholealignment.Theflangesarethenlightlyedgedeburredbeforetheprimarypipesareweldedtotheflangesinaprecisionjigthatorientseachpipeinitsproperplane.
Heatshouldbeevenlydistributedwhentheflangesarewelded.Iftheweldingisperformedproperly,
heatisappliedinasequentialpatternthatspreadstheheatevenly.Insteadoffullyweldingeachtubein
order, the flange is released from the pattern jig and there should not be too much residual stress that
causesaone-pieceflangetowarporcausesmisalignmentofindividualflanges.
Oncetheprimarytubesforaspecificapplicationhavebeenbenttoshapeandcuttolength,theindividualtubesareassembledina
weldingjigthatholdsthetubesinplaceattheflangeandaregroupedtoachievetheproperarrangement.
Whiletheheadergroupissecuredinitsjig,eachprimarytubeiscarefullyweldedtothemountingflange.TIGweldingisgenerally
preferred,whichallowstheweldertofuseelementstogetherintighterspacesandwithminimalweld-beadbuildup.Althougha
talentedMIGweldermaybeabletoaccomplishthetask,TIGlendsitselftoneater,smallerweldseamswithlesschanceofburning
throughthepipe.
Oncetheflangehasbeenweldedtotheprimarytubes,theflangesurfaceisinspectedanddressedtoensureaperfectlyflatsurface.
Mild steel welding may involve either MIG or TIG welds. Stainless steel assemblies are almost
alwaysTIGwelded.
TubeBending
Mandrelbendingisnecessarytopreventkinksandwrinkleswhenbendingheadertubingoranytubing
forthatmatter.Whilevariousformsoftubebendingarepossible,thetwomostcommonincluderamstyle
andmandrelstyle.Hydroformingmayalsobeused.
RamStyle
Rambendingisoftencalledcrushbending.Itusesafemaledietomatchthetubediameterwiththe
angle of bend radius required. The tube is simply hydraulically drawn over the die. In the process, the
outside of the bend stretches to make the outer wall thinner. At the same time, the inside of the bend
compresses,featuringacrushedareathatcreatesanunroundconditionwithinthebendradius.
While this may be acceptable for passenger cars’ exhaust systems, it’s not acceptable for a
performancesystembecausetuberestrictionsaretobeavoided.
MandrelStyle
Mandrelbendingalsousesanouterbendingdie,butadditionallyfeaturesamandrelthatisinserted
into the tube. The mandrel consists of multiple “ball” rings that are connected via a flexible rod. The
mandrelballsarefreetomoveinanarcthatfollowsthepathofthedie.Thisprovidesinternalsupportfor
thetube,preventingitfromcollapsingorkinking,keepingtheinsideofthetuberound.
Mandrel balls are selected not only based on the tubing inside diameters, but also in terms of
thicknessandspacing.Infact,thenarrowerballswithcloserspacingprovidemoreinternaltubesupport.
This allows the manufacturer to create the optimum mandrel for specific materials and degree-of-bend
radius.
Mandrel balls are also available in different materials and hardness qualities. For harder tubing
material,softerballsareused;forsoftertubematerials,harderballsareneeded.Whenusedwithsteelor
aluminum tubing, a harder chrome-plated steel mandrel ball is used. For stainless steel tubing, a softer
aluminum-bronzemandrelball,calledAB18,isused.
Bendscanbemadeinanydegree,uptoandincluding180degrees.Duringthebendingprocess,even
withamandrelbenderinwhichtheinsideofthetubeissupported,averyslightandbarelynoticeable
deformation of the inside of the bend occurs. To remove this minor irregularity, a wiper, positioned
immediatelybehindthedie,appliesmoderatepressureandwipestheinsideofthebendsmoothtoregain
roundness. The CNC operator must precisely adjust the wiper at an angle of 1 degree. Even a minor
deviationofthisanglecanaffectthefinishquality.Thewiperislocatedinawipingdiethatisaligned
withtheentryofthebendingdie.
Although tubes can be bent to shape using a manually operated mandrel bender, high-performance
headermakerscommonlytakeadvantageofCNCmandrelbendingmachines.Oncetheprogramhasbeen
writtenforaspecificprimarytube,asectionofstraighttubeisinsertedintothemachine,whichisfitted
withtheproperdiethatmatchesthetubeOD.
Mandrelbendinginvolvesaflexiblemandrelthatisinsertedintothetube.Thisprovidesclose-tolerancesupportinsidethetube,
preventingitfromcollapsingduringthebendingprocess.Awideselectionofmandreldiametersisinventoriedtosuiteachprimary
tubeIDsize.
Thetuberidesalongaformingdie,whilethemandrelsupportstheinsideofthetube.Atubingwiperispositionedin-linewiththedie,
adjustedtoatypical1-degreeanglerelativetothetube.Asthetuberunsthroughthedie,thewiperdressestheinsideofthebend,
smoothingoutanyminortubewallirregularities,whichmaintainsaconstantinsidetubediameter.
Here’sanexampleofatubemandrel.Aflexiblecoreconnectsaseriesof“balls.”Thisallowsthemandreltofollowthebendofthe
tubewhileprovidingthenecessarysupportfortheinsideofthetube.
HeaderprimarytubebendingisaccomplishedonhighlypreciseCNCmandrel-bendingmachines.Here,anAddison-McKeeCNC
pipebenderisprogrammedtorunaspecificengine-cylinderprimarytubeatStainlessWorks.
Eachindividualprimarytubeforaspecificheaderrequiresitsownbendingprogram.Oncealengthofstraightpipeofthe
appropriatediameterisinsertedintothemachine,theCNCbendercompletesallofthebendsrequiredforthatsectionoftubing,
whiletheCNCtechnicianmonitorstheprocess.
The automated bending machine then performs the desired bends needed for that specific primary
tube.Remember:Dependingontheengineapplication,thisrequiresfour,six,eight,ormoreindividual
primarytubes,eachwithitsownuniqueshape.SeparateprogramsmustbewrittenfortheCNCmachine
foreachprimarypipe.
Hydroforming
Anothermethodofpipebending,hydroformingisoftenusedbymass-productiondirect-replacement
exhaust pipe makers that supply carmakers and the replacement aftermarket. These machines are very
complexandcostisprohibitiveformostheadermakers.
Hydroforming is another type of mandrel bending. Instead of using a metal mandrel inside the tube,
high-pressuredwaterisinjectedintothetube.Thisfillsthetubeand,becauseoftheenormouspressure,
preventstubecollapseatthebendareas.
Hydroformingisalsousedtofabricatevariousshapes,turningastraightpieceoftubeintojustabout
any shape required. The tube is placed into a two-piece die, the die halves lock together, and water
pressureisinjected,pushing(expanding)thetubeintothedie.
HeaderFabrication
Once a design has been established, primary tubes are bent to shape and trimmed for length. In a
positioning jig that holds the flange and all of a bank’s primary tubes in place, the mounting flange is
welded to the entry of the primary tubes. After the welds are dressed (if necessary), the collectors are
installedandwelded.Thisisaverybasicoverview.
When a header manufacturer designs a system for a specific engine and vehicle application, the
prototypingtakesplaceonanactualvehicleandengine.Variousmanufacturersusedifferentapproaches.
StainlessWorksbeginswithaflangeandtack-weldedbendsattheentryareafromtheflange,andthen
works outward using a selection of straight tube sections connected by a tack-welded bendable bridge
tab.
MIGversusTIGWelding
TIG(tungsteninertgas)weldingispreferredforheaderfabrication.OnereasonisthatMIG(metal
inertgas)weldingprovidesaless-stablearcandcanresultin“dirtier”welds.Withoutgoingintotoo
muchdetail,here’sabriefexplanationoftheirdifferences.
MIGweldingfusestwopiecesofmetaltogetherusingaconsumablewirethat’sconnectedtoan
electrode.Theweldingwireandgasisfedthroughtheweldinggunatthesametime.Thegasserves
toprotecttheelectrodefromcontaminants.MIGweldingiseasiertomasterandtheequipmenttends
tobemoreaffordable.
TIGweldingusesatungstenelectrodetoprovidetheelectricalenergywhileaweldingstickis
manipulated at the weld area as filler. TIG welding is much cleaner, with less smoke and
contamination and can result in greater precision, allowing accurate and tidy welds without excess
buildup. A skilled TIG welder can produce welds that need no additional grinding or dressing.
Especiallyforfabricationofstainlesssteelheaders,TIGweldingisthepreferredmethod.
Once the basic shape has been established to fit and clear the application, this pattern is then
transferred to a coordinate-measuring machine (CMM), where a digital probe traces the flow of each
primaryassembly.TheCMMprogramthencalculatesthebendradius,whichisbasedonthechosentube
diameter at each deviation between the straight sections. A program is written for each primary pipe,
whichisthentransferredtoaCNCtubebender.
Once a set (for each engine bank) has been assembled, test fitting and (if necessary) additional
adjustment is made. When the engineer is satisfied with the results, the same primary tubes can be
producedatwillontheCNCbender.
Fabricating your own exhaust headers requires patience, the ability to measure, the ability to weld,
and access to specialty tools. Even if you don’t have access to a tubing bender, you can fabricate your
ownheadersbypurchasingaselectionofpipesectionsbenttospecificangles,thenpiecing/weldingthe
assembly together. Luckily, all components are available separately, including flanges for all popular
cylinderheads,collectors,collectorflanges,andtubinginavarietyofdiameters,lengths,andbends.
ProcedureOverview
Dependingonthecomplexityofeachheaderprimarytubeintermsofthenumberanddegreeofbends,
a primary tube may be formed as a single piece. If the design involves a series of complex bends that
simplycannotbeachievedusingasinglecontinuouslengthoftube,aseriesoftwoormoresectionsmay
berequiredinordertocompleteasingleprimarytube.
Inamanufacturingfacility,inthecaseofaV-8engineapplication,onceallfourprimarytubesfora
givenenginebankhavebeenbenttoshapeandtrimmedtolength,theheaderflangeandtheprimarytubes
areplacedontoapatternjiginordertoholdeverythinginplaceinproperorientation.Allsectionsare
thentackweldedtogetherbyTIGwelding.
Thecollectorisplacedintopositiontoencapsulatetheendsoftheprimarytubesanddouble-checked
forfit.AdditionalTIGweldingisthenperformedinallareasthatareaccessibleonthejig.
Theheaderassemblyisthenremovedfromthejigtocompleteallrequiredwelds.
If the header is being fashioned by a hobbyist who does not have access to a pattern jig and is
fabricatingauniquecustomheader,tubearrangementandroutingmustbedeterminedwiththeenginein
thevehicle.
Start by obtaining a pair of header flanges. Unless you’re dealing with an unusual cylinder head,
flangesarereadilyavailableforallpopularengines,pre-cutandreadytobolton.Onceyou’vedecided
ontheprimarytubediameter,thegeneralapproachistobeginbyweldingashortstubsectionoftubeonto
eachflangeport.Thisprovidesaninitialattachmentpointfortheremainderofeachtube.
By purchasing a selection of straight tubing and various pre-bent tubes (45-, 90-, 180-degree for
example), you have enough material to complete each tube by cutting sections to length for the desired
routing.Thismustbeperformedwiththeengineinthevehicleinordertoobtainclearancesforthetubes
torunpastoraroundframework,steeringshaft,steeringbox,etc.
With the stub-tube-equipped flange mounted to the head, temporarily mount the collector in your
desired location, secured with clamps or with tach-welded braces. Positioning the collector provides
yourtargetfortheendsoftheprimarytubes.Withoutacollectorrigidlypositionedinplace,youendup
withanightmarewhentryingtogrouptheendsofthetubestogetheratalaterpoint.
Now you can begin to add tubing, starting at the header flange. For instance, you may want a 90degreebendattheflangestub.Inordertobeginroutingthetubeatyourdesiredlocation,cutapre-formed
90-degreetubeatthedesiredradiuspointinordertoobtainasharporwideangle.Ifyouneedasharper
diveangle,youcantrimthestubtubeandthematingtubeendsatmatchinganglestomaketheendsmeet
withouthugegaps.
Aseachsectionofprimarytubepositioningisdetermined,onlytackwelditinplace.Thismakesit
easier to remove one or more sections if corrections are needed. Once the tubes are all in place and
you’rehappywiththeresults,addafewtackweldstoeachsectiontomaketheassemblystable.
Carefullymovetheheadertoacleanworkbenchandaddafewseamwelds,thenre-installittoverify
fit.Removeitagainandcontinuetoweld.
Onceallweldingiscomplete,re-installtheheaderandverifythatitfitsproperly.Youwanttomake
surethatthefitiscorrectbeforeyoudressyourweldsandcertainlybeforeyoupainttheheadersorsend
themoutforcoating.
Thisisatime-consumingandpainstakingtask,andreallyisn’tnecessaryunlessapre-madeheaderis
notavailableforyourapplication.
HeaderDesign,FittingandFabrication
Atimesavingapproachtoheaderprototypedesignistotackweldandassembleaseriesoftubesectionsinaspecificvehicle,
ratherthanmakingnumeroustestbends.
Oncetheprimarytubesectionshavebeentackweldedtogether,thetestpipeisthenprofiledonaCMM.
ThisviewoftheCMMmonitorshowsthesectionsofstraighttubethatweremockedtogetherduringtheprototypedesign.The
CMMsoftwarethencalculatesthenecessarybends.
TheCMMcalculatesthebendradiusrequiredforeachbendareaalongthelengthoftheprimarytube.
Aclose-upofthecomputermonitorshowsthebendradiuscalculatedbytheCMMforaspecificbend.
OncethedatahasbeenacquiredbytheCMM,thedataistransferredtotheCNCbenderandatestpipeisformed.Herea
technicianchecksthetestpipewiththeCMMdigitalprobetoverifythatthetubemeetstheinitialdesign.Onceallprimary
tubeshavebeenformed,theheadersetistestfittedtothevehicleagaintocheckforproperfitandclearances.Anyadditional
tweaking(ifneeded)isthenperformed.Afterthedesignisapproved,thesoftwaredataisreadytobeusedontheCNCtubingbender
toproducethefinishedtubes.
Oncethemergespikehasbeeninstalled,thecollectorispositionedandcarefullyTIGwelded.
AthreadedbungmaybeinstalledonthecollectorforthoseapplicationsthatrequiretheuseofanEGTsender.
Insomeapplications,becauseoftherequiredclearanceandfitmentissues,thecollector-to-primarytubegroupneedstobe
angled.Herea4-into-1collectorentranceisangle-cutwhileheldinaprecisionjig.
Thisbatchofstainlesssteelheadersawaitsfullpolishing.Makersofstainlesssteelheadersoftenofferanatural(satin)finish
orafullpolish.Sincestainlesssteel304providesaneffectivethermalbarrier(thematerialdoesnotabsorbheatasmuchassteel
tubes),there’sreallynoneedforadditionalheatbarriercoating.
PortMatchingandScavenging
Portmatchingtheheaderprimarytubeentrytothecylinderheadexhaustportexitisn’tascriticalas
whenmatchinganintakemanifold’sportstothecylinderhead,butitisimportantifyouhopetoextract
maximumenginepower.Whileitatfirstappearsthatexactlymatchingthecylinderhead’sexhaustportto
theprimarytubeentranceisagoodidea,thisisn’tnecessarilythecase.
Themostimportantconsiderationistoavoidrestrictingtheexhaustpath.Youneedtobesurethatthe
primary tube entrance does not block any portion of the cylinder head exhaust port. Simply put, don’t
obstructthehead’sexhaustports,witheithertheprimarytube/flangeortheexhaustgasket.Theprimary
tubeinletshouldbethesamesizeorlargerthanthecylinderhead’sexhaustport(don’tchokethehead).
Ideally,theentranceoftheprimarytubeshouldprovideamirrorimageofthecylinderhead’sexhaust
port. This port has the same dimensions and is either round, D-shaped, or square. This is critical for
racerstryingtoextracteveryounceofpower.Forperformancestreetapplications,theportsreallydon’t
needtomatchexactly,aslongastheentranceoftheprimarytubesdon’tblockanyareaofthecylinder
headexhaustports.
Ideally,theprimarytubeentrancematchesthesizeandshapeofthecylinderhead’sexhaustport.Inpracticalterms,it’svitalthat
theprimarytubeIDisthesamesizeoratadlargerthantheexhaustport.Thepointistoavoidobstructingtheflowfromtheexhaust
port.
Thisisa“snake”headersetupwhereequal-lengthtubingisused,andwithspecificdriver-andpassenger-sideprimarytubes
crossingovertoopposite-sidecollectors.Thisissometimesdonetoimproveexhaustscavenging,basedontheenginefiringorder.
Afteronecylinderfiresandtheexhaustvalveopensandexhaustgasmovesthroughtherightcollector,thenextcylinder(onthe
oppositebank)takesadvantageofthescavengingeffectatthesamecollector.Basically,eachsuccessivepairofcylindershelpseach
otherpullexhaustgasthroughthesystem.
Withthatsaid,youneedtoconsiderexhaustreversion.Thiscanoccurwhentheexhaustpulsebangs
aroundasitflowsthroughtheprimarypipesandcanbouncebacktowardthehead’sexhaustport.AsI
mentionedearlier,ideallyyouwanttheprimarytubestoextendasstraightaspossiblefromtheexhaust
portby2to3inches.Aseverebendimmediatelyfromthecylinderheadcanresultintheexhaustpulse
hittinga“wall”andtheexhaustrevertingbacktowardtheportsandexhaustvalves.However,becauseof
therealitiesoffittingtheheadersintheavailableunderhoodspace,thisisn’talwayspossible.
Preventing exhaust pulse reversion is one aspect of exhaust tuning. Another aspect is scavenging,
where exhaust pulse is used to pull or scavenge exhaust from other cylinders. The scavenging can take
place where two or more primary pipes merge onto a secondary pipe or main collector. The pulse or
pressurewaveofgasesexitingoneprimarypipecanbeusedtopull/siphonexhaustgasoutofanadjacent
primary pipe, which is sort of like a turbocharger or supercharger action in reverse. This scavenging
effect,ifbalancedproperly,improvestheexitofexhaustgasfromallcylinders.
