Engine Dyno Guide

DeskTop Dynos Mini Guide1

Engine BuilderEngine Builders Guide s Guide TTooFilling & Emptying SimFilling & Emptying SimulationulationAnd High-PAnd High-Perferformance Designormance Design

MINI-GUIDEMINI-GUIDEDYNOSDYNOSCOMPLIMENTARYYDYNOSDeskTopDeskTopDeskTop

Motion Software, Inc.535 West Lambert RoadBuilding EBrea, California 92821

ISBN 1-888155-00-0PART No. M43

INCLUDES:Details Of All On-Screen Menu Choices/SelectionsMotions Filling & Emptying Simulation BasicsCylinderhead Flow And Discharge CoefficientsUnderstanding And Calculating Camshaft TimingRam Tuning And Pressure Wave DynamicsExpose Fallacies About Engine ComponentsComplete Glossary And Much, Much More!

2DeskTop Dynos Mini Guide

PLEASE READ THIS LICENSE CAREFULLY BEFOREBREAKING THE SEAL ON THE DISKETTE ENVELOPE ANDUSING THE SOFTWARE. BY BREAKING THE SEAL ONTHE DISKETTE ENVELOPE, YOU ARE AGREEING TO BEBOUND BY THE TERMS OF THIS LICENSE. IF YOU DONOT AGREE TO THE TERMS OF THIS LICENSE,PROMPTLY RETURN THE SOFTWARE PACKAGE, COM-PLETE, WITH THE SEAL ON THE DISKETTE ENVELOPEUNBROKEN, TO THE PLACE WHERE YOU OBTAINED ITAND YOUR MONEY WILL BE REFUNDED. IF THE PLACEOF PURCHASE WILL NOT REFUND YOUR MONEY, RE-TURN THE ENTIRE UNUSED SOFTWARE PACKAGE,ALONG WITH YOUR PURCHASE RECEIPT, TO MOTIONSOFTWARE, INC. AT THE ADDRESS AT THE END OF THISAGREEMENT. MOTION SOFTWARE, INC. WILL REFUNDYOUR PURCHASE PRICE WITHIN 60 DAYS. NO REFUNDSWILL BE GIVEN IF THE DISKETTE ENVELOPE HAS BEENOPENED.

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Motion Software, Inc.535 West Lambert, Bldg. EBrea, CA 92821714-255-2931; Fax 714-255-7956

1995, 1996 By Motion Software, Inc. All rights reserved. Motion Soft-ware, Inc., MS-DOS, DOS, Windows, and Windows95 are trademarks ofMicrosoft Corporation. IBM is a trademark of the International BusinessMachines Corp. DeskTop Dyno and DeskTop Dynos is a trademark of Mo-tion Software, Inc.

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M


MINI-GUIDEMINI-GUIDEDYNOSDYNOSDYNOSDeskTopDeskTopDeskTopCOMPLIMENTARYY

BY:LARRY ATHERTON


Copyright 1996 by Motion Software, Inc.,535 West Lambert, Bldg. E, Brea, CA 92821.All rights reserved. All text and photographsin this publication are the copyright prop-erty of Motion Software, Inc. It is unlawfulto reproduceor copy in any wayresell,or redistribute this information without theexpressed written permission of MotionSoftware, Inc. Printed in U.S.A.

The text, photographs,drawings, and otherartwork (hereafter referredto as information) con-tained in this publication issold without any warrantyas to its usability orperformance. In all cases,original manufacturersrecommendations, pro-cedures, and instructionssupersede and takeprecedence over descrip-tions herein. Specificcomponent design andmechanical proceduresand the qualifications ofindividual readersare

beyond the control of the publisher, therefore the publisher disclaims all liability,either expressed or implied, for use of the information in this publication. All risk forits use is entirely assumed by the purchaser/user. In no event shall Motion Software,Inc. be liable for any indirect, special, or consequential damages, including but notlimited to personal injury or any other damages, arising out of the use or misuse ofany information in this publication.This book in an independent publication. All trademarks are the registered propertyof the trademark holders.The publisher (Motion Software, Inc.) reserves the right to revise this publication orchange its content from time to time without obligation to notify any persons of suchrevisions or changes.

Motion Software, Inc., 535 West Lambert, Bldg. E, Brea, CA 92821

ISBN 1-888155-00-0PART No. M43

TECHNICAL CONSULTANT:CURTIS LEAVERTON

WRITTEN AND PRODUCED BY:LARRY ATHERTON

TECHNICAL DRAWINGS BY:JIM DENNEWILL

MINI-GUIDEMINI-GUIDEDYNOSDYNOSDYNOSDeskTopDeskTopDeskTopCOMPLIMENTARYY

Engine BuildersGuide To

Filling & EmptyingSimulation


SOFTWARE LICENSE .................................................. Inside Front Cover

INTRODUCTION.......................................................................................... 6

USER'S GUIDE TO THE FILLING-AND-EMPTYING SIMULATION ......... 8FILLING-AND-EMPTYING BY ANY OTHER NAME ........................... 8MOTION ENGINE SIMULATION BASICS ........................................... 9THE ON-SCREEN MENU CHOICES ................................................. 10THE BORE/STROKE MENU .............................................................. 11WHATS A SHORTBLOCK ................................................................ 11BORE, STROKE, & COMPRESSION RATIO ................................... 11BORE AND STROKE VS. FRICTION ................................................ 12FALLACY ONE: STROKE VS. PUMPING WORK ............................ 14FALLACY TWO: LONG VS. SHORT STROKE ................................ 18CYLINDERHEAD AND VALVE DIAMETER MENUS ....................... 20CYLINDERHEADS AND DISCHARGE COEFFICIENTS .................. 21RAM TUNING AND PRESSURE WAVES ......................................... 24SORTING OUT CYLINDERHEAD MENU CHOICES ........................ 28WHEN TO CHOOSE SMALLBLOCK OR BIG-BLOCK HEADS ...... 31VALVE DIAMETERS AND AUTO CALCULATE .............................. 33THE COMPRESSION RATIO MENU ................................................. 34COMPRESSION RATIO BASICS ...................................................... 35CHANGING COMPRESSION RATIO ................................................ 36WHY HIGHER C/R PRODUCES MORE POWER ............................. 37OTHER EFFECTS OF INCREASING C/R ......................................... 38COMPRESSION RATIO ASSUMPTIONS.......................................... 38THE INDUCTION MENU .................................................................... 39AIRFLOW SELECTION ...................................................................... 39AIRFLOW MENU ASSUMPTIONS .................................................... 41INDUCTION MANIFOLD BASICS...................................................... 42MANIFOLD SELECTION ADVICE ..................................................... 44THE EXHAUST MENU ....................................................................... 52WAVE DYNAMICS IN THE EXHAUST SYSTEM .............................. 53EXHAUST MENU SELECTIONS ....................................................... 56THE CAMSHAFT MENU .................................................................... 61CAM BASICS ...................................................................................... 62VISUALIZING & CALCULATING VALVE EVENTS .......................... 63HOW VALVE EVENTS AFFECT POWER ......................................... 68CAMSHAFT MENULIFTER CHOICES ........................................... 75CAMSHAFT MENUSPECIFIC CAMSHAFTS................................. 78ENTERING 0.050-INCH & SEAT-TO-SEAT TIMING ........................ 81CAMSHAFT ADVANCE AND RETARD ............................................ 84CALCULATING VALVE EVENTS ...................................................... 86

APPENDIXA: COMMON QUESTIONS AND ANSWERS .................... 88SOFTWARE SUPPORT FAX PAGE.................................................. 97

APPENDIXB: GLOSSARY .................................................................... 98APPENDIXC: BIBLIOGRAPHY........................................................... 108OTHER MOTION SOFTWARE PRODUCTS .......................................... 110

CONTENTS


INTRODUCTIONE n g i n e B u i l d e r ' s G u i d e T o S i m u l a t i o n & D e s i g n

his is not a typical hot roddingbook. Yet it is written specifi-cally for you: The amateur orprofessional automotive enthu-

ing engine simulations is simply anotherapproach to understanding IC engines.

But engine simulations have beenaround for years, and there are manytalented engineers, developers, and uni-versity professors that understand thephysics and math that make the IC en-gine tick. Whats very rare is the indi-vidual who has this knowledge and un-derstands racing engines and the needsof the high-performance engine builder.That individual is Curtis Leaverton, thedeveloper of Motion Softwares enginesimulation and my associate in the de-velopment of these books. This MiniGuide is, to a great extent, a transcrip-tion of a small portion of his vast knowl-edge, with my contribution being one oforganizer and presenter.

Despite the inauspicious beginningsand its odd evolution, these books havebecome true hot rodders engine guides.Engine booksthe better ones, atleastare a compilation of the learnedexperiences of the author/racer. Theydescribe what components and proce-dures are known to work, and they con-jecture about what is less understood orwhat seems to defy logic. This outside-in approach is completely understand-able. It is a direct result of the mostcommon method of engine developmentused by engine builders and racers: Trial-and-error. The DeskTop Dynos booksfollow a new path. They describe enginefunction from the inside-out. You wontfind a list of bolt-on parts that work ononly a few engines, rather youll discoverinformation that will help you understand

Tsiast interested in both engines and com-puter simulations. This book, and its big-brother the complete DeskTop Dynosbook, may reveal more to you about high-performance and racing engines thanyouve discovered in all the other en-thusiast books and magazines youveread. I know this to be true, because Ihave been an engine and book enthusi-ast all my life, and the nearly two-yearprocess of writing these books taught memore about high-performance enginesthan my thirty years of engine building,tinkering, and racing.

Most books, like many products aimedat the consumer, begin with a publisher(a manufacturer) discovering a need inthe marketplace. Every once in a while,though, a book is published that falls out-side of the typical form. It is born from aunique collaborationthe result of fortu-itous eventsthat could never have beenplanned. This is one of those books.

Two years ago, the DeskTop Dynosbook and Mini Guide were simply to bea comprehensive guide for Motion Soft-ware users of Filling-And-Emptying en-gine simulation software. What the au-thor failed to realize at first was that thereis very little difference between under-standing engine simulations and under-standing engines themselves. In retro-spect, its obvious: If a simulation is agood one, it must mimic the physics atwork inside the IC engine. Understand-


WHY engines need parts of a specificdesign to optimize horsepower.

Admittedly, these books only scratchthe surface of the vast and complexfields of engine-pressure analysis, ther-modynamics, and gas dynamics, butthats actually what makes these bookswork. There are many other publica-tions that delve deeply into these sub-jects, the Bibliography on page 108 lists

This book simply would not havebeen possible without the technical as-sistance of Curtis Leaverton. Over the lastfew years I have spent many enjoyabledays with Curtis discussing engines andsimulation science. He is one of only ahandful of people Ive encountered in mylife who can explain complicated subjectswith such enthusiasm that they not onlybecome clear to the listener, but the sheerjoy of being guided toward the under-standing becomes an indelible memory!In short, Curtis is a terrific teacher (and Ilike to think, my friend). Occasionally, heconducts seminars throughout the coun-

Acknowledgments

try, and I encourage every reader of this book to avail themselves of the unique opportunity tobe taught by one of the masters.

I also wish to thank the many manufacturers that provided photos and information for thisbook, especially Harold Bettes of SuperFlow Corporation. Harold, a lot like Curtis Leaverton,has no shortage of enthusiasm and excitement about engines and performance. He has alsobeen extremely generous with his valuable time, and I have learned a great deal from themany hours of conversation we have had (over some great Chinese dinners!). Thanks Harold.

A heartfelt thanks to all the manufacturers that helped with this publication:Borla Performance, Bonnie SadkinCarroll Supercharging, Sheryl DavisChilds & Albert, Raymond AkerlyEdelbrock, Tom DuferHKS Performance, Howard LimHooker Headers, Jason Bruce

Intel Corporation, Rachel StewartMoroso Performance, Barbara MillerSnap-on Tools, David Heide, Tami ValeriSuperFlow Corporation, Harold BettesTPS, Bob HallWeiand, Jim Davis

Another special thanks to Jim Dennewill for the many fine drawings throughout this book.Jims talents and patience are both rare and boundless. Thank you for all your help and yourfriendship.

Finally, a sincere thanks to Paul Hammer. I cant imagine what this book project wouldhave been like without Pauls dedicated help in putting out the fires around me (and I canimagine some pretty awful stuff!).

a few of these tomes, and many are notespecially approachable.

Hopefully, what you find here will bethe right mix of theory and practical ap-plication. If you are able to gain a deeperunderstanding of the IC engine fromthese pages and from using the Desk-Top Dyno software, I will consider thisproject a success.