AnanalogyistopicturetwostreamsofequalsizethatconvergeinaYintersection.Theenergyfrom
eachstreamhelpstopulltheotherstreamalong.Theopeningoftheexhaustvalvesgeneratestheengine
pulsesandthelengthoftimethosevalvesareopen.Excessivebackpressureand/orpulsereversiontends
to restrict the engine’s breathing, as spent gases being trapped or pushed back into the combustion
chamberarecontaminatingtheexhaustgaschargethatit’stryingtospitout.
Onewaytopromotescavenginglieswithinthedesignofthecollector(wheretheprimarytubesenter
the collector). A 4-into-1 collector is an open collector that simply accepts the bundle of four primary
tubes,collectingthemintoasinglelarger-diameterpipe.
Thisisanexampleofatypical4-into-1collector,whereagroupoffourprimarytubesarejoinedinacollector.Dependingonthe
modelvariation,thetubesmaybeinsertedandwelded,oramergespikemaybefirstinstalledontotheexitendsoftheprimarytube
groupbeforethecollectorisinstalled.
Aperformancefeatureusedbysomeheadermanufacturersisamergecollector.Thisprovidesanuninterruptedsmoothflowof
exhaustgasthroughthecollector,preventinganyreversionpulsesthatmightoccuriftheexitendsoftheprimarytubeswereopento
eachother.
Thisisatypical4-into-1collector,whereallfourprimarytubesononebankenteracommoncollector.
AparallelTri-Ycollectorjoinstwoprimarytubestoacommoncollector.Inan8-cylinderengineapplication,apairofTri-Ycollectors
maybeusedoneachbank.Bypairingtwocylindersintoacommoncollector,you’reabletopaircylindersthatfirefarthestaway
fromoneanothertoimprovescavenging.Thisshouldimprovetorqueinthelow-andmid-rangepowerbands.Thisfollowsthesame
theoryasofferedbyTri-Yheaders,whereprimarypipesarejoinedtogether(mergingprimariespriortothecollector).
Theflatendofthismergespikeisweldedtotheendsoftheprimarytubegroup.Thefour-sidedspikefeaturesradiusedgroovesthat
provideacontinuationoftheexhaustflowforeachofthefourtubes.Thespikeisalsoslightlytwisted,whichpromotesswirl
turbulenceastheexhaustgasesflowalongthespike’swalls.Thisimprovesexhaustscavenging,helpingtopullthegasesthroughfor
superiorflow.
Hereamergespikeistack-weldedtotheprimarytubeends.Thespikeisfirstpositionedonacenteringjigforprecisecenteringand
orientation.
Thismergespikehasbeentack-weldedintoposition.
Ifyourheadersfeatureaone-piecemountingflange,expecttoremovetheoildipsticktubeinordertoinstalltheheaders.When
installingthetube,makesurethatyouhaveadequateclearance.Ifthedipsticktubecontactsaprimaryexhausttube,youcould
transmitexcessiveheattothedipstickandsubsequentlytotheoilinthesump.
FlatheadFordenginesfeatureeightcylindersbutonlysixexhaustports(threeperside).Thecentertwocylindersexhaustthrougha
commonport.ShownhereisaheaderfromRed’sHeaders,finishedinchromeplating.
Amergecollectorissimilar,buthadataperedmergesurfacethatpromotesasmoothergasflow.This
features a center spike point, with tapered walls that serve to extend the outlets of the primary pipes
beforethegasentersthecollector.Thismergingeffecthelpstospeeduptheexhaustgasvelocity.
Atri-Ycollectorfeaturestwobanksoftwoshortsecondarypipeswheretwoprimarytubesenterone
pairandtheothertwoprimarypipesenterthesecondpair.Bothpairsofsecondarytubesenteracommon
collector.Thisspeedsuptheexhaustgasevenmore,forincreasedlow-endtorque.
Amergespikemaybeaddedtothebundleofprimarypipesbeforetheinstallationofthecollector.
Thisisataperedfour-sidedspikethatalsopromotesanextendedandsmoothertransitionfromtheendof
theprimarytubesintothecollector.ThemergespikesusedbyStainlessWorks,forexample,alsofeature
aslighttwist,whichcreatesavortexspinofexhaustgasinsidethecollector.Thisimprovesscavenging,
promotingavacuumeffecttohelptopulltheexhaustgasthroughthesystem.
GasketsandFasteners
Sealingtheexhaustheaderflangestothecylinderheadiscritical.Aleakingexhaustflangeproduces
annoyingexhaustnoise,aswellasvisibleburn/scorchmarksontheheadsandcanevenresultinburning
theexhaustvalves.Agasketisthetickethere.
Exhaustheadergasketsvaryinmaterial,includingmulti-layersteel(MLS),copper,andcomposites.Ifthecylinderheadexhaust
deckandtheheaderflangeareperfectlyflat,anyappropriategasketshouldwork.Ifleaksareaproblem,anextreme-hightemperatureRTVcanbeusedtofillvoids.Somefolksrelyonahigh-temperatureRTValongwithacutgasket.Shownhereisan
MLSexhaustgasketonaGMLSengine.
Althoughvariouslow-clamping-loadfastenerapplications(suchasvalvecovers,timingcovers,and
thermostat housings) may be accommodated by common Grade-5 bolts, nuts, or studs, critical fastener
applications absolutely demand the use of the highest-quality fasteners. Examples of high-stress
applicationsincludemaincapboltsorstuds,connectingrodbolts,cylinderheadboltsorstuds,flywheel
bolts,crankshaftbalancerbolts,superchargerhousingfasteners,etc.
Everythreadedfasteneronthevehiclemustprovidethepropertensilestrengthrequiredforanygiven
application.Componentsthatareexposedtoextremestressessimplycannotbecompromisedwiththeuse
ofquestionablefasteners.
Avoid generic or bargain fasteners for critical high-stress applications. Use only high-quality
fasteners from firms such as ARP, A1 Fasteners, AEBS, and other high-performance fastener
manufacturerswhospecializeinhigh-stressautomotivefasteners.
In terms of threaded fasteners for exhaust systems, keep in mind that bolts or studs must be able to
handle the required tightening and clamping loads. They are also exposed to higher levels of heat and
routinelyencounterperiodsofthermalexpansionandcontraction.Neveruseanyboltorstudthatisrated
lessthanaGrade8.
Specificallyforexhaustmanifoldorheaderflangetocylinderheadmounting,it’sbesttousefasteners
that are specifically made for that purpose. Again, firms such as ARP, A1, and Totally Stainless offer
them.
Wherepractical(intermsofinstallationclearance),theuseofexhauststudsisagreatidea.Studsprovideaconvenientguidefor
headerpositioning.Inaddition,forsituationswheretheheadersareremovedandinstalledfrequently(aswithracecars),theuseof
studseliminatesthreadwearthatispossiblewithcontinuedremoval/installationofbolts.Installstudsfinger-tight,withaslight
preload.Neverdouble-nutastudandattempttoovertightenit.Theclampingforceofthenutsecurestheflange.
Mypreferenceistousethehighest-qualitythreadedfastenersforeveryapplication,regardlessofthe
desiredtorqueload.There’sanoldadagethatappliestothisapproach:“Forlackofanail,thehorseshoe
waslost.Forlackofthehorseshoe,thehorsewaslost.Forlackofthehorse,thekingwaslost,andfor
lackoftheking,thebattlewaslost.”I’mparaphrasinghere,butyougetthepoint.
Whetheryouuseboltsornuts,thecorrecttighteningforceiscriticalinordertoachievetherequired
clampingforce,tobothsecuretheheaderflangeandtopreventgasketleaks.
Gaskets
To obtain a proper seal, the two mating surfaces (the cylinder head’s exhaust deck and the header
flange)mustbeflatandparallel.Anywarpingineithersurfaceisaleakwaitingtohappen.Variousgasket
materialsareavailable,includingfiber,composite(perforatedmetal/fiber),MLS(multi-layersteel),and
bead-formedcopper.
Studsgreatlyeaseheaderinstallationbecausetheyprovideaguidesoyoudon’thavetofiddlewithboltsasyoutrytoaligntheholes.
WhiletheuseofanRTVtoaidsealingmayatfirstseemcrudeandviewedasa“backyard”fix,hightemperatureRTVisavailablethatcanenhanceexhaustsealingforproblem-leaksituations.Oneexample
isPermatex’sUltraCopperRTV.Addingathinbeadofultra-high-temperatureRTVtoanexhaustgasket
caneliminatetroublesomeleaks.I’msurethatsomepeopledisagreewiththis,butI’vesuccessfullyused
UltraCopperforproblemleaksonseveraloccasions.
Believeitornot,ultra-high-temperatureRTVcanalsobeusedasthesolemeansofsealing(withouta
gasket)theheaderflangetothecylinderhead.Beforeinstallingtheheaderflangegaskets,testfitthemto
yourcylinderheads.Ifyouplantouseagasket,makesurethatthegasketportsdon’tblockanyportionof
thecylinderheadexhaustports.Iftheydo,don’tusethosegaskets.Findgasketsthatmatch(orareslightly
largerthan)thecylinderheadexhaustports.
If you purchase fiber gaskets and the fit is too tight (where the gasket opening is smaller than the
cylinder head exhaust port), you can try trimming the gasket opening using a new and very sharp razor
knife.Inaddition,testfitthegaskettotheheaderflange.Ifanygasketmaterialoverhangsintotheexhaust
path,trimthegasketaccordingly.Thepointistoavoidanygasketmaterialstickingoutintothepathforthe
exhaustflow.
Eitherstudsorboltssecuretheheaderflangetothecylinderheads,butwhichtousedependsonthe
applicationoryourfastenerpreference.Ipreferusingstudswhenpossible,butinsomecases,studsdon’t
clearprimarytubebends.
Studs
Studsoffertwodistinctbenefits.Theyprovideaninstantflangelocationwhenpositioningtheheader
onthecylinderhead,makingiteasiertoplacetheheaderandgasketsinposition.Inaddition,theyallow
youtohangtheheaderontotheheadwhileinstallingthenuts.Inthecaseofaluminumcylinderheads,the
useofstudsreduceswearandtearonthecylinderheads’femalethreadedholesinthosecaseswherethe
headersareremovedandreinstalledonaregularbasis.
Ifyoudecidetousestuds,thechoiceofbullnose/radius-tipstudsprovideseasiernutinstallation.The
exposedstudtiphasaradiused-noseprofileabovethethreads.Thisallowseasiernutthreadengagement,
with far less chance of cross threading (the smooth tips provide an initial guide for the nuts). When
installingstuds,thereisnoneedtoforce-tightenthemintotheheads.Installthestudsfinger-tight(orat
most,withaveryslightpreloadofmaybe5to8ft-lbs).Theclampingactiontakesplacebytighteningthe
nuts.Thereisnoneedtodouble-nuteachstudtoseverelytightenthestudintothehead.
Bolts
Ifyoudecidetousebolts,youhaveseveralchoicesregardingheadsizeanddrivestyle,including6-
pointhex,12-pointhex,andSHCS(socketheadcapscrews).Asidefromappearancepreferences,thisis
dictated by fastener head clearance and wrench access. Depending on header design, regular-size hex
headboltsmaynotcleartightlyspacedprimarytubes.Reduced-sizehexand12-pointboltsareavailable
withsmallerheadsforimprovedtubeclearance.
Forexample,atraditional3/8-inchbolttypicallyrequirestheuseofa9/16-inchwrench.Areducedsize3/8-inchboltisavailablethatfeaturesasmallerhexhead,requiringtheuseofa3/8-inchwrench.
Dependingonspacerestrictions,youmaybeabletouseasocketwrenchoryoumightberestrictedtoan
open-endorboxwrench.AnotheroptionistheuseofSHCS,whichfeaturearoundheadperimeterand
require the use of a male hex wrench. Again, it all depends on the clearances (or lack thereof) in any
givenapplication.
TorqueSpecifications
Alwaysfollowthetorquevalueprovidedbytheheadermakerorbythecylinderheadmaker.Begin
tighteningatthecenter-mostfasteners,workingyourwayoutwardtowardthefrontandrearends.Dothis
in several steps; don’t fully torque any given fastener before proceeding to the next. Tighten in gradual
steps in order to spread the clamping load evenly across the header flange. For instance, if the torque
specificationis30ft-lbs,firsttighten(inthepropersequence)to10ft-lbs,then20ft-lbs,andthen30ftlbs.
Whenroutingsparkplugwiresonaheader-equippedengine,carefulplanningavoidsburnedwires.Thegoalistokeepthewiresas
farawayfromtheprimarytubesaspossible.Dependingonthesparkpluglocationandangle,andtheproximityofprimarytubes,
thiscanbedifficult,butgenerallyspeaking,trytomaintainatleasta1/2-inchclearanceifpossible.
Granted,theremaybesituationswhenitisnotpracticaltouseatorquewrenchorsocketwrench,due
topotentialaccessproblems.Ifastraightsocketwrenchdoesn’tfitbecauseofprimarytubeobstruction,
youmaybeabletouseanoffsetboxwrenchonyourtorquewrenchtool.Keepinmindthatwheneveryou
useanoffsetwrench(whereyouareeffectivelyextendingtheoveralllengthofthetorquewrenchbody),
youneedtocompensatefortheadditionalleverage.
Ifanadapterisusedthateffectivelylengthensthewrench(suchasaboxwrenchextensionorcrow’sfootwrench),acalculationmustbemadetoachievethedesiredtorquevalue.
Theuseofanoffsetadapterchangesthecalibrationofthetorquewrench,whichmakesitnecessaryto
calculatethecorrecttorquesettings.Followingaretheformulasforcalculatingthischange:
Theadaptermakesthewrenchlonger:
TW=L÷(L+E)×desiredTE
Theadaptermakesthewrenchshorter:
TW=L÷(L-E)×desiredTE
Where:
TW=torquesettingonthetorquewrench
L=leverlengthofthewrench(fromcenterofthewrenchdrivetothecenteroftheadapter’sgriparea)
E=effectivelengthofextension,measuredalongthecenterlineofthetorquewrench.
TE=torqueappliedbytheextensiontothefastener
For example, if the fastener torque value is specified as 30 ft-lbs, but a 2-inch-long box-wrench
extensionisaddedtoyourtorquewrench,youneedtoadjustyourtorquewrenchsettingtoalowervalue;
otherwiseyouovertightenthefastener.Let’ssaythatyourtorquewrenchlength(measuredfromthecenter
ofthetorquewrenchgriptothecenterofthetorquewrenchdrive)is14inches.You’readdinga2-inchlongwrenchextension.
Here,youusetheformulawheretheadaptermakesthewrenchlonger.
TW=L÷(L+E)×desiredTE
14÷(14+2)×30=26.25ft-lbs
Withthe2-inchextensioninplaceandthetorquewrenchsetat26.25ft-lbs,theappliedtorqueis30
ft-lbs.
If you want to know where to set the torque wrench when using an adapter that alters the effective
lengthofthewrench,youmustcalculatetocompensatefortheadapter.Ifthedistancefromthewrench
drive to the center of the bolt makes the wrench longer, the final wrench setting must be adjusted to a
lower value in order to compensate. If the distance from the wrench drive to the bolt center makes the
wrenchshorter,thewrenchmustbesettoahighervaluetocompensate.
Onceallheaderflangefastenershavebeentightenedproperly,you’renotdone.Plantore-tightenall
fasteners a few times in the beginning after engine heat cycling. Bring the engine to full operating
temperature, allow the engine to cool, and re-tighten. Repeat this process at least two or three times,
allowingthegasketandfastenerstostabilize.Re-tighteningafterheatcycleshelpstoensurealeak-free
installation.
DesignYourOwnHeaders
Ifyouwanttomakeyourowncustomexhaustheaders,modelingkitsareavailablefromtheinnovators
atIcenengineworks.ThesekitsareakintoaLegosetandaredesignedtoallowyoutodesignyourown
tubularprimarypipesinaveryprecisemanner.
As I’ve mentioned earlier, custom-making your own headers is simply not necessary if you have a
production vehicle. Shorty headers are readily available for specific engines to accommodate engine
swapsintocustomorstreetrodapplications.
Today’s header manufacturers offer ready-to-install headers for most popular vintage and modern
vehicles,andevenforretrofitapplicationssuchastheinstallationofalate-modelengineintoavintage
vehicle.Theonlyinstanceforwhichacustomheadermayberequiredisinthecaseofcustompurposebuiltracecarsorhighlymodifiedstreetrods.
Ifoff-the-shelfheadersareavailableforyourapplication,it’sfoolishtoinvestthetimeandmoneyfor
customheaders,especiallywhenyouconsidertheneededspecializedtoolingandweldingequipmentthat
mosthobbyistsdon’thaveaccessto.
Toproperlytorquehard-to-reachheaderflangeboltsornuts,awrenchextensionmaybeneeded.Ifso,beawarethatwhenyou
lengthenthedistancebetweenthetorquewrenchgripandwrench,leverageincreases.Becauseofthis,youmustadjustyourtorque
settingonthetorquewrenchtoavoidover-tightening.
HeaderModelingKits
Thekitsareavailableinavarietyofdiametersandconsistofacollectionof1-inch-longroundplastic
blocksthatsnaptogethertocreatealengthofpipe.Theseincludestraightpiecesaswellasavarietyof
radiusbendblocks.Onceyouhaveanexhaustflangeboltedtothecylinderhead,withashortsectionof
straightmetaltubingweldedtoeachportontheflange,yousimplybeginbyattachingtheplasticmodeling
blockstocreateeachprimarytube’sshapeandlength.Thisallowsyoutomock-upthecompleteprimary
seteitherwiththeengineonastandorinthevehicle(dependingonyourapplicationneeds).
Oncetheblocksaresnappedtogether,eachblockcanalsoberotatedforfine-tuningadjustment(for
example,rotatingacurvedblockchangestubedirection).Oncethebankoftubeshasbeenmockedupto
yoursatisfaction,youhaveyourtemplateasaguidewhenreproducingeachtubeinyourchoiceofmetal
(steelorstainlesssteel).
Eachblockislabeledwithseveralindexingarrowsandmatchmarksthatareplaced30degreesapart
thatcanbeusedasreferences.Inordertokeepeverythingorganizedbeforeyoudisassemblethelock-up,
it’s a good idea to create your own reference marks. Using a felt-tip pen (Sharpie, etc.), you can label
eachblockaccordingtocylindernumberandlocationfromtheheaderflange.