Larry Atherton


Filling & EmptyingBy Any Other Name

E n g i n e B u i l d e r ' s G u i d e T o S i m u l a t i o n & D e s i g n

USER'S GUIDE

Motion Software recently releasedengine simulation software forIBM-compatible PC computers.These simulation programs are

based, as of the writing of this book, onthe Filling-And-Emptying method of full-cycle analysis as applied to the 4-strokeIC (internal combustion) engine. These soft-ware packages are available under vari-ous names, including the DeskTop Dyno(available from Mr. Gasket Corporation andfrom Motion Software, Inc. and its distribu-tors) the GM PC Dyno Simulator (availablefrom GM Performance Parts) and others.Each of these packages has individualfunctional differences, but all of these prod-ucts use the same simulation methodologyand many of the menu choices are com-parable. Because of these similarities, theinformation in this guidebook appliesequally to version 2.0, 2.5, and later simu-lation software releases. Minor functionaldifferences will be discussed as required.Most screen illustrations in this book de-

MotionSoftware,Inc., hasrecentlyreleasedenginesimulationsoftware forIBM-compat-ible PCcomputers.Theseprograms are

pict the latest version of the software (ver-sion 2.5.7in the final stages of develop-ment when this book was published). Thisguide is intended to help you obtain aclearer understanding of overall programfunction, what is covered by each menugroup, the implications of individual choiceswithin the menus, and how to interpretsimulation results. In addition, we will illus-trate what can and cannot be modeled andwhy, the drawbacks of the current simula-

based on theFilling-And-Emptying methodof full-cycleanalysis. They areavailable undervarious names,including theDeskTop Dyno, theGM PC DynoSimulator andothers.


tion software, and what advances may bepossible in the near future.

Motion Engine Simulation Basics

Motion engine simulations have beenoptimized for rapid what if testing. En-gine component parts are grouped inmenus along the top of the screen. One ormore selections from each menu groupeffectively builds a test engine (called apaper engine in simulation parlance).Then, simply pressing RUN begins a full-cycle analysis of cylinder pressures. Thesimulation plots a graphic, on-screen rep-resentation of horsepower and torque foreasy visual analysis. It is a simple proce-dure to save the test, select a new part orcomponent dimension from one of themenus, and rerun the simulation to per-form back-to-back testing. The changes inpower and torque that would occur if areal engine were built and tested with thecomponents at hand are clearly displayedin the graphic curves. Remarkably, thisentire process often takes less than oneminute on most computer systems.

Ease of use has always been an impor-tant design element. Without it, many ofthe tens of thousands of automotive en-thusiasts that have successfully assembledand tested engines would not have beenable to use our simulation programs. Imag-ine if one of the necessary inputs re-quested: Enter the intake port flow at each0.010-inch of the valve lift up to the maxi-

Motions Filling AndEmptying simulationoffers an inexpensive andrapid way predict powerand torque to within 5%of actual dyno figures.The various choices inthe Bore/Stroke menu areshown here (version2.5.7in developmentwhen this guidebook waspublishedincludesexpanded menu choicesand several other en-hancements, including aCam Math Calculator).

mum valve lift. Yet this information isabsolutely necessary to perform a true en-gine simulation. To make this softwaretechnology available to as wide an audi-ence as possible, the information requestedby the program is straightforward and gen-erally known by most enthusiasts. Theprogram uses this basic information toderive flow curves, cam profiles, frictionalmodels, induction characteristics, and othertechnical details. While the widely under-stood terms in the menus have made theseprograms accessible, their less-than-exactwording has left some experienced usersscratching their heads. Engine builders andother advanced users are often very awareof the internal complexities of the ICengine and may not realize at first glancehow Motions simulations handle many ofthese important considerations.

Furthermore, some performance enthu-siasts are disappointed when they realizethat engine simulations do not include someof the components they were expecting totest. A short list of these might include:block and head metallurgy, piston typesand dome shapes, head-gasket thickness,combustion-chamber shapes, oils, oil-pandesigns, ignition timing, bolt torque loads,and many more. Modeling many of theseelements would require very complex tech-niques (with concomitant complex inputsfrom the user, not to mention extend cal-culation times) and would reveal only rela-tively small power differences. These limi-tations are discussed throughout this guide,


but a good example of complexity vs. prac-ticality can be found in oil pan testing. Inorder to simulate the conditions inside anoil pan, here are a few of the inputs thatthe user would have to address: dimen-sions of pan and lower crank/block con-tours, position of oil pump, positions andsizes of baffles, trays and screens, oil vis-cosity and temperature, level of oil in pan,acceleration and directional vectors (andhow these vectors change over time), andmore. Simply gathering together theneeded information would be quite aproject!

While it would be wonderful if an inex-pensive computer program could simplyand quickly zero-in on the optimum combi-nation of all components for any intendedapplication, that time has not yet come.However, most of the engine componentsthat play a major role in power productionare modeled in Motions Filling-And-Emp-tying simulations. Cam timing, compressionratio, valve size, cylinderhead configura-tion, bore and stroke, induction flow, andmanifold type are just some of the mod-eled elements that have major effects onengine power.

The simulation discussed in this guideis easy to use and provides a remarkablelevel of predictive power. The designershave purposely avoided complex areas thatwould either make data entry difficult orgreatly extend computational times. If youhave a tendency to dismiss these simu-lations because they seem to consist only

The simulation de-scribed in this guide is

easy to use and pro-vides a remarkable level

of predictive power. Ifyou have a tendency to

dismiss this simula-tion because it seems to

consist only of simplepull-down menu choices,

we encourage you totake a second look. Webelieve this software, atunder $50, offers terrific

value!

of simple pull-down menu choices, weencourage you to take a second look.Motions Filling And Emptying simulationoffers an inexpensive and rapid way toselect component combinations that pro-duce power and torque curves often within5% of optimum for applications that liewithin the range of the simulation. We be-lieve thats quite an accomplishment for asoftware program you can load in your PCfor under $50!

That brings us to the main purpose ofthis guidebook. The information presentedhere revolves around detailed descriptionsof every item listed in the on-screen com-ponent menus. Youll discover the assump-tions made by Motion programmers withrespect to each of the possible choices andcombinations. Within sections that discusseach menu category, youll also find sub-stantial background information that can behelpful for both your simulated and real-world engine building projects. This infor-mation was compiled from the feedback ofthousands of users, hundreds of betatesters, and countless hours of testing andexploration. We are confident that what youfind here will make using Motion enginesimulation software easier and your engineanalysis more productive.

THE ON-SCREEN MENU CHOICES:

After starting Motions engine simulation,the user can follow two paths of enginetesting: To recall a previous test (using the


UTILITY menu) or to build an engine fromscratch. Assuming the latter selection, acommon choice is to start with the leftmostBore/Stroke menu then work, menu-by-menu, from left to right. While there are norestrictions dictating the order in whichmenus must be opened, the upcomingsections in this guide follow the naturalleft-to-right progression taken by most en-gine builders.

THE BORE/STROKE MENU

The Bore/Stroke menu is located onthe left end of the menu bar. By openingthis menu, you are presented with a vari-ety of pre-loaded engine configurations.If any one of these choices is selected, theappropriate bore, stroke, and number ofcylinders will be loaded in theSHORTBLOCK section of the on-screenComponent Selection Box. In addition toselecting an existing engine configuration,you can scroll to the bottom of the Bore/Stroke menu and choose the Other...option. This closes the menu and positionsthe cursor in the Component Selection Box,permitting direct entry of bore, stroke (ininches to three decimal places) and num-ber of cylinders. At each data input posi-tion, a range of acceptable values is dis-played at the bottom of the screen. As withall numerical input, only values within therange limits will be accepted by the pro-gram.

Whats A Shortblock

When a particular engine combinationis selected from the Bore/Stroke menu, thebore, stroke, and the number of cylindersare loaded into the on-screen ComponentSelection Box. These values are subse-quently used in the simulation process ofpredicting horsepower and torque. TheBore/Stroke menu choices should be con-sidered a handy list of common enginecylinder-bore and crankshaft-stroke values,not a description of engine configurations(e.g., V8, V6, straight 6, V4, etc.), materialcomposition (aluminum vs. cast iron), thetype of cylinderheads (hemi vs. wedge) orany other specific engine characteristics.The Bore/Stroke menu only loads bore,stroke, and the number of cylinders intothe program database.

Bore, Stroke, & Compression Ratio

After making a selection from the Bore/Stroke menu, or when the individual bore,stroke, and number of cylinders have beenentered manually, the swept cylinder vol-ume (in cubic centimeters) and the totalengine displacement (in cubic inches) willbe calculated and displayed in the on-screen Component Selection Box. Theswept cylinder volume measures the vol-ume displaced by the movement of a singlepiston from TDC (top dead center) to BDC(bottom dead center). This full-strokevolume is one of the two essential elements

Choices loaded and calculatedafter Bore/Stroke Menu selection

When a selection ismade from the Bore/Stroke menu, the bore,stroke, and the numberof cylinders are loadedinto the on-screenComponent SelectionBox. In addition, theswept cylinder volume(in cubic centimeters)and the total enginedisplacement (in cubicinches) will be calculatedand displayed.


required in calculating compression ratio.Well discuss compression ratio in moredetail later in this guide, but for now itshelpful to know that compression ratio isdetermined with the following formula:

Compression Ratio =

Swept Cyl Vol + Combustion Space VolCombustion Space Vol

In other words, the total volume that existsin the cylinder when the piston is locatedat BDC (this volume includes the SweptVolume of the piston and the CombustionSpace Volume) is divided by the volumethat exists when the piston is positioned atTop Dead Center. For the time being, itsimportant to keep in mind that your selec-tions of bore and stroke dimensions greatlyaffect compression ratio. When the stroke,and to a lessor degree the bore, is in-creased while maintaining a fixed combus-tion-space volume, the compression ratiowill rapidly increase. And, as is the case inMotions simulation software, if the com-pression ratio is held constantbecause itis a menu selection and, therefore, fixedby the userthe combustion space vol-ume must increase to maintain the desiredcompression ratio. This may seem moreunderstandable when you consider that ifthe combustion-space volume did not in-crease, a larger swept cylinder volume (dueto the increase in engine displacement)would be compressed into the same finalcombustion space, resulting in an increasein compression ratio.

One example of how bore and strokecan have a significant affect on compres-sion ratio is demonstrated in a destrokedracing engine. Engine designers have re-alized for many years that shorter-strokeengines waste less power on pumping-work(more on this later), leaving more horse-power available for rotating the crankshaft.Furthermore, if engine displacement is heldconstant (by a concomitant increase in boresize), a larger cylinder diameter will ac-commodate larger valves, and increasingvalve size is one of the most effective waysto improve breathing and horsepower. Allof these changes taken together can pro-

duce considerable power increases, pro-viding the compression ratio is maintainedor increased. Unfortunately, it is difficult tomaintain a high compression ratio in shortstroke engines because: 1) Overall sweptvolume is often reduced, 2) combustionspace volume is proportionally larger, and3) short stroke engines move the pistonaway from the combustion chamber moreslowly requiring increased valve-pocketdepth to maintain adequate piston-to-valveclearance.

It is easy to use your PC to simulate a1-inch stroke, 5-inch bore, 8-cylinder en-gine of 157 cubic inches with an 11:1 com-pression ratio that produces over 500hp at8000rpm (with the power continuing toclimb rapidly!). Unfortunately, it may benearly impossible to build this engine withmuch more than 9 or 10:1 compressionbecause the volume needed for the com-bustion chamber, valve pockets, and headgasket on a 5-inch bore is large comparedto the swept volume produced by the short1-inch stroke. If we installed typical small-block race heads on this short-stroke con-figuration and great care was used inmachining the valve pockets, a total com-bustion space of about 55ccs might bepossible. Unfortunately, this would onlyproduce 7:1 compression. The entire com-bustion-space volume would have to beless than 32 cubic centimeters to generatea compression ratio of 11:1 or higher!

Bore And Stroke Vs. Friction

You know that selecting stroke lengthwill determine, in part, the cubic-inch dis-placement of the engine. And from theprevious section you now realize that ob-taining high compression ratios with shorterstroke engines can be quite difficult. Whatyou may not realize is that setting the strokelength determines, to great extent, theamount of power lost to friction. The strokefixes the length of the crank arm, and thatdetermines how fast the piston and ringpackages rub against the cylinderwalls atany given engine speed. And heres an-other rub (pardon the pun): 70% to 80% ofall IC engine frictional losses are due topiston and ring-package drag against the


cylinderwall! If a 1-inch stroke engine run-ning at 8000rpm looses 10 horsepower dueto cylinderwall friction, the same engine willloose 25hp with a 2-inch stroke, 40hp witha 3-inch stroke, 70hp with a 4-inch stroke,90hp with a 5-inch stroke, and 120hp witha 6-inch stroke at the same 8000rpm crankspeed. That means this 6-inch stroke en-gine consumes 110 more horsepower thanits 1-inch stroke counterpart just to drivethe pistons and rings up and down theirbores!