For instance, at cylinder number 1, mark C-1 on each modeling block. Then mark the first block
(closesttothehead)as“A.”Markthenextblockonthatcylinder’srunas“B,”etc.Inordertoprovidea
referenceregardingclockpositiononeachblock(handyincaseyouaccidentallyrotatetheblocksduring
handling),placematchmarksfromblocktoblock.
Now you’re ready to duplicate each primary tube model in metal. You can duplicate the model by
piecingtogethersectionsfromreadilyavailablestockofstraight,45-degree,90-degree,and180-degree
mandrel-formedpieces.Theseareavailablefromavarietyofperformanceexhaustraw-partsmakersand
speed-equipment retailers. By having a selection of pre-formed sections, you’re able to cut and trim to
achieveavarietyofshapesandangles.
ManualConstruction
Inaprofessionalfabricatingshop,headersareconstructedbyusingaCMMthatusesadigitalprobe
totracetheentirelengthofthetubemodel.ThedatacollectedisthentransferredtoaCNCpipebender.
However,evenaCNCbendermaynotbeabletoformextremelycomplexbendsfromasingletube,sothe
tubemayhavetobeformedinsectionsthataresubsequentlyweldedtogether.Becausemostofusdon’t
haveaccesstosuchhigh-techequipment,theprocesscanbehandledmanually.Itmaybetime-consuming,
butit’srelativelyeasy.
First, purchase a collection of metal tubing in your desired material, diameter, and wall thickness
(straightsections,J-bends,andU-bends).Youcanthencutthemetaltubestomimicsectionsofthemodel.
It’simperativetomakeyourcutsperpendiculartothecenterlinereferencemarkonthemodelinorderto
achieveaflushmatingofeachsection.
To provide a precision cut, Icengineworks offers a pivoting table (this works with any vertical
bandsaw)thatallowsyoutoproperlyindexeachcut.
Onceyoucutasection,labelitforcylindernumberandlocation.Forinstance,ifyoucutalengthof
tubing that mimics blocks C-1A, C-1B, C-1C, C-1D, label that piece of tubing accordingly so that you
know exactly where this section is to be placed. Also, be sure to indicate the flow direction, to avoid
weldingasectionupside-down.Youcaneitherdrawanarrowtoindicateflowdirection(fromheadto
tube outlet) or you can label the cylinder numbers and position numbers in order (exactly the order
markedonthemodelingblocks).
After all of the metal pieces have been cut, you need to weld them together. Rather than driving
yourselfnutsbytryingtoholdthepiecestogetherbyhand(whichrequiresthreehands),takeadvantageof
thespecialtubeclampsfromcompaniessuchasIcengineworks.Thesepositioningclampsareuniqueand
aredesignedspecificallyforthisjob.Eachclampconsistsoftwoclampingstrapsthatareconnectedbya
slotted/adjustable-lengthcrossbar.Oneclampstrapattachestoeachadjacenttube,andthespacebetween
thestrapsisadjustedtoprovideweldingaccess.
Oncetheclampistightened,thetwosectionsofmatedtubesareheldsecurelyinposition.Continueto
assemblethepre-cutmetaltubesectionsbysimplyreproducingyourpreviousplastic-blockmodel.Once
the entire header bank has been fully assembled using the special clamps, you have the opportunity to
slightlylooseneachclamptorotatetubesectionsasnecessarytofinalizeyourshapes.
Whenyou’resatisfiedthatyou’veduplicatedthemodelandyou’rehappywiththefit,tackweldthe
tubejointsasneededtolockallpiecestogether.Oncetheheaderbankissecurelytackweldedtogether,
removetheclampsandremovetheheaderfromtheengine.Placetheheaderonaclean,safeworkbench
andfinishweldalljoints.
Justtobesafe,reinstalltheheadertoverifyfit.Atthispoint,youcandressthewelds(ifnecessary)
andapplytheappropriatecoating(suchasaceramiccoatingorhigh-temppaint).
UsingaHeaderFabricationKit
Eachmodelingblockisexactly1-inchlongandfeatureshandyindexingarrowsthatindicatethecenterline.Blocksarealso
colorcoded,indicatingtubeOD.Theblockshownhereisfor1.625-inchODtubing.(PhotoCourtesyIcengineworks)
Bothstraightandradiusedblocksareincludedinthekit,allowingyoutopiecetogetheryourexacttubeshapes.Thetwo
blocksshownhereareorange(labeledfor1.750-inch-diametertubing).Noticethatthecurvedblockislabeledashavinga3inchcenterlineradius(CLR).(PhotoCourtesyIcengineworks)
Here’sanexampleofaheaderbankmockedupusingmodelingblocks.Eachblockcanberotatedtofine-tunetubeangles
andclearances.(PhotoCourtesyIcengineworks)
Icengineworks’collectormockuptoolallowsyoutopositiontheexitendsofthetubepackagetogethertosimulateaplanned
collector.(PhotoCourtesyIcengineworks)
Modelingblocksallowyoutomockandtest-fittheplannedtubingeveninthetightestconfines.Theadaptabilityandeaseof
assemblyspeedsupthedesignprocesssignificantly,withouttheneedtofabricatemetaltubingforthedesignstage.(Photo
CourtesyIcengineworks)
Inthisexample,tubingmustclearthesteeringshaftandrequirestheuseofcreativebendshapesinordertoachievetube-totubeclearanceandfit.Bysimplyaddingorremovingstraightorradiusblocks,andbyrotatingradiusblocks,it’seasyto
explorethevariouspossibilities.Oncethefinalshapesandlengthsofalltubeshavebeendetermined,eachtubepackageis
replicatedinthemetalofchoice(steelorstainlesssteel).(PhotoCourtesyIcengineworks)
Specialtack-weldclampsprovideagapbetweentheclampbands,allowingaccessforpipetackwelding.(PhotoCourtesy
Icengineworks)
Worksheetsprovidedinthekitallowyoutokeeptrackofeachsegmentandeachbendradius.(PhotoCourtesy
Icengineworks)
Thisbankwasmockedupusingplasticblocks.Shownhereisonemetalprimarytubethatwasreplicatedusingthemodeling
blocks,clampedtogetherandtackwelded.Thecollectorisattached,keepingthetubesinthecorrectlocation.(PhotoCourtesy
Icengineworks)
Onebyone,eachprimarytubeisreplicatedandreplacedwithasteelcounterpart,terminatedinthecollector,andtacked
together.Onceallsectionshavebeentackedtogether,theentirebankisremovedfromtheengineandfinish-welded.(PhotoCourtesy
Icengineworks)
Amock-upcollectortemplatefromIcengineworksaidsinaccuratelydeterminingprimarytubeterminationpoints.(Photo
CourtesyIcengineworks)
Here’sanexampleofamodel-blockmockupofabankofprimarytubesfittedtoacollectorguide.Theguidemimicsthe
anticipatedcollectorensuringthatallmodelingtubesgatherforaccurateplacementintoaplannedcollector.(PhotoCourtesy
Icengineworks)
Duetotheavailabilityofvarious-diametermodelingblocks,asteppedtubeiseasytodesign.Hereyousee1.625-inchblocks
(blue)steppedupto1.750-inchblocks(orange).Bypurchasingsectionsoftubinginbothdiameters,asteppeddesigncanbe
obtainedaseasilyasaconstant-diameterdesign.Thespecialtack-weldingclampsareadjustabletoaccommodatetheadjacent
diameters.(PhotoCourtesyIcengineworks)
CHAPTER5
MUFFLERSANDCATALYTICCONVERTERS
Inalmostallcases,mufflersarerequiredinordertomeetsound-levellaws,especiallyforanystreetdriven vehicle. In addition, catalytic converters are required for vehicles that were originally so
equipped. Here, I discuss the role that mufflers and converters play, and offer tips on enhancing both
components from a performance perspective. I discuss various muffler styles, as well as catalytic
convertersthatareavailablespecificallydesignedforhigh-performanceapplications.
Mufflers
The muffler affects more than the engine’s sound level. As with the manufacture of tubular exhaust
headers, materials include mild steel, low-grade alloy, and higher-grade 300- or 400-series stainless
steel.Ifmadeofsteel,mufflerbodiesandnecksaretypicallygalvanizedoraluminum-coated.ThelowgradestainlesssteelusedbymanyOEMscertainlylastslongerthanmildsteel,butnonethelessrustsover
time(andmorequicklythanthevehicleownerexpects).Ifyouwantareplacementperformancemufflerto
lastandkeepitslooks,chooseamanufacturerwhouses304-,321-,or409-gradehigh-densitystainless
steel.
Whereshowappearanceisdesired,stainlesssteelmufflersandtipsareavailablefullypolished.
ThiscutawayviewofFlowmaster’s40SeriesmufflershowstheDeltaFlowtwo-chamberdesign.Theexhaustflowexitsattheend
nearthelowerportionofthisphoto.Thisdesignissaidtominimizevehiclecabinnoisewhileemittinganoticeableexhaustnote.
(PhotoCourtesyFlowmaster)
ThisProSeriesmufflerfromFlowmasterprovidesalaminarflowandadeepexhausttone.Theflowpattern,aidedbythe
perforationsintheinnersurface,allowsgasflowinsomewhatparallellayersthatslideoveroneanother.Thisdesignisakintoa
straight-throughglasspack,butwithadditionalflowmanagement.(PhotoCourtesyFlowmaster)
Variationsofmufflerconnectiondesignsaccommodatesingleinletandoutletstylesforasinglepipeonasingle-pipesystemoroneon
eachpipeinadualsystem.Thisisanexampleofasinglemufflerdesignedforadualsystemthatrunsthroughasinglemuffler
(PhotoCourtesyBorlaPerformanceIndustries)
Mufflerdesignsvarywidely.ShownhereisacutawayviewofaCorsaRSCmufflerthatfeaturesaseriesofreversioncavitiesto
managesoundwhileprovidingminimumbackpressure.Theexhaustflowsthroughastraighttubewhileexhaustnoisepulsesentera
seriesofacoustictuningchambers.(PhotoCourtesyCorsaPerformance)
Thismufflerhasareverse-flowdesignwheretheexhaustentersthroughonetube,runsthroughacentertubeintheopposite
direction,andfinallyexitsthroughathirdtubeintheopposingdirection.Incombinationwiththebleed-throughperforations,specific
backpressureandsoundtuningisaccomplished.
Here’sanexampleofamufflerwithtwininletsandtwinoutlets,accommodatingadual-exhaustsystemrunningthroughasingle
muffler.(PhotoCourtesyBorlaPerformanceIndustries)
Offsetinlet/outletmufflersareoftenneededtoprovideabetterfit,duetopipelocationsandavailableundercarspace.(Photo
CourtesyBorlaPerformanceIndustries)
Ifyoudecidetouseovalexhaustpipe,mufflersareavailablewithovalinletsandoutlets,sonoround-to-ovalmodificationsare
necessary.Ovalexhaustpipeisapopularchoicewheregroundclearanceisanissue.(PhotoCourtesyBorlaPerformanceIndustries)
Thispipetothemufflerhasashort“branch”pipethathelpstoreduceexhaustdrone.Thisisyetanotherwaythatmuffler
manufacturerscanaltertheaudiblefrequenciescreatedbytheexhaustpath.
Insituationswherethepipeentrytothemufflerdiffersinplanefromtheoutletpipes,offsetinlet/outletmufflerdesignssolve
clearanceandfitmentissues.
Theprimarygoalofanymuffleristoreducethenoiselevel(dBA)toatolerablerangetosuitboth
passengers and anyone outside the vehicle. Noise levels are legislated by government and are also
mandatedbymanyracetracksinordertoappeasethosewholivewithinearshotofthosetracks.
Toachievethisnoisereduction,mufflerstylescanfeaturechambers,baffles,perforatedinternaltubes,
and/orasound-deadeningmaterial.
Here,achamberedexhaustpipeservesasboththepipeandmuffler.Aninternalflowtubehasperforations,orlouvers,intowhich
thesoundwavesenter.Thesoundwavesthenhitthecrimps,ordents,placedstrategicallyalongtheoutershell,effectivelytuningthe
exhaustnote.Thesizeoftheperforations/louversandthelocationoftheoutertubecrimpsaffectthesound.Spacingandsizeofthe
perforationsvarydependingonthemanufacturer’sdesign,often“tuned”forspecificvehicle/engineapplications.
Oncetheinnerflowtubeisplacedinsidetheouterpipe,theouterpipeiscrimpedtolocktheflowtubeinplaceandtoestablishthe
exhaustsound.Nopackingsound-absorptionmattingisusedinachamberedexhaustpipe.
Glasspacksarestraight-throughmufflersthathaveaninternalperforatedtubewithsound-absorbingpackingmaterialsandwiched
betweentheinnerandoutertubes.Glasspacksareavailableinavarietyoflengths.Evenextremelyshortglasspacks,suchasthis
one,areavailableforapplicationswherethesoundlevelneedstobereducedabitwhileprovidingthedesiredtone.(PhotoCourtesy
CherryBomb)
Thisisanexampleofarace-applicationchamberedmuffler.Thecompactdesignandfree-flowcapabilityofthisExtrememodel
accommodatesanapplicationwherespaceisatapremium.(PhotoCourtesyCherryBomb)
ThisillustrationshowstheflowpatternofanExtrememuffler.Redindicatesexhaustgasflowandblueindicatestonal,ornoise,
reduction.(PhotoCourtesyCherryBomb)
ThisCorsamufflerfora2012MustangV-6boltsonandfeaturesOEMbendsandwelded-onhangerpegsthatslipintotheOEM
rubberhangerlocations.(PhotoCourtesyCorsaPerformance)
MufflerTypes
Three basic types, or styles, of mufflers are commonly available. These are chambered, turbo, and
straight-through.
Chamberedmufflersfeatureaseriesofinternalchambersand/orbaffles.Astheexhaustpulseenters
themuffler,thesoundwavesbounceandreflectthroughandoffthesewallsorcavities,causingthesound
wavestobounceagainsteachother.Thesesoundwavescanceleachotherout,whichreducesnoise.Each
mufflermanufacturerfollowsitsowndesigninachievingthis.Essentially,chamberedmufflersusesoundcancelingtechnologytoreducenoisewhileproducingadesiredsoundlevelandexhaustnotetone,with
varyingdesignsthatappealtostockreplacementorhigh-performancesoundattributes.
Insteadofhavingtoaddchromeorpolishedexhausttips,somesystemsareavailablewithpolishedstainlesstipsalreadyinstalled.
ThisisanexampleofamufflersetupfortheZR1Corvette.(PhotoCourtesyCorsaPerformance)
ThisillustrationoftheCorsaRSCmufflerprovidesaclearunderstandingofitsnoise-cancelingdesign.Asexhaustnoiseentersthe
maintube,thesoundwavesbleedoffintothelow-frequencychannels,resultinginareflectedout-of-phasepulsethatdiminishes
noisewhileallowinganon-restrictedflowofexhaustpressure.(PhotoCourtesyCorsaPerformance)
Chamberedmufflersfeatureinternalchambers,orrooms,wheretheexhaustflowisdiverted.Muffler
manufacturersusevariousdesigns,butthegoalisthesame:tomanagesoundandpressurebymeansofthe
flowpath,tuningthemufflerdesignwithbothsoundandbackpressureperformanceinmind.Performance
muffler manufacturers go to great lengths to create innovative chambering designs. As far as sound is
concerned,itcanvaryfromextremelyquietstreetversionstoextremelyloudraceversions.
Chamberedexhaustpipeservesasbothpipingandmufflerinoneunit.Achamberedmufflerhasthe
appearanceofanexhaustpipewithaseriesofcrimps,ordents,alongthebody.Insidetheoutertubeisa
coretubethathasperforationsorlouvers,oftencalledaflowtube.Astheexhaustflowsthroughthepipe,
sound waves enter the perforations or louvers, causing them to hit the crimps in the outer tube. This
createsturbulenceinthesoundwavesthat“tune”theexhaustsound.Chamberedpipesdon’tincludeany
sound-absorbingpackingmaterial,andareusuallyniceandloudwitharaspysound.Thesearestraightthroughpipes;theytunethesoundbyalteringthesoundwaves.
The turbo-style muffler commonly features perforated internal tubes that snake the exhaust flow
throughthemufflerinanSpattern,providingaddedtubelengthinsidethemuffler.Aninlettubedirectsthe
exhaust into an open chamber that then diverts the flow 180 degrees through a second tube, where the
exhaustflowisagaindivertedanother180degreesthroughathirdtube,withtheflowfinallyexitingthe
muffler.
Turbomufflershavenothingatallincommonwithaspinningturbine.Thetermissimplyusedasa
marketing catch phrase. A turbo-style muffler commonly has an S-bend flow path inside the muffler
housing.Theflowenters,runsthroughacertaindistanceinastraightpath,thenturns180degrees,runs
straight, then makes a final 180-degree turn. Along the way, perforations may be present along with
sound-deadening packing. Typically, a turbo-style muffler offers increased sound reduction and is
typicallyaquietermufflerascomparedtootherstyles.
Straight-throughmufflerstylesofferadirectpathfortheexhaustfromentrytoexit,withnodiversions
orwave-reflectivewalls.Aninternalperforatedpipeiswrappedinasound-deadeningmaterialfornoise
absorption. This design offers less restriction and is more compact (typically bullet shaped or round
canistershaped),makingastraight-throughmufflereasiertolocateintermsofavailablespace.
Straight-through mufflers feature exactly what the term implies: an uninterrupted flow from entry to
exit,withnochambersorbafflewallsintheway.Thisstylehasaperforatedinnertubewithasounddeadening material, typically fiberglass, packed between the perforated tube and main housing. Sound
wavesentertheperforatedtubeandareabsorbed,toadegree,bythepacking.Straight-throughmufflers
areoftenreferredtogenericallyasglasspacks.
MufflerAssembly
Mufflershellprofilesaretypicallyeitherroundoroval.Roundshellsareusuallymadewitharound
tubeasthemainbody,orbyformingaflatsheetintoaroundprofilewithinletandoutletnecksweldedin
place.Somestraight-throughglasspacksareformedfromatubewiththeinletandoutletnecksreducedto
theappropriatediameterthroughhydroforming.Thisisdoneinatwo-piecedie.Thetubeisplacedinto
the die, the die halves are mated together, and water is injected at extremely high pressure, forcing the
tubetoconformtotheshapeofthedie.
Whenmufflershells,eitherroundoroval,areformed,themanufacturerbeginswithflatstockmetal
thatisrolledtoshape.TheseamisthenTIGwelded,eitherbyhandorbyroboticwelding.
Hydroformingallowsamufflerbodytobemadewithoutmultiplesections,suchasmaincanisterandtips.Thisglasspackmuffler
bodywasformedattheCorsafactorystartingwithapieceofstraighttubing,usinghighwaterpressurewiththestraightpipe
capturedinsideadie.
Hydroformingusesextremelyhighwaterpressuretoexpandandformapieceofpipeintoaspecificshape.Atwo-piecedieis
positionedinthemachine.Withtheprecision-machineddiesmatedtogether,waterpressureisinjectedintothepipe,forcingthepipe
toconformtothedie.Duetotheexpenseofmakingthedies,hydroformingisonlyusedinrareapplicationswhereweldedseamsare
notdesired.
Mufflerbodyshellsareinitiallyformedonaroller.Thisallowsthemakertoformeitheraroundorovalbodyshell.Hereasheetof
stainlesssteelisrolledthroughtheformingmachinetocreatearoundshell.
HereagroupofroundformsarereadytoproceedtoseamweldingatCorsa’sfactory.
ThesestainlesssteelovalshellsarereadyforseamweldingatStainlessWorks.
MufflershellseamsareTIGweldedbeforeassembly.Dependingonproductionschedulesorshelldesign,seamweldingcanbedone
byskilledweldersorwiththeuseofroboticwelders.HereacraftsmanatStainlessWorksseamweldsamufflershell.
Forcertainapplications,perhapshigh-productionruns,roboticweldersareemployedformuffler-shellwelds.
Sound-AbsorptionPacking
Sound-absorption materials are commonly referred to as packing. They are used in a variety of
muffler designs and can vary depending on the manufacturer and the muffler model. Materials include
fiberglass matting, steel wool, stainless steel mesh, and other proprietary high-temperature acousticalsuppressionmaterials.
Mufflersthatuseapackingsound-absorptionmaterialfeatureoneormoreperforatedtubesinsidethe
mainhousing.Exhaustenterstheperforatedtubeandbleedsoutthroughtheperforationsintothepacking
material, which serves to absorb sound. The theory is akin to early-generation silencers used on some
firearms.Theexhaustpressureisabletotravelthroughtheinternalpipe,withsoundpulsesdampenedor
filteredbythepackingmaterial.
Somemufflerdesignsfeatureasinglelayerofsound-absorbingmaterial(fiberglassorothermaterial).
Otherdesignsusetwolayers:asteelwoolorstainlesssteelmeshasthefirstlayerwrappedaroundthe
perforatedtubeandasecondouterlayeroffiberglassorothermaterial.Thereasonforusingsteelwool
orstainlesssteelmeshistobothaffecttheexhaustsoundandtoprovideaprotectiveboundarylayerthat
prevents the more-fragile outer layer of material from “blowout.” This dual-layer approach provides
increaseddurabilitytotheassembly.
Oncefullyassembled,themufflerendcapsareTIGwelded.(PhotoCourtesyCorsaPerformance)
Mufflerassemblytakesplacebeforeend-capinstallation.Hereperforatedinternalpipesarewrappedinasteelwoolsoundabsorptionmattingandinsertedintotheshell.
Thismufflerdesignfeaturesastaggeredinternal-perforatedpipewrappedinsound-absorptionmattingwiththeremainingcavity
alsostuffedwithmatting.Thisisessentiallyastraight-throughmufflerwithplentyofsound-absorbingcapacity.Decibellevelsare
determinedbythedesignoftheapplication.Thereisnostandardintermsofdecibellevels;itvariesbymanufacturer,diameter,and
lengthofthemuffler.
Acollectormufflerisidealforapplicationsinwhichnoadditionalexhaustpipeisusedandthereforeasound-absorbingcollector
mufflerservestoaidinscavengingandprovideminimaltonemanagement.(PhotoCourtesyBorlaPerformanceIndustries)
MattingReplacement
Here’saperforatedinnertube(left)nexttothemainmufflerbody(right).(PhotoCourtesyBurnsStainless)
Here’sanexampleofado-it-yourselfserviceablestraight-throughglasspackmuffler,primarilyintendedforracinguse.Theinlet
headandperforatedinnertubeisaseparatepiece,securedtothemainhousingwithsetscrews.(PhotoCourtesyBurnsStainless)
Theheadandinnertubeisremovedfromthemufflerhousingbyremovingthesetscrews.(PhotoCourtesyBurnsStainless)
Afterremovingtheworn-outmatting,newsound-absorbingmattingiswrappedaroundtheperforatedtube.Ratherthanusing
sound-deadeningmaterialsavailablefromlocalhardwarestores,it’sbesttopurchasethematerialdirectlyfromthemuffler
manufacturer.Choosingincorrectmaterialcanleadtomufflerobstructionorobjectionableexhaustsound.(PhotoCourtesyBurns
Stainless)
Themattingissecuredtotheperforatedtubewithtape.Thetapeissomewhatsacrificial,onlyusedtoholdthemattingin
placeduringinstallation.Althoughmaskingtapeisadequate,heat-resistantfoiltapesimilartothatusedforfurnaceductingis
better.(PhotoCourtesyBurnsStainless)
Thewrappedtubeisinsertedintothemufflerhousing.(PhotoCourtesyBurnsStainless)
Aspiralinsertconsistsofasteeltubewithaseriesofspiralbladesthatserveasexhaustdiverters.
Spiralinsertsareofferedinavarietyoflengths,diameters,bladecounts,andbladepitchestoaccommodatedifferentsoundand
performancelevels.
Onceyou’vedrilledthetwoholesinthepipetoalignwiththeholesinthebaffle’smountingtabs,slidetheinsertintothestraightpipe.
Aligntheholesandsecurethebafflewithstainlesssteelbutton-headscrews.Thismakesfutureremovaleasyifyouwanttoswitch
frombaffledtoopen.
SpiralInserts
Analternativetousingaconventionallyinstalledmuffleristhespiralinsert.Thisisastraightsection
of small-diameter pipe upon which a series of spiral blades are welded, making the unit look like an
auger.Thespiralunitisinstalledasaninsertintoastraightsectionoftheexistingexhaustpipe.
Youcanrefertothistypeofproductasabaffleinsert,whichisinstalledmuchlikeabaffletubeona
motorcycle engine’s exhaust. The baffle slips into a straight section of exhaust pipe and is anchored in
placewithtwosmallsetscrews.Spiralinsertsareavailableinawidevarietyofdiameters,lengths,and
numberofspiralblades.
Selecting the diameter of a spiral muffler is easy. Simply choose a diameter that fits inside your
existingexhaustpipe.Forexample,ifyou’rerunning3-inchpipe,youneedaninsertdesignedtofita3inch pipe ID. As far as length and number of spiral blades is concerned, the experts at Spiral Turbo
Specialtiesandotherscanoffertheirrecommendations,basedonthetypeofsoundthatyoudesire,engine
displacement,enginehorsepower,heads,andcam.
SpiralTurboSpecialtiesmakesthreebasictypes:the3-flightthatis8incheslongandfeaturesthree
spiralblades,the4-flightthatis12incheslongwithfourspiralblades,andthe9-flightthatis30inches
longandhasninespiralblades.The30-inchversioniscommonlyusedforside-pipeapplications,suchas
thosefoundonCorvettesandCobras.
Theirrecommendationforlocationofthebafflesistoplacethemasfarbackaspossible,ascloseto
thetipsaspossible,inordertoproducethebestsoundandtoeliminateanypotentialdroningissues.In
general,thegreaternumberofspiralblades,thequieterthesystem.Asthenumberofbladesdecreases,
sound increases. Another sound-tuning aid is found in the baffles’ center tube, which is capped. Once
installed,ifyoudesirealoudersound,simplyremovethebaffleanddrilloutthecapwitha7/8-or1-inch
hole.
Spiralexhaustbafflescanbeinstalledinanysectionofstraightpipe,includingexhausttips.
One of the extremely beneficial features of a spiral insert involves installation space. Because the
insert is installed inside a section of straight pipe, it’s invisible from the outside. With no increased
dimensionsrelativetothepipe,thisstylesavesroom,withnoneedtolocatethemuffleraccordingtothe
frame, crossmembers, etc. It’s a real space-saver. An added benefit is in appearance. With the baffle
hiddeninsidethepipe,theexhaustsystemlookslikeanopensans-mufflersetup.
Thiscloneofa1964CometAFXfeaturesaverysimpleexhaustsetup,withstraightpipesrunningfromtheheadercollectorsmated
to45-degreepipesthatexitinfrontofthereartires.Whilethecarlookslikeanold-schooldragcar,theownerintendstodriveiton
thestreetonly.Toprovideastraight-throughexhaustappearancewhilemanagingthenoiselevel,the45-degreetipsfeaturehidden
spiralbaffleinserts.Thiswasacleanwaytoprovidearaceappearancewhileavoidingaticketforloudexhaust.
Selectinglengthandnumberofbladesforanygivenpipediameterallowsyoutotuneforsoundand
performance.TheowneranddesignerofSpiralTurboSpecialties,JimLaughlin,hasinvestedincountless
hoursofchassisdynoandroadtestinganddevelopmenttocreatethesebaffleinsertsforawidevarietyof
both street and track fitments, including gas and diesel applications. Jim offers baffle diameters in 11
sizesrangingfrom2.25inchesallthewaytoawhopping10-inchdiameterforcommercialDieselsemitractors.Arecentadditiontothelineisthetaperedbaffleforinstallationtolake-pipeheadersorconeshapedexhaustpipeonstreetrods.
Asanexample,myshoprecentlybuilta1964CometAFXcloneforacustomerwhowantedside-exit
exhaustpipes,locatedimmediatelyaheadoftherearwheels.Wesimplyran2.5-inchpipefromtheheader
collectors,straightbacktowardtherearandadded45-degreebends,withaslashcutatthetips(sothat
thetipendswerecutparalleltothebody).Insteadofaddingin-linemufflers,weinstalledspiralbaffles,
oneineachtip’sstraightsection.
The 45-degree bends with the straight exits were made of stainless steel and polished. The baffles
feature two mounting tabs with threaded holes. The exit pipe was drilled to provide holes that aligned
withthesemountingtabs.Theinsertswerethensecuredwithstainlesssteelbutton-headscrews.
Thebaffleswechoseprovidedaniceraspytonewithminimalrestrictionthatwasidealforthestreet.
Whenevertheownerwishestoremovethebaffles,allthat’srequiredistoremovethetwosetscrewsand
pullthebaffleoutdirectlyfromtheexhaustpipetip,withnoneedtocrawlunderthecar.Especiallyif
spaceisanissue,thisisasound-managementapproachworthconsidering.Backpressureisminimalwith
thespiralinserts.
Mufflersadjustsoundbyanyofanumberofmethods,includingdampingsoundwavesthroughsoundabsorbingpackingmaterialorbyallowingwavestobedirectedbypartialwallsorintochambers.
Years ago, I saw a prototype exhaust system at the research center of one of the major OEM and
aftermarketreplacementexhaustsystemmanufacturers.Ithaddevelopedauniquesound-cancelingsystem
thatusedamplifiedsoundwaves.Thesystemdidn’thaveamufflerinthetraditionalsenseandtheexhaust
passedstraightthroughthesystemwithnorestriction.
Sound control on this prototype was a microphone that picked up the sound waves created by the
engineastheypassedthroughthepipe.Thesoundorfrequencywavesthatweremonitoredwerethenfed
through a processor that created a mirror image of the waves, instantly creating the same wave, which
was transmitted back to the exhaust via an amplifier and speaker. The result was a cancellation of the
exhaustsound.
This noise cancellation process proved to be too expensive and required too much space under the
vehicletobepractical,butthetheorywassolid.Manyoftoday’smufflermanufacturersutilizeaformof
noise cancellation in their designs with the use of diversion walls and resonance chambers, tuning the
designtocancelspecificfrequencies.
Resonators
Some OEM vehicles have a resonator. It is essentially a sound-tuning device that’s added to the
system,eitherbeforeorafterthemuffler,toadjustthesoundfrequency.Aresonatorisastraight-through
canister that may or may not have internal sound absorption. If an OEM vehicle was equipped with a
resonator,removingtheresonatordoesn’taffectpower,butitalterstheexhaustnote(perhapslouderor
withadifferentfrequency).Resonatorsareallaboutsoundquality;thecarmakeraddsthemwhentheyfeel
theneedto“tune”thesound.Fromaperformancestandpoint,eitherforthestreetortrack,theadditionof
aresonatorisnotcommon.
Taperedlake-pipeheaderscanalsoaccommodatethistypeofinsertbaffle.
Thischartshowsthechassisdynoresultsforastock2008PontiacG8GTwithstockexhaustsystem.Horsepoweratthedrivewheels
wasrecordedat285.37hpwithtorqueat308.11ft-lbs.(GraphCourtesySpiralTurbo)
Thesamecaronthesamechassisdyno,withthestockmufflersremovedand3-inchspiralbafflesinstalledproduced302.92hpand
318.88ft-lbsoftorque.(GraphCourtesySpiralTurbo)
Asanexperiment,thesamePontiac,stillequippedwiththe3-inchbaffles,wasthenswitchedfrom87-octanefuelto91-octanefuel.
Thisincreasedhorsepowerto311.70andtorqueat333.48ft-lbs.(GraphCourtesySpiralTurbo)
AnothermethodthatmanyOEMs(andsomeaftermarketexhaustmakers)usetoreduceoreliminate
bothersomeexhaustdroneistouse“branches”alongtheexhaustsystem.Thisinvolvesinstallingashort
pipe 90 degrees to the exhaust pipe, just ahead of the muffler, that is capped off or that enters into a
separatechamberofthemuffler.Thisprovidesasmallsidepocketofvolume;itissimilartoariverthat
has a small branch-off stream. The exhaust noise enters this short branch and is captured within the
capped-offbranch,alteringthefrequencyoftheexhaustnote.Ofcourse,developmenttestingisinvolved
intermsofthebranchdiameterandlength.
Forinstance,iftheexhaustnotethatmatchesthefrequencyofthetiresastheyrolldownthehighway
hitsacertainsoundfrequency,anobjectionablehumordronemayresult.Aresonatormightthenbeadded
toalterthefrequencyoftheexhaustnotetoavoidthisnoise.OEMsusespecializedfrequency-monitoring
data-collector equipment to develop a resonator for a specific application, which is out of reach for
hobbyists. Several performance aftermarket exhaust system manufacturers have already developed
exhaustsystempackagesforspecificvehiclestoaddressthisissueofannoyingfrequencies,byvirtueof
mufflerdesignorbyaddingaresonator.
CatalyticConverters
Internalcombustionenginesessentiallyoperateasairpumps.Inover-simplifiedtheoreticalterms,the
moreairthatcanbeintroducedintothecombustionchamber(alongwiththeproperratioofairandfuel),
the more horsepower that can be produced. OEM and performance aftermarket engineers continually
strivetoincreasetheefficiencyofcylinderheadintakerunnersandcombustionchambers,pistondomes,
camshafts,valves,intakemanifolds,carburetors,andthrottlebodies,etc.Alongwithincreasingintakeair,
the exhaust system must be designed equally as efficient in order to complement the engine as a total
system.
Theoriginalmufflerswereremovedandaspiralbafflewasinstalled.
Straighttipsectionsequippedwithspiralbafflessimplyclampontotheexistingpipes.
There’snoreasonthatacatalyticconverterneedstolookboring.Performanceaftermarketconvertersareavailablewithpolished
stainlesssteelhousingstobettercoordinatewithaperformanceexhaustsystem.(PhotoCourtesyEasternCatalytic)
Componentsthatcontrolorhaveaneffectontheexhaustflowoftheengineare:
•
•
•
•
•
•
•
•
Camshaftprofile
Exhaustvalve
Cylinderheaddesign
Exhaustmanifolds/tubularheaders
Turbochargerorsupercharger
Exhausttubingdiameterandlength
Catalyticconverter
Muffler
An exhaust system that reduces the amount of work required for the engine to push the exhaust gas
translates into reduced energy loss and that (theoretically) translates into power output. Remember:
Horsepowerisn’tfree;asyouincreaseairintake,youalsoneedtoincreasefuel.Theexhaustsystemmust
not only decrease flow restrictions and increase airflow, but also allow the engine’s fuel system to
managethecorrectair/fuelratio(nottooleanandnottoorich).
Airflowisinfluencedbythediameteroftheexhaustsystemaswellasthespeedinwhichtheexhaust
gasismoving.
ExhaustFlow=flowvelocity×area
Where:
FlowVelocity=mass÷speed
Area=mass2
Asyouincreaseairflowonlybyincreasingexhausttubediameter,youlikelydecreaseairvelocity.A
performance exhaust system needs to be designed to produce as little backpressure as possible while
maintainingthehigh-flowvelocityrequiredforcylinderscavenging.Acatalyticconverter’spurposeisto
reduce engine emissions, specifically carbon monoxide, hydrocarbons, and oxides of nitrogen. The
converterattemptstoburnoffcarbonmonoxideandunburnedhydrocarbons,whileconvertingoxidesof
nitrogenbackintoharmlessnitrogen.
Catalyticconverterassembliesareavailablefordirect-replacementfitonproductionvehiclesthatbettersuitstreet-performance
requirements.Thisexamplefeaturesabuilt-inH-pipecrossoverimmediatelyaftertheheadercollectors.Performanceaftermarket
assembliescomeinstainlesssteelaswellasceramic-coatedversionsforincreaseddurability.(PhotoCourtesyEasternCatalytic)
A washcoat applied to a ceramic honeycomb core is used to chemically treat contaminants. These
washcoatscanincludematerialsuchasaluminumoxide,titaniumoxide,andsilicondioxide.Thesereact
withtheexhaustcontenttowashemissionparticles.Thecatalystisusuallymadeofapreciousmetalsuch
asplatinum,palladium,orrhodium.