Try this simulation on your PC to dem-onstrate frictional losses. Build a 603.2cubic-inch engine, well call it a very Long-Arm smallblock, with the following compo-nents:Bore: 4.000 inchesStroke: 6.000 inchesCylinderheads: Smallblock/Stock

Ports And ValvesValve Diameters: 2.02 Intake; 1.60

Exhaust (Valvediameters must bemanually selectedto disable theAuto Calculatefunction. Thiskeeps the valvesize fixed whenthe bore size ischangedrequiredfor the next test.)

Compression Ratio: 10.0:1Induction Flow: 780 CFMIntake Manifold: Dual Plane

Exhaust System: Small-TubeHeaders withOpen Exhaust

Camshaft: Stock Street/Economy

Lifters: HydraulicThis combination produces the power andtorque curves shown in the accompanyingchart (Long-Arm). Note that the powerdrops to zero above 5500rpm. Translated,this means that at 5500rpm the engine isusing all the power it produces to over-come internal friction!

Now build the same displacement en-gine, but change the bore and stroke com-bination to:Bore: 5.657 inchesStroke: 3.000 inchesThis Short-Arm configuration displaces thesame 603.2 cubic inches, but it producesover 100hp at 5500rpm and maintains anon-zero power level until about 6500rpm.Where was the extra horsepower hiding?The majority is freed by lower pistonspeeds and reduced bore-wall friction fromthe 3-inch shorter stroke.

Motion simulations use an empiricalequation to calculate frictional mean effec-tive pressure (Fmep):

Fmep = (12.964 + (Stroke x (RPM /6))) x (0.0030476 + (Stroke x (RPM /6) x 0.00000065476))

Once the Fmep is known, the horsepowerconsumed by friction can be calculated with

This Short-Armconfiguration (solidlines) displaces thesame 603.2 cubic inchesas the Long-Arm test(dotted lines), but itproduces nearly 100hpmore at 5500rpm andmaintains a nonzeropower level until about6500rpm. Where was theextra horsepowerhiding? The majority isfreed by lower pistonspeeds and reducedbore-wall friction.


the following equation:

Friction HP =(Fmep x CID x RPM) / 792,000

Applying these equations to the two previ-ous test engines reveals the following:

Power consumed by friction with the 6-inch stroke engine @ 5000rpm: 120.7hpPower consumed by friction with the 3-inch stroke engine @ 5000rpm: 44.8hpSo far weve discussed how stroke

length affects displacement, compressionratio, and frictional losses within the en-gine. In addition to these effects, bottom-end configuration can have a measurableeffect on power consumed by the pro-cesses of drawing fresh fuel/air mixture intothe engine and forcing burned exhaustgasses from the cylinders (a process calledpumping, consuming what is termed pump-ing work). However, the IC engine is aremarkably complex mechanism that cansometimes fool nearly anyone into believ-ing that they have discovered subtle, se-cret power sources. The logic behind thediscovery may seem to make perfectsense, but eventually turn out wrong ormisleading. This is the case with the beliefpurported by some experts that: 1)Shorter-stroke/larger-bore engine configu-rations have the potential to be more effi-

cient high-speed air pumps, and 2) Shorterstrokes are the key to producing high-speedhorsepower. Lets take an in-depth look athow bore, stroke, and rod length affectpiston speed and pumping work.

Fallacy One:Stroke Length Vs. Pumping Work

The concept of pumping work is moreeasily understood when you remember thatthe IC engine is, at its heart, an air pump.Air and fuel are drawn into the cylindersduring the intake cycle, and burned exhaustgasses are pumped from the cylindersduring the exhaust cycle. The power re-quired to perform these functions is calledpumping work and is another source oflost horsepower.

The intake stroke begins with the pistonaccelerating from a stop at TDC. As itbegins to move down the bore, the sweptvolume within the cylinder increases, cre-ating a lower pressure. This drop in pres-sure causes an inrush of higher pressureair and fuel from the intake system to com-pensate or fill the lower pressure in thecylinder. As the piston continues to accel-erate down the bore, the speed of the in-duced charge also increases, until at about70-degrees after top dead center, the pis-ton reaches maximum velocity. At this point,

TDC

STROKE

BDCBDC

Piston Speed And Frictional LossesIncrease As Stroke Increases

The stroke length deter-mines, to great extent, theamount of power lost tofriction. 70% to 80% of allIC engine frictional lossesare due to piston and ring-package drag against thecylinderwall! A longerstroke increases the lengthof the crank arm, and thatincreases the speed ofpiston and ring contactwith the cylinderwall.


PISTONTRAVEL

PISTONTRAVEL

1/2STROKE

1/2STROKE

BOREDIAMETER

Equal Displacement EnginesSweep Out Equal Cylinder Volumes

During Each Crank Degree

Small BoreLong Stroke

Large BoreShort Stroke

the greatest pressure drop exists within thecylinder, drawing outside charge with thegreatest force. It is this force that is thekey element in understanding pumpingwork. The differences in pressures thatcause the inrush of air and fuel are not afreebie from nature; they consume work.The greater the difference in pressurebetween the intake manifold and the cylin-der, the more pumping work will be re-quired to move the piston from TDC toBDC. Any increase in pumping work di-rectly reduces the power available at thecrankshaft.

Lets consider various techniques thatcan reduce pumping work on the intakestroke and potentially increase power out-put. First of all, pumping work can be sub-stantially reduced by lowering cylinder vol-ume. A smaller cylinder takes less work tofill it. Unfortunately, reducing cylinder vol-ume also reduces the amount of air andfuel that can be burned on the powerstroke, lowering power output more thanany possible gains from a reduction inpumping work. So if power is the goal, re-ducing pumping work must be accom-

While its true thatshorter-stroke engines

consume less frictionalhorsepower, it is nottrue (as believed bysome) that shorter-

stroke engines, withtheir slower piston

speeds, generate lesspumping work. For equal

displacements, equalcylinder volumes are

swept out at eachincrement of piston

travel from BDC to TDC.Short-stroke/large-bore

engines pump just asmuch air as long-stroke/

small-bore engines.

plished without decreasing the swept vol-ume of the cylinder.

The question is how can we fill the samespace using less work? This can be trans-lated to read: How can we move the sameor more air/fuel volume into the cylinderwhile inducing less pressure drop betweenthe cylinder and the intake system? Solu-tions to this problem are widely used byracers and include larger intake valves,freer-flowing ports and manifolds, largercarburetors or injector systems, and tuned-length intake runners. All these techniquesallow the same or more air/fuel mixture toflow into the cylinder while consuming lesspumping work.

While its true that shorter-stroke enginesconsume less frictional horsepower to drivethe pistons up and down the cylinders (asdescribed in the last section), it may alsoseem logical that a shorter stroke engine,with its slower piston speeds, can reducepumping work. Heres how this concept isoften described and how the underlyingthread of truth often remains undiscovered:First, picture a 6-inch stroke, small-boreengine. When the piston begins to move


away from TDC, the long stroke sends itquickly towards a maximum velocity (maxi-mum volume change) that occurs about70 degrees ATDC. At this point, the longstroke has accelerated the piston to veryhigh speeds that generate a strong pres-sure drop in the cylinder. It is claimed thatthis low pressure generates flow velocitiesso high that the cylinderheads, valves, andthe induction system become a significantrestriction to flow. The rapid buildup in flowand high peak flow rates lower pumpingefficiency, and the overall picture getsworse as engine speed and piston speedincrease. Now consider a 1-inch stroke,large-bore engine. Because the stroke ismuch shorter, peak piston speedandtherefore peak pressure dropare believedto be considerably lower. The reasoningcontinues that induction flow never reachesas high a rate but is spread out more evenlyacross the entire intake cycle. The shorterstroke is believed to allow the inductionsystem to more efficiently fill the cylinder,consuming less pumping work. As enginespeed increases, this improved efficiencylets the engine produce higher power lev-els at greater speeds than a longer-strokeengine.

The accompanying Chart-A (drawn froma simulation based on the underlying phys-ics) illustrates the actual differences in in-cremental flow between an engine with a

long stroke and the same displacementengine with a destroker crank and a largerbore. Notice how the shorter stroke enginedevelops less peak flow and spreads thesame total flow volume slightly more evenlyacross the entire intake stroke. This reduc-tion in peak flow allows the induction sys-tem to operate more efficiently, slightlyimproving cylinder filling and reducingpumping work. So that would seem to proveit, right? The chart shows that a shorterstroke pumps more efficiently. While thatmay seem to be the case, take a look atChart-B. Heres a plot of the same twoengines, but this time the rod lengths havebeen adjusted so that they have exactlythe same rod ratio, that is, the length of allconnecting rods are now equal to 1.7 timesthe length of the strokes. With identical rodratios, the cylinders in both engines pumpexactly the same volume at each degreeof crank rotation. The proven benefits ofa short stroke disappear.

The misunderstanding that longer strokeengines are less efficient breathers prob-ably occurs because during dyno tests ofvarious combinations, identical rod ratiosare not maintained when stroke lengths arechanged. So measured differences inpower are mistakenly attributed to changesin stroke, rather than the real cause: Varia-tions in rod ratio. If you have any doubtsabout this, take a look at the simulation

While pumping work isnot directly affected by

bore or stroke, largerengines, like this 600cid

big block, generatetremendous pumping

work at high enginespeeds. The pumping

losses are due to restric-tions at the valves, ports

and runners, andthroughout the intake and

exhaust systems. Atsome point in engine

speed, there no longerexists enough time to

move the gasses throughpassages of fixed sizes.

When this happens,power takes a nose-dive.


This graphillustrates thedifferences inflow as strokechanges. Noticehow the shorterstroke enginedevelops lesspeak flow andspreads the sametotal flow volumeslightly moreevenly across theentire intakestroke.

Heres a plot ofthe same two

engines, but thistime the rod

lengths have beenadjusted to

produce the same1.7 rod ratio. With

identical rodratios, the cylin-

ders in bothengines pump

exactly the samevolume at eachdegree of crank

rotation.

The graph showsthe flow per crankdegree for 3-inchstroke vs. 24-inchstroke engines.Both engineshave the same603cid and sweepout the samevolume at each 2-degree incrementof crank rotationthroughout theentire intakestroke, despite the21-inch differencein stroke lengths!


depicted in Chart-C. Heres a test of a 3-inch stroke, 5.67-inch bore engine com-pared to a very long 24-inch stroke enginehaving a 2-inch bore. Both engines havethe same total displacement of 603ci andthe same rod ratios (2.0 times the strokelength). The graph shows that the pistonsin both engines sweep out the same vol-ume at each 2-degree increment of crankrotation throughout the entire intake stroke,despite the 21-inch difference in strokelengths!

So its not stroke, but rather rod ratio,that has subtle, measurable effects onpumping work and peak flow rates. Thelonger the rod ratio, the more spread outthe induction flow and, potentially, thegreater the high-speed power. The shorterthe rod ratio, the higher the peak flow andthe earlier in the intake cycle the flow peakis reached. Considering these flowchanges, short-rod combinations couldshow potential gains on engines that ben-efit from a strong carburetor signal at lowerengine speeds, like short-track and streetengines. But even rod ratio cannot beconsidered in isolation. When the rod lengthis changed it also affects lateral loads onthe pistons, and it can change the way thepistons rock in their bores and the waythe rings flutter or seal against the cylin-der walls. So, depending on the lubricantsused, the cylinder block and piston con-figurations, and several additional factors,the theory of rod ratio and the powerchanges that are predicted may or maynot be found on the dyno. At least younow have an understanding of the under-lying theory, even though it makes its pre-dictions in isolation of many other interre-lated variables.

While were on the subject of rod ratio,many readers may be wondering why thisvariable was not included in one of thepull-down menus in Motion Softwaresengine simulation. There are two reasonsfor this: 1) As just stated, the effects ofvarying rod length are very subtle and of-ten masked by other variables within theengine (such as piston side loads, bore-wall friction, etc.) making accurate model-ing extremely difficult, and 2) The subtlechanges in swept volume primarily require

wave-action dynamics for rigorous analy-sis (a process discussed in the completeDeskTop Dynos book available from Mo-tion Software). So rather than add a com-ponent that could not be modeled as accu-rately as the other variables in the pro-gram, rod length is not presented as a tun-able element (although it is included withinthe simulation process).