Asfarasacatalyticconverter’sroleinexhaustsystembackpressureisconcerned,researchershave
foundthatperformancedoesnotincreasewiththeincreaseofbackpressure.Evenpressurelevelsof2to
5psiintheexhaustsystemcanadverselyaffectengineperformance.Thispositivebackpressureslowsthe
exhaustvelocity,causingunevenscavengingofthecombustionchamberandmakingthepistonfightharder
on the combustion stroke, as well as reducing or diluting the oxygen content coming into the cylinder
through the intake plenum. Further complicating the situation, during overlap of the intake and exhaust
valve timing, the positive pressure from a poorly designed exhaust system can actually dilute adjacent
cylinders,pushingexhaustgasintotheintakeplenumandthrottlebodies.
Inrecentyears,evenOEMshaverecognizedtheneedforcatalyticconvertersthataddresstheneeds
oftheperformanceenthusiast.Thishasresultedinhigh-flowconvertersbeingintroducedonsomenew
vehicleswhilemaintainingrequiredemissionslevels.
The catalytic converter is simply part of the exhaust system, with the specific responsibility of
reducing emissions. When creating a performance exhaust system, the common trend involves using
larger-diameterexhausttubingandhigh-flowmufflersinanefforttoreducebackpressureandtopromote
exhaustscavenging.
However,asfarascatalyticconvertersareconcerned,youactuallywanttheexhaustgastogothrough
theconvertersubstrateatasslowarateaspossibletoallowtimeforthechemicalreactionstotakeplace.
Whilethesereactionstakeplaceinmicroseconds,acatalyticconverterrequirestheexhaustgastoremain
intheconverterforaperiodoftimethatisdeterminedbythelengthoftheconverter’ssubstrateandthe
exhaust flow velocity. Converter bodies typically feature a larger diameter than the inlet pipe, which
slowstheexhaustflowvelocity,whichinturnprovidesadditionaltimeforthecatalystreactiontotake
place.
Additionally,thelargervolumeinsidetheconverterpermitstheuseofalargersubstratetopromote
exhaustgasemissionsreduction.Astheexhaustflowleavestheconverter,theconverternecks-downin
diameter to increase the exhaust velocity once again. Manufacturers of high-quality catalytic converters
areevermindfulofsizingtheconverterstomatchtheenginemanufacturer’srequirementsforairflowand
backpressure.
Volumetric efficiency and maximum engine RPM can play an important role in the selection of a
correctlysizedcatalyticconverter.Goingtoalargerconverterdoesn’tgainanything.
Cat-BackSystems
Acat-backsystemreferstoaftermarketexhaustsystempipingandmufflersthatcanbeinstalledafter
thecatalyticconverters.Itisusedbyownerswhowishtoenhanceexhaustflowandsoundonaconverterequippedstreetvehicle,withoutdisturbingtheconverter’semissionshandling.Thosewhoprefernotto
upgradetotubularheadersuseit.Ratherthanstartingfromscratchbyreplacingtheentireexhaustsystem,
acat-backsetupallowsamildupgradefortheleastamountofmoneyandlabortime.
Cat-backsystemsenhancetheexhaustsystembyreplacingthepipesandmuffler(s)withoutremoving
orreplacingtheconverters.Theyarereadilyavailableforawiderangeofproductionvehiclesasadirect
boltonfromtheconverterstotheendofthetailpipes.Acat-backsystemallowsaneasyupgradeinterms
ofaddedperformanceandalteredsound.
Some systems are quiet, with minor performance and sound change, while others provide notable
changes. The results largely depend on the specific engine and the design of the replacement mufflers.
Selectingacat-backsystemissimple.Chooseareputablebrandthatfeaturesthemufflerstyleyouprefer.
Cat-backsystemsarepre-madetofitpopularproductionvehicles,soyoudon’tneedtoworryabout
modificationsinordertoaccommodatetheinstallation.Theyareeasytoinstallandtypicallymountinthe
stockpipeandmufflerlocations,usingtheexistingOEMhangerlocations.Acat-backsystemthat’smade
ofstainlesssteelispreferableintermsoflongevity.Cat-backsystemsalsocommonlyfeaturecrossover
pipes.
Acatalyticconverter’sjobistoscrubharmfulemissions,includingcarbonmonoxide,hydrocarbons,andoxidesofnitrogen.Awelldesignedperformanceconverteravoidsanincreaseinbackpressurewhilealsocapturingtheflowlongenoughforthechemicaland
thermalreactionstakeplace.(PhotoCourtesyEasternCatalytic)
DiagnosingCatalyticConverters
Whenaconverterfailsbypluggingup,orexperiencingreducedconversionefficiencythatresultsin
not being able to pass an emissions test, the root cause of the failure usually lies elsewhere. In other
words,somethingupstreamfromtheconverterprobablycausedtheconvertertofail.Possibleculpritscan
include an overheated engine, chemical contamination, vacuum leaks, the EGR system, fuel injectors,
sparkplugs,oxygensensors,improperengineassemblytechniques,etc.
Some converter failures should make you look elsewhere in the engine for the real cause of the
problem. Unfortunately, some failures caused by these problems are not covered by the manufacturer’s
warranty.
Overheating
Catalyticconvertersaredesignedtotoleratesubstantiallyhightemperatures,oftenupto2,000degrees
F.Severeoverheatingcandestroyconvertermattingandevenmeltsubstratecoatings,andcanevenlead
to clogging of the converter. For example, if the engine has experienced leaks in the exhaust system,
upstream of the converter, it can result in elevated converter temperature. Typically, a converter
temperatureneedstobebroughttoabout350to500degreesFto“lightoff”(forthesubstratetodoits
job). This range varies depending on the application and the location of the converter relative to the
cylinderhead(s).
Themeltedceramicbrickinthisdamagedconverterindicatesthattheenginewasrunningtoohot.Anexcessivelyleanair/fuel
mixturemayhavecausedthis.Replacingthebrickisnotanoption;theconvertermustbereplaced.(PhotoCourtesyEastern
Catalytic)
Thisoil-fouledceramicbrickisatelltalesignthatengineoilhadenteredtheexhaustsystem.Thiscouldbeduetowornpistonrings,
wornvalveseals,excessivecrankcasepressure,etc.Anoil-fouledconvertermustbereplaced.(PhotoCourtesyEasternCatalytic)
EvidenceofRTVonthisexhaustmanifoldflangeisanindicationofoxygen-sensorcontamination.(PhotoCourtesyEasternCatalytic)
ThisoxygensensorhasbeencontaminatedbyanattempttouseTeflontapetosealthesensorthreads.(PhotoCourtesyEastern
Catalytic)
ConverterContamination
Several things can easily contaminate the substrates used in converters. For example, if antifreeze
enterstheexhauststreamastheresultofacrackedcylinderhead,crackedblock,warpedcylinderhead,
and/or failed head gasket, the substrate can be contaminated easily, rendering the converter ineffective.
The converter can also foul if engine oil enters the exhaust system as a result of worn piston rings,
valveguides,orseals.
Ifincorrectsealantsareusedduringassemblyofintakemanifolds,exhaustmanifolds,etc.thatcontain
highlevelsofsiliconeorTeflon,thesystemcanoutgaswhenexposedtohighheat.Thisoutgassingcan
thenleavearesidueontheoxygensensorsandintheconverter.
Whenever you’re using a sealing compound on an engine that features a catalytic converter and/or
oxygensensors,makesurethatthesealantis“sensorsafe.”Usingthewrongsealantcanruinaconverter
byreducingitsefficiency.
TestingtheConverter
Ifyoususpectablocked/pluggedcatalyticconverter,youcanperformavacuumtest.First,connecta
vacuumgaugetoavacuumportontheintakemanifold,carburetor,orthrottlebody.Readandrecordthe
vacuum at engine idle. Next, run the engine to 3,000 rpm and hold it at that speed. The vacuum should
dropwhenyoufirstopenthethrottle,butshouldthenriseandleveloff.Ifthevacuumreadingstartsto
drop,thiscanindicatethatpressureisbuildingintheexhaustsystem,whichpointstoarestrictioninthe
exhaust system. The converter, the muffler, or a foreign object stuck inside the piping can cause the
restriction.
Thediscolorationonthisconverterbodyindicatesexcessivelyhighexhausttemperatures.(PhotoCourtesyEasternCatalytic)
Ifyoususpectanon-workingconverter,acommontestusedtobetocheckthetemperaturebeforeand
after the converter because it was accepted practice to expect about a 100-degree F difference in
temperaturefromtheinletoftheconvertertotheoutlet.However,today’slate-modelfuelinjectedengines
have become so efficient that this test may not provide an accurate means of determining converter
function.
Inlate-modelsystems,theconvertermaycooldownsorapidlythatyouarenotbeabletoreadmuch
of a temperature difference at idle, and you might only see a difference of 50 degrees or less with the
enginerevvedto2,500rpm.Withthisinmind,atemperaturecheckisstillworthwhiletodetermineifthe
converterisperformingproperly.
Using an infrared pyrometer (a point-and-shoot non-touch thermometer), check the inlet temperature
(atthefrontweldringoftheconverter).Thetemperatureshouldbe350degreesForhigher.Thencheck
thetemperatureattheoutlet,attherearweldring.Thetemperatureshouldbeabout100degreeshigher.If
the temperature is more than 500 degrees F, it may indicate overheating caused by a number of issues
upstream.
Beforeyoublametheconverter,checkforvacuumleaks,improperfuelmixture,andexhaustleaksat
themanifold,headerflange,cylinderhead,etc.
Thepluggedceramicbrickinthisconverterindicatesincompletecombustionby-products,likelyresultingfromatoo-richengine
condition.Catalyticconvertersarenotrepairable;anyconverterthathasbeendamagedorfouledmustbereplaced.(PhotoCourtesy
EasternCatalytic)
BreakIn
Breaking in a converter may sound odd, but it’s important for avoiding long-term problems. Just as
newpistonringsneedtobebrokeninandseatedtothecylinderwallsandflat-tappetcamshaftsneedto
bebrokeninwiththeirlifters,anewcatalyticconverteralsorequiresabreak-inperiodtoprovidethe
expectedservicelifeandefficiency.Theissuedealswiththerequiredexpansionoftheconvertermatting
thatholdsthesubstrateinplace.
Thisfueltrimdataindicatesthatarichconditioncommandstheon-boardenginemanagementcomputertoleanthefuelmixture.A
leanconditioncommandsarichermixture.(ChartCourtesyEasternCatalytic)
Thisisasamplescreenshotonadiagnosticanalyzerwhileloadtestingtheefficiencyofacatalyticconverter.Thepercentof
efficiencythateachconvertermanufacturertriestoachievemaydiffer.Iftherearoxygensensoroscillatesupanddown,itis
indicativeofaninefficientconverter.Aproperlyoperatingcatalyticconvertershouldbe90percentefficient.Inotherwords,90
percentofthehydrocarbonsandcarbonmonoxidethatenterstheconvertershouldbescrubbedbytheconverter.Inthisexample,a
usedconverterthatisdeterminedtobeonly53percentefficienthasfailedandshouldbereplaced.(PhotoCourtesyEastern
Catalytic)
Hereisanexampleofcatalyticconverterefficiencyduringano-loadcondition.(PhotoCourtesyEasternCatalytic)
Acatalyticconverter’sceramicbrickiswrappedwithmattingthatinsulatesitandholdsittogether.
Themattingismadefromthemineralvermiculite,whichissecuredbyafibermatandanorganicbinder.
Astheconverterheatsupduringitsfirstuse,thefibermatandbinderburnoffandthemattingactually
becomes looser before it expands to fill the converter cavity to hold the ceramic brick in place. If the
properbreak-inwarmupisnotperformedcorrectly,theceramicbrickcanloosenandcanrattleinsidethe
converterhousing.
WarmupHeatCycling
Forthemattingtoexpandproperlyandtoprovidelongconverterlife,followthesesimplesteps:
•
•
•
•
•
•
Starttheenginebutdonotrevit.
Idletheengineandallowittowarmupslowly.
After5minutes,increasetheenginespeedto2,500rpm.
Holdat2,500rpmfor2minutes.
Allowtheenginetocooldown.
Roadtesttoconfirmcorrectinstallation.
Thistemperatureexpansiongraphillustrateshowtheconvertermattingexpandsasconvertertemperatureincreasesduringthe
initialwarmupcycle.Ignoringcatalyticconverterbreak-incanresultininadequatemattingexpansion,whichmayleadtobrick
vibrationanddamage.(GraphCourtesyEasternCatalytic)
Thisisacutawayviewofatypicalcatalyticconverterwithmattingandaceramicblock.(PhotoCourtesyEasternCatalytic)
Mattingisinstalledinitsinitialunexpandedstate(right)toeaseinstallationandassemblyoftheconverter.Onceexposedtoexhaust
heat,thematerialexpands(left)tofillthevoids.Thisexpansionoccursduringtheconverter’sbreak-inperiod.(PhotoCourtesy
EasternCatalytic)
CHAPTER6
SUPERCHARGEDANDTURBOCHARGEDSYSTEMS
Forced induction, through the use of a turbocharger or a supercharger, forces additional air to the
intakechargeThisforcedairisreferredtoasboost.Thiscreateshighercylinderpressure,allowingthe
enginetoproducemorepower.Bycontrast,anaturallyaspiratedengineusesavailable(ambient)airto
entertheengine,mixwithfuel,andigniteinthecombustionchamber.
When I talk about forced induction, I’m referring to either supercharging or turbocharging. The
purposeofeithersystemistopackmoreairintotheenginethanisavailableundernormalatmospheric
pressure.Atmosphericpressureis14.7psi(atsealevel).Ifaturbochargerorsuperchargecreates,say,7
psi of boost, you’re packing about 21 psi into the engine (nearly a 50-percent increase). However,
becauseneithersystemoperatesat100-percentefficiency,youlikelydon’tseea50-percentincreasein
horsepower.Morethanlikely,powerincreasesbyaround40percent.Oneofthewaystohelpmaximize
efficiencyistotailortheexhaustsystemformaximumexhaustgasscavenging.
Asuperchargerpressurizestheintakesystemandflowsmorefuelintothecombustionchamber.Ifyourengineisfittedwitha
supercharger,youneedtoselectacompatibleexhaustsystem.
Supercharging
While turbocharging packs additional air into the cylinders with the use of an exhaust-gas-driven
compressor, a supercharger accomplishes the same task by being mechanically driven by the engine’s
crankshaft. In many cases, depending on the specific engine design, a supercharged engine’s crankshaft
may have a longer snout than a naturally aspirated version of the same type of engine. Additional
crankshaftsnout/noselengthmayberequiredinordertoacceptthesupercharger’sbeltdrive.Ifyou’re
building an engine intended for supercharger use, talk to the crankshaft maker to determine if the
crankshaftacceptsasuperchargerdrive,basedonthetypeofengine,assomeenginedesignsmayoffera
shortorlongsnout.
Aturbochargerusesexhaustgasestopressurizetheintakesystem.Turbosgenerateheatsoheatmanagementisatoppriority
becauseexcessiveheatrobsanengineofpower.
Mostsuperchargerkits,suchastheoneshownhere,includeeverythingrequiredfortheinstallation.Dependingonthetypeof
superchargerandthespecificmanufacturer,kitsmayinclude(inadditiontothesuperchargeritself)anintercooler,programmer(for
EFIapplications),electricfuelpump,regulator,specialairintake,andserpentine-beltidlerpulley.(PhotoCourtesyEdelbrock)
TypesofSuperchargers
Therearethreetypesofsuperchargerstoconsider:twin-screw,Roots,andcentrifugal.Twin-screw
superchargershaveapairofinter-meshingmale/femalehelicalrotors,verysimilarinnaturetoatwinscrewshopaircompressor.Thematedpairofhigh-precisionmatchedrotorsprogressivelycompresses
theairbetweenthescrewsastheyspininopposingdirections.Thescrewsdon’ttoucheachother.Atight
clearancebetweenthescrewscompressestheairasitisdrawnfromoneendofthescrewstotheother
end.Atwin-screwsupercharger“squeezes”theairmoreefficientlythanaRoots-typesupercharger.
TheRoots-typesuperchargerhasopposingmalerotorlobesinsideahousing.Asthelobesrotate,air
is pushed between the outside of the lobes and the housing case, blowing additional air into the intake
system. Twin-screw superchargers are said to produce less heat than the Roots type for a cooler air
charge, but with intercooler aid this may be a moot point. The twin-screw type of supercharger is
theoreticallymoreefficient.Bothatwin-screwandRoots-typesuperchargermountstotheintakemanifold
locationontheengine.
ARoots-typesupercharger’srotatinglobescreateforcedairbetweenthelobesandthesupercharger’shousing.Atwin-screw-type
supercharger(shown)squeezesairbetweentherotatingscrews,creatingamore-efficientpressuregeneration.(PhotoCourtesy
WhippleIndustries)
Asatwin-screwsupercharger’shelicalscrewsrotatetowardeachother,airisprogressivelycompressedasitisroutedalongthe
lengthofthescrews.(PhotoCourtesyWhippleIndustries)
Thetwin-screwdesign’smale/femalescrewsareprecisionlocatedwithaverytightclearanceandneveractuallytoucheachother,so
wearbetweenthescrewsisnotanissue.(PhotoCourtesyWhippleIndustries)
ThePaxtoncentrifugalsuperchargersysteminstalledonthisFordMustangGTprovides6to8psiofboost,whichdoesn’trequirea
larger-diameterexhaust.However,increasingexhaustsizeonevenalight-boostsuperchargersystemmaybenefithorsepower.A
centrifugalsuperchargerlooksverysimilartoaturbocharger.Insteadoftheimpellerbeingdrivenbyexhaustgas,abeltdriveis
used.
Acentrifugal-typesupercharger,atfirstglance,looksandfunctionssimilarlytoaturbocharger,inthat
theunithasaninternalimpeller.Insteadofmountinginplaceoftheintakemanifold,acentrifugal-style
unitismountedanywhereatthefrontoftheengineaslongasitspulleyalignswiththeengine’sbeltdrive.
Thismakesthecentrifugalstylemoreversatileintermsoftheinstalledlocation.
Theimpelleroperatesatahighspeed,drawingairintothevolute(compressorhousing).Thehighair
velocityhasarelativelylowpressure.Theairtravelsthroughadiffuserthatconvertstheairintoalowspeedhigh-pressurechargethatisforcedintotheengine’sairintake.Aninternallygearedtransmission
providestheincreaseinimpellerspeed.
Depending on the kit design for a given vehicle, the air discharge plumbing from the centrifugal
superchargermaybelocatedbetweentheengine’sthrottlebodyandtheintakemanifold.Somecentrifugal
kitsaredesignedtoplumbtheairchargeintothethrottlebody,eliminatingtheneedtorelocatethethrottle
body.
Smokey’sDynoinAkron,Ohio,installedthisbelt-drivencentrifugalsupercharger.Today’ssuperchargerkitsarerelativelyeasyto
installbecausetheyincludeallmountinghardware.Typically,littleornofabricationisrequired.
ProChargerrecentlyintroducedtheire-1programmable-ratiocentrifugalsupercharger,withinternaltransmissiongearingratios
selectableviaanon-boardcontroller,allowingvariablelevelsofboosttosuitindividualrequirements.(PhotoCourtesyProCharger)
SelectionTips
Allthreetypesofsuperchargersproducethesameendresultofpackingmoreairintotheengine,but
they accomplish it by different mechanical designs. In theory, a screw-type or Roots-type supercharger
producesbetterlow-RPMpowerthanacentrifugaltype,whichshouldproducebetterhigh-RPMpower.
Theboostproducedbyasuperchargerisdirectlyaffectedbythesize/displacementoftheengineas
wellasthespeedatwhichthesuperchargerisdriven.Forinstance,ifthesuperchargerisdrivenatthe
same ratio as the crankshaft, a supercharger with a larger displacement produces more boost than a
smallerbloweronthesameengine.
Thisisacutawayviewofafixed-ratiocentrifugalsupercharger.(PhotoCourtesyProCharger)
Ingeneral,youshouldselectasmallerblowerforasmaller-displacementengineandalargerblower
for an engine with bigger displacement. The boost can be tuned by altering the diameter of the drive
pulleyinordertomaketheblowerrunslowerorfaster.
Alargesuperchargeronasmall-blockenginecanbetunedbydrivingthesuperchargerataslower
speed to reduce boost, in order to prevent detonation. However, operating the supercharger too slowly
canresultininadequateaircompression/boost.
Ifthesuperchargeristoosmall,itmustspeeduptoapointwheretheintakechargetemperatureistoo
high,resultinginpoorairdensity.
FuelSystem
Packing more air into the engine requires increasing fuel delivery during boost. Depending on the
amountofboost,aforced-inductionsystemmayrequirelargerinjectorsandreprogrammingthefueltrim
by reflashing or programming the ECM. Engine timing may also require less advance, which aids in
avoidingpre-ignition/detonationandpermitstheuseofadditionalboostpressure.
Pressure-ReleaseValves
A turbocharged system requires two adjustable pressure-release valves in order to avoid overboostingandmanagementofpressurelimits.Thisincludesbothawastegate(WG)andablow-offvalve
(BOV).
A wastegate valve is installed in the exhaust stream before the gas enters the turbocharger, which
slowlyopensif/whenexhaustpressureexceedsapre-setlimit.Thiscontrolsboosttotheturbo.
A blow-off valve is installed in the air charge path that runs from the turbocharger to the engine’s
intake,regulatingthelimitofboostpressuredeliveredtotheintakemanifold.Thisislocatedintheaircharge pipe between the turbo and the intercooler. When throttle is lifted, the blow-off valve prevents
forcedairfrombeingpackedintotheengineasenginespeeddecreases,protectingtheenginefromexcess
pressurewhenthethrottleisreleased.
Asuperchargerrequirestheuseofabypassvalve,whichregulatesthelimitofairpressuredelivered
totheengine.
CompressionRatio
When air intake and combustion pressure are increased by a forced-induction system, it effectively
increases the engine’s compression ratio. An engine with a specific compression ratio in naturally
aspirated form experiences an increase in compression ratio when forced induction is used. When
building an engine that features a forced-induction system you need to consider the initial compression
ratiotohandletheincreasethatwillbeprovidedbyaturbochargerorsupercharger,toavoidpre-ignition,
or“knock.”Inthecaseofanenginespecificallybuiltforforcedinduction,loweringthecompressionratio
allowsmoreboostwiththesameoctane.
Anengine’sstaticcompressionratio(CR)referstothecompressionratiooftheenginewithoutforced
induction. An engine’s final compression ratio (FCR) is experienced during forced-induction operation
undertheavailablefull-boostlevel.
Thiswastegatevalveisplumbedintotheexhaustpipingbeforeenteringtheturbochargeronarear-mountturbosystematSmokey’s
Dynoshop.Smokey’sspecializesinforced-inductioninstallationsandchassisdynotuning.
Abypassvalveforaturbosetupisaffectedbyairpressure.Onceapredeterminedpressurelimitisset,thevalveispushedopen,
allowinganyexcessairpressuretoescapethroughtheopenslotsinthevalvebody.
Aturbochargerwastegatehasavalvethatremainsclosedwhentheboostpressurerunsatapredeterminedlevel.
Whenboostpressureexceedsthepre-setlimit,thewastegatevalveopens,divertingexcessexhaustpressure,tolimittheamountof
pressurethattheturbocangenerate.
FinalCompressionRatio
Thischartshowshowthefinalcompressionratio(FCR)correspondstobothstaticcompressionratio
(CR)andboost.Touseit,findyourCRontheleft-handsideandyourboostacrossthetop.The
intersectingnumberindicatesthetheoreticalfinalcompressionratio(FCR).
Here’stheformulaforcalculatingFCR:
FinalCompressionRatio=(boost÷14.7)+1×CR
Variablesofeachenginebuildneedtobeconsidered,suchasthestrengthofthecrankshaft,rods,and
pistons, but in general terms the FCR of any forced-induction engine should not exceed approximately
12:1iftheengineistobeoperatedonpumpgas.HigherFCRsdemandtheuseofhigher-octaneracefuel.
For most street applications, CR should be limited to about 10:1. It’s important to understand that the
amountofboostthatyouplantousedirectlyaffectstheFCR.
Turbocharging
A turbocharger contains a turbine and a compressor. In essence, the turbo is fed by the engine’s
exhaust gas, which spins the turbine. As exhaust increases, the turbine spins faster. The exhaust-driven
turbine drives the compressor side of the turbo, which serves as a centrifugal pump, and packs
pressurizedairintotheintakemanifold.Thisforcedinductionpacksmoreair,andsubsequentlyagreater
air/fuelcharge,intothecylindersthanisavailableonanaturallyaspiratedengine.Thegreatertheair/fuel
charge,themorepowertheenginecanproduce.
Turbochargingdoesnotprovide“free”horsepower.Becauseplacingtheturbochargerintheexhaust
stream creates added restriction in the exhaust, the engine is required to fight against this added
backpressure during its exhaust stroke. For this reason, a turbocharged system benefits from a largerdiameterexhaust.
Engineupgradesneedtobeconsideredforanybuildwhereoneortwoturbochargersareinstalled.
Becauseofhighercylinderpressureandresultinghighertemperatures,stronglyconsidertheuseofforged
orbilletpistons,whichwillwithstandtheseincreasedpressuresbetterthancastorhypereutecticpistons.
Alsoconsidertakingadvantageofspecialtythermalbarriercoatingsforthepistondomes,exhaustvalves,
cylinderheadcombustionchambers,andcylinderheadexhaustports.
Aturbochargersystemrequirestheuseofawastegate,whichlimitstheamountofexhaustpressurethatenterstheturbo.A
turbochargerfeaturestwosections.Thecast-ironsectionacceptstheengine’sexhaust,drivinganimpeller.Thisinturndrivesthe
impellerlocatedinthealuminumair-chargesection,producingpressurizedairthatissenttotheengine’sintake.
When a turbo is under boost and forces a denser charge into the cylinders, the air is pressurized.
Whenairispressurized,theairtemperaturerises.Iftheairtemperaturebecomestoohot,itcanignitethe
chargebeforethesparkplugfires.Thisis,ofcourse,acaseofpre-ignition,whichproducesengineknock.
This is why turbocharged engines typically feature a lower compression ratio: to help compensate for
potentialpre-ignition.Turbochargedenginestypicallyalsorequireahigher-octanefuelratingforthesame
reason:toavoidpre-ignition.
Hereyoucanseetheexhaustentryflangethatattachestotheexhaustmanifold.Theair-chargeoutletisatthetopleft.Theexhaustoutportisatthefarright.
Thisturbocharger(sizeT-76)offersarelativelylargeexhaustdiameter.Thisapplicationacceptsanexhaustpipeof4.5-inchID.The
largeflangelipallowstheuseofaV-grooveclamp.
Aturbochargerneedsrotationaltimetogenerateboost.Asmallturbospoolsfasterthanalargeturbo,
reducingthechanceforlag.Usingasmallerturboalongwithawastegatevalvepreventstheturbofrom
spinningtooquicklyathigherenginespeeds.
EngineHigh-BoostUpgrade
If you plan to increase engine horsepower by 150 hp or more by using a forced-induction system,
consider potential engine upgrades that allow the engine to both perform and survive under these
conditions.Appropriateupgradesdependontheengine’sexistingcomponents.
Theoutletofthisturbocharger,wheretheairchargeispushedout,hasaflangethatallowstheuseofaV-bandclamp.Thisflange
ODmeasures4.125inches,requiringtheuseofamatching-diameterflangefortheair-chargepipe.
Selectingtheright-sizeturboforagivenapplicationinvolvesanumberofvariables.First,youneedtodeterminehowmuch
availablespaceyouhavefortheturbolocation,whichcanoftenbealimitingfactor.Thenyouneedtoestablisharealistichorsepower
goal,themaximumengineRPMrange,andhowmuchboostpressureyouexpecttouse(whichmaybelimitedtothesturdinessof
yourengine’scomponents).
Onefactortoconsideristheturbineandcompressorarearatio(A/R).AsmallA/Rincreasesexhaustgasvelocityintotheturbo,
resultinginfasterturbinespinandaquickerboost.AsmallA/Ralsoincreasesbackpressureandreduceshorsepowerathigher
RPM.AlargeA/Rlowersexhaustvelocity,delaysboosttime,andprovideslessbackpressureforincreasedpowerathigherengine
RPM.
Youreallyneedtofollowtheadviceoftheturbomanufacturerwhenchoosingturbosize.
Oncetheappropriate-sizeturbochargerhasbeenselected,measureoverallwidthtohelpdeterminewheretheunitmaybelocatedin
thevehicle.
Thisturbochargerhasafresh-airintake“bell”thatmeasures6.25inchesindiameter.Youneedtohaveaccesstoasourceforpipe
inarangeofdiameters(andbends)toaccommodatethevarietyofturbosetupsthatyou’relikelytoencounter.
For example, consider the following: If the engine is currently equipped with cast or hypereutectic
pistons,Istronglysuggestswitchingtostrongerforgedorbilletpistons.Dependingontheengine’scurrent
compressionratioandtheamountofboostyouplan,youmayneedtolowerenginestaticcompression.
Talktotheturbochargerorsuperchargermakerforarecommendation.
If you’re going to pack in more than about 10 psi of boost, static compression may need to be a
maximumofabout10:1.
Also consider the use of thermal barrier coatings for piston domes, combustion chambers, exhaust
valves, exhaust ports, and exhaust manifold or headers. Connecting rod bolts can be a weak link, so
changingtohigh-tensile-strengthaftermarketrodboltsisalwaysagoodmove.
Ifconnectingrodsarecurrentlycastorpowderedmetal,it’swisetoupgradetoforgedrods.Thesame
holdstrueforthecrankshaft.Ifit’scurrentlyacastcrank,movingtoaforgedcrankiswise.Dependingon
the engine design, using MLS cylinder head gaskets and cylinder head studs in place of bolts may be
recommended.
Also, depending on engine block design, it may be necessary to strengthen the main cap system by
movingtosteelbilletmaincapsandpossiblyaddingamainstudgirdletobeef-upthebottomend.Ifthe
engineisexpectedtorevharderandquicker,considerupgradingtohigher-strengthvalvespringsandfullroller rocker arms, as well as upgrading the timing chain system (where applicable) to a heavy-duty
double-rollerchain,crankgear,andcamgear.
Askinganenginetoproducesubstantiallymorepowercanplaceunduestressonanumberofinternal
components,soconsideryourspecificengine’spotentialweakpoints,andaddressthemaccordingly.
Ifhighcylinderpressuresareobtained(throughforced-inductionand/ornitrousinjection),seriousconsiderationshouldbegivento
upgradingcriticalcomponents,suchasusingforgedpistons,pistonringsthataredesignedforforced-induction,forgedconnecting
rods,andaforgedcrankshaft.
TurbochargerTemperatureManagement
Turbocharging is a hot topic, literally. The turbocharger itself is exposed to intense heat, as are the
pistons,exhaustvalves,andexhaustports.Thermalbarriercoatings,availablefromaftermarketcoating
specialists,aredefinitelyworthconsidering.Specifically,theturbochargerhousing,theexhaustmanifold,
pistondomes,combustionchambers,exhaustvalvefaces,andcylinderheadexhaustportscanbenefit.
ThermalBarrierCoating
Whyhavetheturbohousing,manifold,downpipe,andtheremainderoftheexhaustpipingcoated?The
applicationofatruethermalbarriercoatingretainsmoreheatinsidethesecomponents,resultinginless
heattransfer.Thismeansthattheexhaustcomponentsurfacesarelesshottothetouchandkeeptheheat
inside. Keeping the exhaust gas hotter, with less transfer to the outside, helps to speed the exhaust gas
flowandimprovesexhaustscavenging.
Moreover,greaterexhaustgasvelocitymakestheturbospoolquicker.Inotherwords,theapplication
of effective heat retention/thermal barrier coatings in the combustion chamber, pistons domes, turbo
housing,andexhaustpipingcandirectlyaidturboperformance.
Addingathermalwrapontheturbochargerhousingservestwopurposes:Itlowersthetemperatureofthesurroundingarea,whether
theturbochargerisinstalledunderhoodorattherearofthevehicle.Italsoenhancesexhaustefficiency.Thisturbochargeris
adornedwithaform-fittingthermalbarrierheatshield,readilyavailablefromDEIandothersources.
For the exhaust stream, different levels of thermal barrier coatings are available. Some are more
cosmeticanddesignedtoenhanceappearance,andmaybeasthinas.002inch.Othersaredesignedfor
function, with little regard to appearance, and can be as thick as .015 inch or more. By using applied
layersofrealceramic(appliedtothecomponentsinamoltenstate)andbyusingathickerapplication,
heatretentioncanbeimprovedbyasmuchas40to50percent.Thincoatingsthatemphasizeappearance
ratherthanfunctionmayonlyimproveheatretentionbyaslittleas5percent.
Swain Tech Coatings, for example, manufactures and applies all coatings in-house. Their thermal
barrier coatings feature a pure ceramic content applied in layers to a substantial thickness. The only
negative,ifyouwanttousethattermatall,involvesappearance.TheirWhiteLightningthermalbarrier
coatingprovidesasomewhatrough,whiteappearancethatmaynotbeacceptableforshowcarfolks.
Analternativetoathermalwrapistohavetheturbochargerhousingcoatedwithahigh-temperatureceramicthermalbarrier.(Photo
CourtesySwainTechCoatings)
However, you need to ask yourself one question: Do I want to look good without regard to
performance,oramImoreconcernedwithfunctionandincreasedperformance?TheWhiteLightninghas
a melting point of 3,000 degrees F, which certainly withstands extreme turbo temperatures. If you’re
seriousaboutperformance,youshouldn’tcareaboutcolororsurfacetexture.
I’vehadmanycomponentscoatedbySwainandIcanbearwitnesstothefactthatthestuffworks.This
isnottosaythatothercoatingservicescan’tprovidethesamelevelofperformance.Examplesofother
skilled coating services include Cerakote, Polydyn, and Calico. Just do your research before deciding
whichcoatinghousetouse.
Forpistondomes,ceramicthermalbarriercoatingsaregenerallyappliedatamaximumthicknessof
about.002inch.Typically,pistondomethermalbarriercoatingsfeatureathermalbarriercoatingceramic
(TBC-ceramic). This is effective for any naturally aspirated application. Forced-induction, including
turbocharging, supercharging, and nitrous applications, often call for a higher resistance to heat. Many
specialty coaters offer a dedicated ceramic coating for turbo pistons (sometimes called a gold coat),
whichisappliedabitthicker,ataround.003inch.Thiscoatingisspecificallydesignedfortheextreme
temperaturesthataregeneratedinaforced-inductionengine.
Heat-EmitterCoating
Nowlet’sturnourattentiontotheturbosystem’sintercooler.Forthiscomponent,youwanttorelease
heatinsteadofretainingit.Theforcedairthat’sbeingpackedintotheengineneedstocooldown(that’s
the reason for the use of an intercooler in the first place). Applying a heat-emitter coating to the
intercooleranditspipingimprovesheattransfer.
Withthesubjectofheatmanagement,youneedtoconsiderbothheatretentionwithathermalbarrier
coatingandheattransferusingaheat-emittercoating.Byapplyingaheat-emittercoatingtotheintercooler
anditsairplumbing,youcanimproveheattransferbyapproximately5to10percent.Forinstance,ifthe
air running through the intercooler normally is around 200 degrees F, a properly applied heat-emitter
coatingcanpullanadditional20degreesfromtheaircharge,reducingthetemperatureto180degreesF.
ThermalWrap
Anotheroptiontoretainheatinsidetheturbochargeristoconsiderathermalwrapthatinstallsover
thehousing,capturingheat.Thislowersunderhoodtemperatureaswell,inadditiontoretainingheatinthe
exhaustforimprovedexhaustgasscavenging.
DEI,asanexample,offersthermalbarrierwrapsandcoversforamultitudeofexhaustcomponents,
includingturbochargers,downpipes,headers,andexhaustmanifolds.Forcedinductionraisestheexhaust
gastemperature.Managingthisheataffectsbothcomponentdurabilityandexhaustscavenging.Themore
efficientlyyoucanpulltheexhaustoutofthecombustionprocess,thequickermoreaircanbepackedin.
Sinceaforced-inductionengineiscreatingthisheatanyway,itjustmakessensetotakeadvantageofitby
keeping it where it belongs: in the exhaust stream, instead of being soaked into the cylinder heads and
pistons.