Fallacy Two: Long-Stroke TorqueVs. Short-Stroke Horsepower

Another remarkably widespread beliefhas to do with stroke length vs. enginespeed and torque vs. horsepower. A greatmany performance enthusiasts believe thatlong-stroke engines inherently developmore low-speed torque and short-strokeengines generate more high-speed horse-power. This understanding (probably as theresult of reading various magazine articlesover the years) is almost completely incor-rect, and some enthusiasts are quite defi-ant when confronted with the fact thatstroke, by itself, has little to do with high-speed or low-speed power potential. But,like the fallacy that short-stroke enginesare more efficient air pumps, there is athread of truth lying hidden under the sur-face.

To start off, long- and short-stroke en-gines of equal displacement will producethe same cylinder pressures during thepower stroke for the same swept volumes(assuming that they have consumed equalquantities of air and fuel; and weve al-ready demonstrated that stroke, by itself,has almost nothing to do with induction ef-ficiency). While the pistons in a longer-stroke engine are smaller in diameter and,therefore, experience less force from thesame cylinder pressure (force on the pis-ton is directly related to the surface area ofthe piston and pressure in the cylinder),the crank arms are longer and have agreater mechanical advantage. The result,believe it or not, is that equal displacementengines of unequal stroke lengthsexpe-riencing the same cylinder pressureswillproduce the same torque at the crankshaft.Stroke, however, is not an isolated vari-able; it affects the design of many other


engine components. Its in this interrelat-edness that we find the roots of misunder-standing about stroke vs. horsepower.

Probably the most direct reason for thebelief that longer-stroke engines are lower-speed torque generators, not capable ofproducing as many horsepower per cubicinch, is that many longer-stroke engineshave smaller bores. An engine with smallerbores almost always has smaller combus-tion chambers, and smaller combustionchambers have smaller valves. So mostlonger-stroke engines are forced, by thedesign of the cylinderheads and the entireinduction system, to produce less power athigher engine speeds. Its not the length ofthe stroke that limits power potential, itsreduced flow from smaller valves, ports,and runners.

For more concrete proof, refer back tothe simulation we performed in the earliersection on stroke vs. friction. In that test,we compared identical displacement en-gines of 603ci, one with a 6-inch stroke(dotted lines on graph) to one with a 3-inch stroke (solid lines). Both engines usedthe same size valves and the same 780cfm

induction flow capacity. As previously dem-onstrated, the increase in horsepower fromthe shorter-stroke engine is due to a re-duction in bore-wall friction, adding about100 horsepower at 5500rpm. But look atthe shape of the solid-line curves. Theymatch the shape of the dotted-line curvesfor the longer-stroke engine. There is nonoticeable boost in low-speed torque fromthe engine with a 6-inch stroke. A steadily-increasing power loss from additional cyl-inderwall friction is clearly visible, but no-tice that the short-stroke engine producedno less torque between 2000 and 2500rpm,a range that many believe the longer-strokeengine should easily out-torque its short-stroke counterpart.

From a real-world mechanical stand-point, longer-stroke engines have draw-backs that limit their high rpm potential. Ascan be seen in our simulation, bore-wallfriction becomes a substantial power rob-ber. Ring seal also becomes a seriousproblem. As the stroke increases, higherand higher piston speeds cause the ringsto flutter against the cylinderwalls decreas-ing their sealing ability. Its also possible

SMALLERVALVES

SMALLERPORTS SMALLERRUNNERS

SMALLERBORE

TOP VIEW

Smaller-Bore/Longer-Stroke EnginesTypically Have Smaller Valves and Runners

And Produce Peak Power AtLower Engine Speeds

Many people believethat longer-stroke

engines are lower-speedtorque generators, notcapable of producing as

many horsepower percubic inch as shorter-

stroke engines of equaldisplacement. What is

often overlooked is thatmost engines with

smaller bores almostalways have smaller

combustion chambers,smaller valves, and

smaller runners. Its notthe length of the stroke

that limits powerpotential, its reduced

flow through the induc-tion system.


that at very high piston speeds during thepower stroke, the piston moves so quicklythat the rings cant maintain a seal betweenthe bottom of the ring and the ring land inthe piston, increasing blowby and furtherdecreasing horsepower. The mechanicalloads on the piston and rod assembly inlong-stroke engines also become a seri-ous factor. As the piston is accelerated fromTDC down the bore, extremely high ten-sion loads are imparted to the rods andpistons, and added component weight tocompensate for the additional loads fur-ther increases stress.

While there are good reasons whylonger-stroke engines are ill suited to high-rpm applications, dont confuse these an-cillary problems with the basic design rela-tionships between bore and stroke. Anddont fall into the trap of believing, like thou-sands of enthusiasts, that selecting a longerstroke will automatically boost low-speedpower.

THE CYLINDERHEADAND VALVE DIAMETER MENUS

The Cylinderhead pull-down menu, lo-cated just to the right of the Bore/Strokemenu, contains two sub-menus that allowthe simulation of various cylinderhead de-signs and a wide range of airflow charac-teristics. The first submenu, Head/PortDesign, lists general cylinderhead charac-teristics, including restrictive ports, typicalsmall- and big-block ports, and even 4-

valve cylinderheads. Each category in-cludes several stages of port/valve modifi-cations from stock to all-out race. A selec-tion from this menu is the first part of atwo-step process that Motion simulationsuse to accurately model cylinderhead flowcharacteristics. This initial selection deter-mines the airflow restriction generated bythe ports. That is, a selection from the firstsubmenu fixes how much less air than thetheoretical maximum peak flow will passthrough each port. What determines peakflow? Thats selected from the secondCylinderhead submenu: Valve Diameters.Valve size fixes the theoretical peak flow(called isentropic flowmore on that later).Most cylinderheads flow only about 50%to 70% of this value. The Valve Diametersubmenu allows the direct selection of valvesize or Auto Calculate Valve Size maybe chosen, directing the simulation to cal-culate the valve diameters based on boresize and the degree of cylinderhead port-ing/modifications.