Granted, a forced-induction system allows the engine to generate more power, but with that power
comes a concern for the durability of various engine components. With increased boost and with the
enginecrankingoutitsmaximumpower,thereciprocatingandrotatingassemblyissubjectedtoadditional
abuse. This includes the pistons, piston rings, piston pins, connecting rods, connecting rod bolts,
connectingrodbearings,crankshaftandmainbearings,andtheengineblock’smaincaps.Generatingmore
powercreatesadominoeffectthatexposesthesecomponentstoaddedstress.
Inastreet-drivenenginewithminimalboostofabout6to8pounds,theconcernisrelativelyminor.
Butwhenyoubegintouseboostabovearound18pounds,andstarttoreallypushtheenginetogenerate
maximumpower,mechanicalstressescanreachapointwherepistondomesfail,bearingstakeabeating,
connectingrodshaveagreaterpotentialforbendingorcracking,crankshaftsaremorepronetofailure,
and main caps are subjected to potential distortion or failure. For this reason, any serious forcedinduction build should consider forged or billet pistons, more durable rod and main bearings, forged
connectingrods,forgedcrankshaftsandsteelbilletmaincaps,high-tensile-strengthconnectingrodbolts,
andhigher-strengthcylinderheadandmaincapfasteners.
HeadersversusCast-IronManifolds
In some cases, available space may dictate the choice between cast-iron exhaust manifolds and
tubular headers. The benefits of using cast manifolds include durability because they’re stronger and
shouldbelesspronetocracking.Inaddition,cast-ironexhaustmanifoldsretainmoreheat,evenwithouta
thermalbarriercoating,whichallowstheturbochargertospoolmorequickly.
However,OEMcast-ironmanifoldscommonlyofferamorerestrictiveexhaustgaspath,whichcanbe
detrimentaltohorsepower.Ifyou’relimitingthepowerboostwiththeuseofaturbochargerandplanto
generatenomorethananadditional50to100hp,castmanifoldsarelikelyacceptable.Ifyouplantoadd
morethan100horsepowerbyvirtueofyourturbosetup,youreallyneedtousetubularexhaustheaders.
OnaV-8engine,a2-inch-diameterprimarypipeislikelythebestchoice.Ineithercase,whetheryouopt
forcastmanifoldsortubularheaders,it’sadvisabletohavethemcoatedwithaceramicthermalbarrier
coatingforheatretention,plustheaddedbenefitofloweringunderhoodtemperatures.
ExhaustPipeDiameters
The installation of a supercharger does not require a larger-diameter exhaust pipe system, but
horsepowerandtorquebenefitfromincreasingthepipediameter,generallytothe3-inch-plusrangeonV8 engines. Ideally, exhaust headers should have 2-inch primary tubes, which can add around 10 hp and
approximately30ft-lbsoftorque.Granted,free-flowingheadersmayslightlylowertheboost,butthatcan
be remedied by slightly raising the supercharger boost through a management approach, such as
decreasingthepulleydiametertomakethesuperchargerspinfaster.
Forthisprototypesportsracer,OEMcast-ironexhaustmanifoldswerefittedwithturboflangestoaccepttheturbochargers.
On the other hand, a turbocharged system does require larger-diameter exhaust piping, which is
criticalforbothenginedurabilityandtheproductionofhorsepowerandtorque.Ingeneralterms,aV-8
enginethatrunsasingleturbointherangeof6to15poundsofboostrequiresaminimum3-inchexhaust,
whileatwin-turbosetupexhaustpiperequiresonlya2.5-inchexhaust.Asyouincreaseboost,theexhaust
pipediameterneedstoincreaseproportionately.
Runninganexhaustpipethatislargerthannecessarydoesn’thurt,butrunningatoo-smallexhaustwill
bedetrimentaltobothpowerandtheengine’smaximumRPM.Asaferuleofthumbistorunturboexhaust
pipinginaslargeadiameteraspracticallyfitsintheavailablespacealongthechassis.
Also,foraturbochargedsystem,it’sbesttorunthesame-diameterexhaustpipefortheentirelength,
withnoneedtostepupordownalongthepath.Becauseaturbochargerfeedsoffexhaustpressuretospin
theturbine,theturboneedstobeplacedclosetothecylinderheadexhaustports.Castexhaustmanifolds
designedforturbouseallowashortrunpathofexhausttotheturboandprovidesasubstantialmounting
toaccommodatetheweightoftheturbocharger.Iftubularexhaustheadersareemployed,theyshouldbe
relativelyshortsothattheyfeedtheturboasdirectlyaspossible.
It’s common for some builders to use shorty headers installed in reverse, depending on engine type
andavailablespace.Withthisarrangement,thecollectorsaimforwardtoalloweasiermountingofthe
turbochargerinthefrontoftheengine.
TurbochargerLocation
Becauseasuperchargerismechanicallydrivenbythecrankshaft,itmustbemountedonthefrontof
theenginetoaccommodatethebeltdrive.
Theinstallationofaturbcharger,however,offersadegreeoflatitude.Sincetheturboisdrivenbythe
engine’s exhaust gas, the turbocharger unit may be located wherever space permits, as long as exhaust
piping and air charge piping can be accommodated. As a result, a turbo may be installed within the
confinesoftheenginebayorattherearofthevehicle.
Arear-mountturbosetupisoftenfavoredwhendealingwithacrowdedandcrampedenginebay.A
typicalrear-mountsystemhastheexhaustpipesrunningtoapointbehindtherearaxle,withtheturbo(s)
mountedinaconvenientlocation(forexample,atthelocationoftheoriginalrearmuffler(s).Afresh-air
intake with an air filter is plumbed to the fresh-air intake side of the turbo. The air charge plumbing,
whichdeliversthecompressedairchargetotheengine,isthenroutedfromtheturbototheintercooler
andtotheintakesystem.
A typical V-8 rear-mount turbo system should feature 2-inch header primary tubes, 3-inch exhaust
pipingtotheturbo,a3.5-inchfresh-airintaketotheturbo,and3-inchchargepipeexitingtheturboupto
theintercooler.
Advantages of a rear-mount turbo include reduced underhood temperatures (as compared to an
underhood installation), lower oil temperature through the turbo, and, theoretically, a superior pressure
differential across the turbine, as the extended exhaust lowers the outlet pressure and in turn raises the
turboinletpressure.Thepressuredifferentialistherelationshipbetweeninletandoutletpressureatthe
turbo.
Theexhaustgasinlettotheturbochargerisimportant,butnotascriticalastheair-chargeoutletfrom
theturbo.Iftheair-chargepipeistoosmallindiameter,itcanresultinexcessivebackpressure,which
canleadtooilleakage,blowby,oilsmokeexitingtheexhaustwhenoff-throttle,reducedhorsepower,and
lowermaximumengineRPM.
Rear-mountturbochargersetupshavebecomeincreasinglypopular,especiallywhereunderhoodspaceisseverelylimited.Locating
theturbocharger(s)attherearofthevehicleisofteneasierthantryingtoobtaininstallationspaceinanalready-crowdedenginebay.
Rearmountlocationsalsomaketheturbocharger(s)moreeasilyserviced.ShownhereisasingleturboinstallationonaNissan350X.
Notethethermalheatshieldblanketinstalledonthisrear-mountturbochargerhousing.Itinsulatesthetrunkareafromexcessturbo
heatandenhancesturboperformance.
Installingarear-mounttwin-turbosystemonthislate-modelDodgeChargerrequiredaminoramountoffabrication.Thissystemwas
purchasedasakit,whichincludedallmountinghardwareandpre-fitpiping.Properlyinstallingthemountingbracketrytosupport
theweightofeachturbochargerwascritical.
OnthisNissan350Xrear-mountturboinstallation,thesingleexhaustpiperoutesthroughthewastegatelocationandintothe
turbocharger.Theair-chargepiperunsfromtheturbotothefront-mountedintercooler.Thelarge-diameter180-degreepipeconnects
theturbotoitsairfilter(withintheblackairfilterhousing).
Thepipesthatwereincludedinthisturbokitwerepre-bentandaffordedarelativelyeasyinstallation.Kitscangreatlyreduce
installationtime,withoutneedingtohavepipescustomfabricated,whichmeansthatyoucanmoreeasilyhandletheinstallationon
yourownratherthanfarmingoutthepipebending.
Withtheturbochargersinstalledattheveryrearofthevehicle,noadditionalmufflerswereneeded.Theturbos’restrictionservesto
reducetheexhaustnote.
Eachturbochargeronthisprototypehasitsownintercooler.Locatingeachturbochargeranddesigningtheexhaustpipingpresented
achallenge.
This1969Camaroisalsooutfittedwithtwinturbos.Quiteoften,becauseofunderhoodspacelimitations,exhaustheadersareeither
fabricatedorturnedupsidedown(shown)inordertoaccommodatetheturbochargers.Thewastegatesareinstalledbetweeneach
headeranditsturbo.
Thisisatwinturbochargerrear-mountsetuponalate-modelDodgeCharger.Theair-chargepipethatrunsfromtherear-mount
turbostotheintercoolermergesinaY-connectionbeforeenteringtheintercooler.
Intercoolers
Aturbochargerorasuperchargergeneratesheattothepressurizedairchargethat’sbeingpackedinto
the engine. An intercooler is simply a heat exchanger (or radiator) that’s plumbed between the
turbochargerorsuperchargerairoutletandtheengine’sintakesystem.Thisheatexchangerlowerstheair
temperature entering the engine to create a more dense air charge. Lowering the intake air temperature
(IAT)producesmorepower.
Thisexoticmid-enginesportsracerisactuallyintendedforthestreet.TheLSengineisequippedwithtwinturbochargersinavery
crampedspace.Alloftheexhaustandair-chargepipingrequiredcustomfabrication.Thisone-offsetupwasdesignedandinstalled
atStainlessWorks.
Anintercooleracceptsthecompressedairgeneratedbytheturbochargerandlowerstheintakeair-chargetemperaturebeforeit
reachestheengine’sintake.Acooler,denserchargepromotesengineefficiencyandpower.Asfarasintercoolersizeisconcerned,
youshouldchoosethelargestintercoolerthatcanbefittedtothevehicle.
Thisisanexampleofatwin-turbosystemfittedtoanLS7engine,whichincreasesrear-wheelhorsepowerto1,000underfullboost.
Giventheinstalledlocationsoftheturbochargers,custom-formedexhaustpipesarerequiredtorouteexhaustfromtheturbostothe
remainderoftheexhaustsystem.
Ifatwin-turbochargersetupcanbeinstalledincrampedquarterssuchasthese,youcancertainlyaccommodateaturboinany
productionvehicle.Thisexampleisacustom-builtstreetcarthatemulatesanexoticroadracecar,featuringamid-mountLS7
enginewithtwinturbochargers.Becauseofthetightconfinesandexoticsuspensionsystem,aswellastheengine/transaxledriveline,
designingtheexhausttubingpresentedquiteachallenge.Theentireexhaustsystemwasdesignedandfabricatedin-houseat
StainlessWorks.
Thislate-modelCamaroturbosystemhasanintercoolerwithbothinletandoutletair-chargepipesatthetopoftheintercooler,dueto
spacerestrictions.Thebypassvalve(onthedriver’sside)limitsandprotectstheenginefromanoverboostcondition.
Inanair-to-airintercoolersetup(left),theairrunningfromthesuperchargeriscooledbyasimpleair-to-airheattransferbeforeit
enterstheengine.Anair-to-water-to-airsystem(right)ismorecomplex,asheatedairfromthesuperchargerentersapump-driven
water-coolersystemandthenrunsthroughasecondaryair-to-airintercooler.(IllustrationCourtesyProCharger)
Howlargeshouldtheintercoolerbe?Ingeneral,youshouldselectaslargeanintercoolerasfitsinto
theavailablespace.Thisisonecomponentdecisionthatfollowsthe“biggerisbetter”approach.
TurboAir-ChargePipe
The air-charge piping that runs from the turbo outlet to the intercooler to the engine is plumbed in
sectionsinordertofacilitateassembly.Theconnectionsatpipetopipe,pipetointercooler,andpipeto
intakemustbesecuretoavoidaccidentaldisconnection/leakagecausedbyseparationoftheconnections.
Commonlyahigh-temperature-resistantsiliconehoseisusedateachconnection,securedbyabandclamp
or a wide T-bolt clamp. These silicone hose connectors are available in a variety of diameters and
shapes,includingstraight,45-,and90-degreebends.
Air-chargepipingroutedfromtheturbochargertotheintercoolermaybeconnectedusinghigh-temperaturesiliconesleeves,which
areavailableinavarietyofdiameters,includingsteppedadaptersizes.ThesleevesaresecuredtothepipesusingT-boltband
clamps.
If slipping or disconnection is possible, it most likely occurs at a straight connection. To provide
additionalinsuranceagainstaccidentalconnectionpopoff,aboostbracemaybeadded.Thisissimplya
steel,aluminum,orstainlesssteelbracethatconnectsmatingpipes.Addingoneinvolvestackweldinga
posttoeachpipeoneachsideofthehoseconnection.Thepostsarethensecuredtogetherwithabolt-on
orclip-onbracebar.
Vibratech’sboostbraceisagoodexample.Eachposthasaballendandeachendofthebracebarhas
aspring-loadedfemalecouplerthatsnapsontoeachpostball.Thisiseasilyremovedforpipeservicing.
Basically, any locking system that prevents the air-charge pipe connections from slipping loose is
worthwhile.
For instance, you could simply weld a tab onto each mating pipe behind the silicone-rubber sleeve
locations.Thensecurethetwotabstogetherwithaboltandalockingnutorbyconnectingthetwotabs
withaheavyspring.Thisprovidesafail-safeconnectionthatpreventsthetubesfromwalkingoutofthe
connectionsleeveswhenthetubeisunderchargepressure.
Theair-chargepipeshouldberoutedtoavoidclosecontactwithpotentialrublocationsandhigh-heat
locations.Ifthepipingisclosetoaheatsource,athermalwrapmaybeaddedtothepipeinthatlocation.
Here,aredbypassvalveislocatedontheair-chargepipebeforetheintercooler.Awastegatevalvelimitstheexhaustpressurethat
enterstheturbos;abypassvalveprotectstheenginefromoverboost.
ProvidingasecureconnectionbetweentheheadercollectorflangeandexhaustpipeisaV-bandclamp.Thistypeofclamppositively
engagesthematedflanges,eliminatingthepotentialforpipeslipping.Thesensorinthecollectorisanexhaustgastemperature
(EGT)sensor,whiletheothersensorisanair/fuelratio(oxygen)sensor.Thisprovidestuninginformationtomonitorbothexhaustgas
temperatureandtheengine’sair/fuelratio.OnanEFIsystemthisinformationaidstheECMinregulatingbothfuelandspark
delivery.
Topreventpossibledisconnectionofajoiningsleevebetweenair-chargepipesonaturbochargedsystem,afail-safemechanical
systemshouldbeinstalled.ShownhereisVibratech’sBoostBracethatfeaturestwoball-tippedstands,whichareweldedtothepipes.
Attachingthetwostandsisaquick-releasebrace.Analternativeistoweldtabstoeachpipeandconnectthetabswithalongboltand
lockingnut.
Regardlessofdesign,wheneverair-chargetubesareconnectedwithsiliconesleeves,aback-upmechanicalsecuringsystemisa
goodideatopreventthepipesfromslippingoutofthesleevesunderboost,especiallyiftheturbosystemisexpectedtoproduce
seriousboostpressure.(PhotoCourtesyVibratech)
Air-chargepipingmustfeaturesmoothbends.Avoidshort90-degreebendswherepossible.Pipefittinginthisapplicationwas
relativelyeasy,asthebuildersimplypurchasedpre-benttubinginavarietyofangles.Eachsectionwastrimmedtotheneededlength
andconnectedwithhigh-pressuresleeveconnectionsandclamps.
Theownerofthiscarhadthealuminumair-chargepipingfabricatedbeforethecarwasdeliveredtoStainlessWorks.Theair-charge
pipeswerefabricatedbyweldingsectionstocreatethefinishedproduct.Turningtoacustompipebender,suchasStainlessWorks,
couldhaveresultedinCNC-formedair-chargepipeswithnoweldedsegments.
CHAPTER7
EXHAUSTSYSTEMMATH
Choosing exhaust pipe diameter and header primary-tube diameter and length can be confusing.
Generally,peopletendtocopywhatothershavedone,oftenchoosingtoo-largetubeandpipediameters
with the belief that bigger is always better. Not so. Choosing diameters that are too large can rob the
engine of torque and horsepower at lower engine speeds; diameters that are too small can choke the
engine.
Theengineneedstobreathe,buttheexhaustpathneedstobematchedtotheengine’soperatingspeed
(the RPM at which you want maximum torque and/or horsepower) and the engine’s VE. Selecting the
proper header primary tube diameter and length affect where maximum torque and/or horsepower are
achieved in a specific powerband range. Matching header tube size to your engine is a major factor in
maximizingpoweratlow-end,mid-range,ortop-end.
Sizinganexhaustsystemdoesn’tneedtobeaguessinggame.Afewformulaswillaidyouindeterminingtheoptimumpipediameter
andlength.
FindingSweptVolume
Asabasisforseveralcalculations,youfirstneedtoknowhowtodeterminethesweptvolumeofone
cylinderandtotalenginedisplacement.
Eitherofthefollowingtwoformulascanbeusedtodeterminethesweptvolumeofeachcylinder:
SweptVolume=Pi×(bore÷2)×(bore÷2)×strokeSweptVolume=bore×bore×stroke×.7854
Where:
Pi(π)=3.14159
.7854=mathconstant
Asanexample,let’ssaythattheenginefeaturesaborediameterof4.030inchesandastrokeof4.000
inches.Usingthefirstformula:
3.14159×(4.030÷2)×(4.030÷2)×4.000
3.14159×2.015×2.015×4.000=51.022
Usingthesecondformula:
4.030×4.030×4.000×.7854=51.022
Theresultingfigureof51.022ciindicatesthedisplacementofasinglecylinder.Iftheenginefeatures
eightcylinders,simplymultiplyby8.Inthisexample,totalenginedisplacementis408ci.
ExhaustpipeissizedbasedonOD.Shownhereisa3-inchpipe.
Thisisanexampleofmeasuringpipewallthickness.Thispipefeatures.060-inchwallthickness.
TheIDofthis3-inchexhaustpipemeasures2.88inches.Thisistypicalforpipethatfeaturesabouta.060-inchwallthickness.
Measuringpipediametershouldbedonewithacaliperormicrometer,ratherthaneye-ballingwithatapemeasure.
CalculatingPrimaryTubeSize
Wave tuning is realistically the realm of high-level race engine builders, where variables such as
valveopeningtiming,connectingrodlength,andoverlapareconsideredinrelationtotubediameterand
length. For instance, peak horsepower is usually about 1,500 to 1,700 rpm above the engine’s torque
peak.ThisRPMspreadbetweentorqueandhorsepowerisinfluencedbytheengine’srod-to-strokeratio.
With stroke remaining the same, using a shorter connecting rod should broaden the separation between
peaktorqueandhorsepower.Alongerrodshouldlessenthisspread,withtorqueandhorsepowerpeak
RPMbeingclosertogether.
Ratherthandrivingyourselfnutstryingtounderstandthecomplexitiesinvolvedinwavetuning,you
canusethefollowingformulasasaguideintherightdirection.
CylinderVolume
Because primary tube diameter affects engine torque, you also want to consider the engine’s peaktorqueRPM.Tocalculatecylindervolume,youcanusethisformula:
CylinderVolume=displacement÷numberofcylinders
Forexample,a408-cienginewith8cylindershasavolumeof51ccpercylinder(408÷8).
Cross-SectionArea
If you already know (or anticipate) the engine’s peak-torque RPM, you can use this formula to
determinetheprimarytube’scross-sectionarea,whichisalsoknownastheprimarypipearea(PPA):
Cross-SectionArea=(cylindervolume×RPM)÷88,200
Where:
88,200=mathematicalconstant
RPM=here,RPMatpeaktorque
For example, a 408-ci engine at 4,200 rpm has a cross-section area of 2.428 ci [(51 × 4,200) ÷
88,200].
Youwanttoselectthebesttubediameterforyourapplication,withaspecificpeak-torqueRPMin
mind.Asanotherexample,let’susea350-cienginethatfeaturesasingle-cylinderdisplacementof43.75.
Usingyourplannedpeaktorqueat5,000rpm,thecross-sectionareais2.480[(5,000÷88,200)×43.75].
Youcanalsofindthecross-sectionareabyfirstdeterminingthepipe’sinsidediameter.Asyourecall,
tubesizesaredesignatedbyoutsidediameter,soyouneedtosubtractthetotalwallthicknessofthepipe
(wallthickness×2).Thisformulaforfindingthecross-sectionareais:
Cross-SectionArea=πR2
Where:
π=3.14159
R=thetube’sinsideradius,whichishalfoftheID
If a tube measures 1.75 inches in outside diameter and features a wall thickness of .040 inch, the
insidediameteris1.67inch.Theradiusishalfofthis,.835inch.Sousingtheformulaabove,thecrosssectionareais2.19039(3.14159×.8352=3.14159×.697225.ThisisabitsmallfortheintendedpeaktorqueRPM.
If you bump up to a primary tube OD of 1.875 inches with a .040-inch wall thickness, the crosssectionareaincreasesto2.530(3.14159×.89752=3.14159×.8055).
Giventhatthetheoreticalgoalwastoobtainacross-sectionareaof2.480,youcancompromiseby
choosingthe1.875-inchprimarytubesize,whichtheoreticallyallowspeakRPMtooccuraround5,100
rpm,whichisprettyclosetotheinitialtarget.
Youcanfurthertailorwherethepeaktorqueoccursbychangingthelengthoftheprimarytubes.By
lengthening the tubes, you emphasize torque below the 5,100-rpm range. If you go with shorter tubes,
torqueisemphasizedathigherRPM.
InsideDiameter
Once the cross-section area has been determined, you can then calculate the primary tube inside
diameterusingthisformula:
Continuingtheexampleabove,yougetaprimarytubeIDof1.98inches[√2.428×1.273)].
Considering commonly available tube diameters, this indicates a choice of a tube with a 1.5-inch
insidediameter.
Dependingonthetubewallthickness,youcanalsocalculateIDbasedonthetube’sODanditswall
thickness.Forinstance,ifyoupurchaseasetofheaderslabeledashaving1.75-inchprimaries,thismeans
thattheODis1.75inches.
The industry standard uses OD to refer to tube diameter and ID to refer to pipe diameter. Header
manufacturerstypicallyrefertoODwhenidentifyingprimarytubediameters.
Forexample,headerslabeledashaving1.5-inchprimariesactuallyhaveanIDofaround1.42to1.37
inch, depending on wall thickness of the material. If you buy headers labeled as having 1.75-inch
primaries, the inside tube diameter is slightly smaller, but that doesn’t mean that you’re being cheated.
TubingissimplysizedaccordingtoitsOD.
OutsideDiameter
HereareafewexamplesofprimarytubeOD(assuminga.040-inchwallthickness)with5,000-peak
torqueasthetarget.
ExhaustPipeDiameter
Youcangothroughthetroubletocalculateexhaustpipediameterbasedonyourspecificneeds,
butageneralruleistomatchthepipediameterbasedonenginedisplacementandanticipatedengine
horsepower.Thefollowingchartisaguidelineforselectingexhaustpipediameter.
Notethatpipediametersalwaysrefertotheoutsidediameter.
Here’sanotherformulaforcalculatinginsidediameter:
InsideDiameter=OD–(2×wallthickness)
Forexample,ifthetubeODmeasures1.75inchesandthewallthicknessis.047inch(18gage),the
IDis1.656inch[1.75–(2×.047)=1.75–.094].
Here’sanotherexample:IfthetubeODmeasures1.75inchesandwallthicknessis.0625(16gauge),
theIDis1.625inch[1.75–(2×.0625)=1.75–.125].
PipeCFM
Aruleofthumbistodetermineengineintakevolumeandthenapproximatelymatchthisvolumeforthe
exhaust.Tocalculateityoucanusethisformula:
ExhaustVolume=(RPM×.001)×displacement÷2
For instance, if your target is to obtain optimum performance at, say, 4,000 rpm, and the engine
displacementis350ci,theformulaworksoutto700cfm[(4,000×.001)×350÷2].
Theexhaustpipe(s)totalCFMshouldbeinthesamerange.
PipeCFM
Thefiguresinthefollowingchartrepresentestimatesbecauseadditionalvariablescanincludepipe
gauge(wallthickness)andpipethermalexpansion.Thehorsepowerestimatesforsingle-anddualpipesystemsrepresentasuggestedmaximumdiameter.
ExhaustWaves
Moststreetvehicleownerswhointendtoinstallasetofexhaustheadersforimprovedperformance
aremoreconcernedwithinstallationandfitratherthanthenuancesinvolvedintuningforpeakpowerand
torque. Generally speaking, smaller primary tube diameters and longer primary tubes with collectors
providebetterlow-endtorque,whichisbestsuitedforthestreet.However,compromisesarecommonly
madesimplybecauseofavailablespaceandfitinthevehicle.Seriousracersaremoreinclinedto“tune”
theexhausttomaximizepowerandtorquetosuittheirengine’soperatingparametersonthetrack.
Exhaustsystemmathcanbedifficulttocomprehend.Iexplainherehowwavepulsesaffecttheexhaust
systeminunderstandableterms.
When the engine is running, pressure waves run in both directions through the primary tubes of the
headers.Theseincludecompressionwavesandexpansionwavesthattendtomovefasterthantheactual
exhaustgasparticles.Compressionwaves(orpulses)arepositivewavesthatpushexhaustparticlesfrom
the cylinder head, through the remainder of the system. Expansion waves are negative waves that push
gases in the opposite direction. The expansion wave is created at the end of the exhaust outlet, where
negative pressure reflects back up into the tube, which creates a pressure drop when it reaches the
combustion chamber. This can create a pressure differential (or vacuum) at the cylinder area. This
pressuredropthenallowstheatmospheric(orforced-induction)airtoincrease,packingmoreairandfuel
intothecylinder.
ExhaustPipeWallThickness
Inotherwords,acombinationofapositivepressurewaveandnegativepressurewavecanimprove
exhaustscavenging,resultinginanincreaseinpoweroutput.
As far as exhaust tuning is concerned, this pressure/expansion wave phenomenon is influenced by
engineRPM,primarytubediameter,andlength.Smallerdiameterprimarytubescreatemorepressureand
betterscavenging,butifsizedtoosmall,theprimarytubescancauseflowrestrictionandcanmakethe
engineworkhardertopumpthecharge.Largerdiametertubesresultinlowerpressuresthatcanbeless
efficientatscavenging.
Connectingrodlength(crank-to-rodratio)isyetanotheraspecttoconsiderintermsofhowclosein
theRPMrangethatpeaktorqueandpeakhorsepoweraregenerated.Withthesamecrankstroke,shorter
rodstendtowidentheRPMdifferencebetweenpeaktorqueandpeakpowerwhilelongerrodstendto
narrow this gap (moving peak torque and peak horsepower closer together in terms of engine speed).
Alteringrodlengthdoesn’tmakeadifferenceinpeakvalues,butitcanbroadenortightentheRPMrange
betweentorqueandpower.
Again,rememberthebasics:Smaller-diametertubescatertolow-endtorque,whilelarger-diameter
tubestendtofavorpeaktorqueatahigherRPMband.Intermsofprimarytubelength,shortertubesfavor
higherenginespeedswhilelongertubesfavorlowerenginespeeds.
Althoughallofthistheoryisinterestingtonote,therealityisthatmoststreetapplicationsbenefitfrom
smallerdiametersandlongertubes(consideringthevariableofpracticalfitmenttothevehicleathand).
Racersexperimentwithdiametersandlengthstosuitspecificenginerequirementsbasedonwherethey
needthemosttorqueandpoweratagivenRPM.
PrimaryTubeLength
Hereistheformulafordeterminingprimarytubelength:
PrimaryTubeLength=[850(360–EVO)÷RPM]–3
Where:
EVO=exhaustvalveopeningpoint(refertoyourcamcard)
Forexample,let’ssaythatyourengine’scamhasanexhaustvalveopeningpointof82degreesand
yourdesiredpeaktorquehitsatabout4,200rpm.Usingtheformula,thelengthis53.26inches([850(360
–82)÷4,200]–3).
In general, longer primary tubes move the powerband below the engine’s peak torque point and
shorterprimarytubestendtomovethetorquevaluetoabovethepeak-torqueRPM.
CollectorDiameterandLength
Acollector(wheretheprimarytubesterminateasagroup)ismostbeneficialatorbelowpeak-torque
RPM.Intheory,asyouaddcollectorvolume,youincreasetorque.Asyoureducecollectorvolume,you
slightly reduce torque. If you’re more concerned about operating the engine at peak horsepower, a
collectormaynotbeneeded.Ifyou’remoreconcernedaboutrunningtheengineatornearitspeak-torque
RPM,acollectormakessense.
Herearetheapplicableformulasforestimatingcollectordiameterandlength:
CollectorDiameter=1.9×primarytubediameter
CollectorLength=.5×primarytubelength
Where:
1.9=mathconstant
.5=mathconstant
Forexample,let’ssaythatyourprimarypipesare1.75inchesinoutsidediameter,andtheprimaries
are38inchesinlength.Usingtheformulasyougettheseresults:
CollectorDiameter=1.9×1.75=3.325inches
CollectorLength=.5×38=19inches
Ingeneral,acollectorwithasmallervolumeshouldbenefitmid-rangepower,whilealargervolume
collectorshouldbenefittop-endpower.
SOURCEGUIDE
AeroExhaust
13053S.MinutemanDr.
Draper,UT84020
801-301-0408
aeroexhaust.com
aFePower
252GraniteSt.
Corona,CA92879
888-901-7693
afepower.com
AmericanRacingHeaders
26ElmPl.
Amityville,NY11701
631-608-1986
americanracingheaders.com
ANSAExhaustTechnologies
300DixieTr.
Goldsboro,NC27530
800-841-9166
silverlineexhaust.com
ARP
1863EastmanAve.
Ventura,CA93003
800-826-3045
arp-bolts.com
BanksPower
546DugganAve.
Azusa,CA91702
800-601-8072
bankspower.com
BassaniXhaust
2900E.LaJollaSt.
Anaheim,CA92806
714-630-1821
bassani.com
BBKPerformance
27440BostikCt.
Temecula,CA92590
951-296-1771
bbkperformance.com
BillyBoatExhaust
23045N.15thAve.
Phoenix,AR85027
888-598-3688
bbexhaust.com
BorlaPerformanceIndustries
701ArcturusAve.
Oxnard,CA93033
805-986-8600
borla.com
Bosal
bosalna.com
BurnsStainless
1013W.18thSt.
CostaMesa,CA92627-4557
949-631-5120
burnsstainless.com
CalicoCoatings
5883BalsomRidgeRd.
Denver,NC28037
866-901-3717
calicocoatings.com
CherryBombPerformanceExhaust
2400MaremontPkwy.
Loudon,TN37774
615-221-7426
cherrybomb.com
CerakoteCeramicCoatings
70506thSt.
WhiteCity,OR97503
866-774-7628
cerakotehightemp.com
ClassicTube
80RotechDr.
Lancaster,NY14086
800-882-3711
classictube.com
CorsaPerformance
140BlazeIndustrialPkwy.
Berea,OH44017
800-486-0999
corsaperformance.com
DEI
604MooreRd.
AvonLake,OH44012
800-264-9472
designengineering.com
DougThorleyHeaders
803EastParkridgeAve.
Corona,CA92879
951-739-5900
dougthorleyheaders.com
Dr.GasPerformanceExhaust
1630W.2225S.
WoodsCross,UT84087
801-563-1111
drgas.com
Dynatech
977HyrockBlvd.
Boonville,IN47601
800-848-5850
dynatechheaders.com
DynomaxPerformanceExhaust
OneInternationalDr.
Monroe,MI48161
734-384-7806
dynomax.com
EasternCatalytic
2201CabotBlvd.West
Langhorne,PA19047
800-553-7199
easterncatalytic.com
ExtremeMufflers
2280ShastaWay,Ste.115
SimiValley,CA93065
805-584-2984
extrememufflers.com
Flowmaster
100StonyPointRd.,Ste.125
SantaRosa,CA95401
800-544-4761
flowmastermufflers.com
FlowTechExhaustSystems
1801RussellvilleRd.
BowlingGreen,KY42101
270-781-9741
flowtechheaders.com
GoodFabs
170-ARacewayDr.
Mooresville,NC28117
704-696-0077
goodfabs.com
HeartthrobExhaust
60819USHwy.12
Litchfield,MN55355
800-642-0105
heartthrobexhaust.com
HedmanHedders
12438PutnamSt.
Whittier,CA90602
562-921-0404
hedmanperformancegroup.com
HookerHeaders
1801RussellvilleRd.
BowlingGreen,KY42101
270-781-9741
hookerheaders.com
HyTechExhaust
12HammondDr.,Ste.203
Irvine,CA92618-1641
949-581-2181
hytechexhaust.com
Icengineworks
14704CrosscreekDr.
Austin,TX78737
512-858-2232
icengineworks.com
Jardine’sHeaderPalace
944EastASt.
Casper,WY82601
307-234-6499
jardinesheaderpalace.com
Jet-HotHighPerformanceCoatings
2611LaVistaDr.
Burlington,NC27215
800-432-3379
jet-hot.com
KooksCustomHeaders
141AdvantagePl.
Statesville,NC28677
866-586-5665
kookscustomheaders.com
MagnaflowExhaustProducts
22961ArroyoVista
RanchoSantaMargarita,CA92688
800-824-8664
magnaflow.com
NICIndustries
70506thSt.
WhiteCity,OR97503
866-774-7628
cerakotehightemp.com
PerformanceTubeBending
5462DiazSt.
Irwindale,CA91706
626-939-9000
bendtubing.com
PolymerDynamics
11211NeeshawDr.
Houston,TX77065
888-765-9396
polydyn.com
Pro-Fabrication
4328TripleCrownDr.
Concord,NC28027
704-795-7563
profabrication.com
PypesPerformanceExhaust
2705ClemensRd.,Bldg.105A
Hatfield,PA19440
800-421-3890
pypesexhaust.com
RCIHeaders&Components
7349MillikenAve.
RanchoCucamonga,CA91730
909-483-3893
rcimustangs.com
RCIHeaders&Components
211SouthHamptonDr.
Jupiter,FL33458
516-248-6793
racecomponentsinc.com
Red’sHeaders
31-410ReserveDr.,Ste.4
ThousandPalms,CA92276
760-343-2590
reds-headers.com
RemflexExhaustGaskets
121MapleLn.
Mineral,WA98355
866-641-7325
remflex.com
SchoenfeldHeaders
605S.40thSt.
VanBuren,AR72956
479-474-7529
schoenfeldheaders.com
SPD
11252SuncoDr.
RanchoCordova,CA95742
916-635-8108
spdexhaust.com
SpinTechMufflers
4768FelsparSt.
Riverside,CA92509
951-360-2474
spintechmufflers.com
SpiralTurboSpecialties
P.O.Box186
Seville,OH44273
330-321-1918
spiralturbobaffles.com
Stage8LockingFasteners
4318RedwoodHwy.,Unit200
SanRafael,CA94903
800-843-7836
stage8.com
StainlessHeadersMfg.
3130FiechtnerDr.S.,UnitA
Fargo,ND58103
701-356-9467
stainlessheaders.com
StainlessWorks
9899E.WashingtonSt.
ChagrinFalls,OH44023
440-543-6593
stainlessworks.net
StahlHeaders
1515Mt.RoseAve.
York,PA17403
855-846-6800
stahlheaders.com
SuperTrappIndustries
4540W.160thSt.
Cleveland,OH44135
216-265-8400
supertrapp.com
SupraSport
2515WhiteBearAve.,Ste.A8-118
Maplewood,MN55109
651-276-6299
suprasport.com
SwainTechCoatings
963NorthRd.
Scottsville,NY14546
585-889-2786
swaintech.com
TechLineCoatings
26844AdamsAve.
Murrieta,CA92562
972-775-6130
techlinecoatings.com
TotallyStainless
P.O.Box3249Gettysburg,PA17325
800-767-4781
totallystainless.com
TubularSolutions
7518StoneRd.
Medina,OH44256
330-723-7662
tubularsolutions.com
UltimateHeaders
682WestBagleyRd.,Unit12
Berea,OH44017
440-234-9600
ultimateheaders.com
Download PDF
Similar pages