While it may seem as if the variousCylinderhead menu choices simply refer toranges of airflow data stored within theprogram, that is not the case. If each menuselection fixed the flow capacity of thecylinderheads to a specific range of values(like typical flow bench data measured at astandardized pressure drop), the simula-tion would be severely limited. While asimulation based on this technique mightadequately calculate mass flow for enginesthat used nearly identical cylinderheads, ac-

The Head/Port Designmenu, under the Cylin-derhead main menu, listsgeneral cylinderheadcharacteristics, includingrestrictive ports, typicalsmall- and big-blockports, and even 4-valvecylinderheads. Eachcategory includesseveral stages of port/valve modifications fromstock to all-out race.


curacy would diminish rapidly for engineswith even modestly larger or smaller portsand valves.

There are several reasons why the de-termination of port flow in sophisticatedengine simulations can not be based strictlyon flow-bench data. First of all, flow gen-erated in the ports of a running engine isvastly different than the flow measured ona flow bench. Airflow on a flow bench is asteady-state, measured at a fixed pressuredrop (its also dry flow, but a discussion ofthat difference is beyond the scope of thisbook). A running engine will generate rap-idly and widely varying pressures in theports. These pressure differences directlyaffectin fact, they directly causethe flowof fuel, air, and exhaust gasses within theengine. An engine simulation program cal-culates these internal pressures at eachcrank degree of rotation throughout thefour-cycle process. To determine mass flowinto and out of the cylinders at any instant,you need to calculate the flow that occursas a result of these changing pressuredifferences. Since the variations in pres-sure, or pressure drops, within the engineare almost always different than the pres-sure drop used on a flow bench, flow benchdata cannot directly predict flow within theengine.

Before we delve deeper into the differ-ences between flow-bench data and themass flow calculated by an engine simula-tion program, lets permanently put to restthe idea of storing the needed airflow data

The Valve Diametermenu allows the direct

selection of valve size oryou can select Auto

Calculate Valve Size,directing the simulation

to calculate the valvediameters based on bore

size and the degree ofcylinderhead porting and

other modifications.

within the program. Consider for a momentan engine simulation program thats basedon internally-stored airflow data. If youassume that the program is capable ofmodeling a wide a range of engines (likeMotion Software simulations), then it mustalso store a wide range of cylinderheadflow data. Flow would have to be recordedfrom a large number of engines, startingwith single-cylinder motors and working upto 1000+ cubic-inch powerplants. If it werepossible to accumulate this much data,there are additional shortcomings in alookup model. Since the pressures insidethe IC engine are constantly changing, alookup program would have to containranges of flow data measured from zero tothe maximum pressures differentials devel-oped within the running engine. Even if thismuch data could be assembled and filedaway, additional data for each engine andcylinderhead would be needed to predictthe flow changes when larger valves wereinstalled; and even more data would beneeded to model port modifications. It soonbecomes clear that the sheer bulk of flowdata needed for a comprehensive lookupmodel would probably consume morespace than exits on your hard drive (andthats considering that most of us now owngigabyte and larger drives)!

Cylinderhead ChoicesAnd Discharge Coefficients

While it is impractical to base an engine


simulation on extensive flow-bench data,measured cylinderhead flow figures are,nonetheless, commonly used in sophisti-cated engine simulations. Rather thanstored in extensive lookup tables withinthe programs, flow-bench data can be usedas a means to compare the measured flowof a particular port/valve configurationagainst the calculated isentropic (theoreti-cal maximum) flow. The resulting ratio,called the discharge coefficient, hasproven to be an effective link between flow-bench data and the simulated mass flowmoving into and out of the cylindersthroughout a wide range of valve openingsand pressure drops. Furthermore, the dis-charge coefficient can be used to predictthe changes in flow for larger or smallervalves and for various levels of port modi-fications. In other words, its the dischargecoefficient, not flow bench data, that pro-vides a practical method of simulating massflow within a large range of engines. Sincethis is such a powerful and often misun-derstood concept, the following overviewshould prove helpful in understandingwhats happening behind the scenes

when various choices are made from theCylinderhead pull-down menu.

The choices in the Cylinderhead menuare purposely generic. This can be frus-trating if you are interested in modeling onlyone engine or a single engine family. Inthese cases it would be ideal to list theexact components you wished to test, inname or part-number order. But if you areinterested in simulating different engines,including popular 4-, 6-, and 8-cylinderpowerplants, the choices provided in theCylinderhead menu (and many of the othermenus) offers considerable modelingpower. The menu choices move from re-strictive heads to smallblock, big block, andfinally to 4-valve-head configurations. Ifeach of these choices loaded a higher ab-solute flow curve into the simulation, theywould cover only a very narrow range ofengines. Instead, each of the menu choicesmodel an increasing flow capacity and re-duced restriction. This makes it possible toaccurately simulate a lawnmower engine,a high-performance motorcycle engine, anall-out Pro Stock big block, or a mild streetdriver, each having different port and/or

PRESSURE

CFM

SIMULATEDPRESSURE-WAVE FLOW

FLOW BENCHSTEADY-STATE FLOW

Airflow on a flow bench is steady state, but Motion's engine simulation program cal-culates internal pressures and mass flow at each degree of crank rotation throughoutthe four-cycle process. Since the variations in pressure, or pressure drops, within theengine are rarely equal to the pressure drop used on a flow bench, flow bench datacannot directly predict flow within the engine.


valve sizes, using selections from the samemenu!

The basis for this universality is thateach menu selection uses a different dis-charge-coefficient curve rather than airflowcurve. While the discharge coefficient datais derived from flow-bench data, it is di-mensionless (has no length, weight, mass,or time units) and is applicable to any cyl-inderhead of similar flow efficiency. Toclarify this concept, picture two large roomsconnected by a hole in the adjoining wall.When pressure is reduced in one room, airwill flow through the hole at a specific rate.It is possible to calculate what the flowwould be if there were no losses from heat,turbulence, etc. This flow rate, called theisentropic flow, is never found in the realworld, but is, nevertheless, a very usefulterm. Its the rate of flow that nature willnever exceed for the given pressure dropand hole size. If we measure the actualflow through the hole and divide it by thecalculated isentropic flow, we will havedetermined the flow efficiency or dischargecoefficient of the hole:

Discharge Coefficient (always less than 1)= Measured Flow / Calculated Flow

Since every orifice has some associatedlosses, the discharge coefficient is alwaysless than 1. If the calculated discharge

coefficient for our hole in the wall was0.450, this would mean that the hole flows45% as much air as an ideal hole. In otherwords, its 45% efficient.

Lets see how this concept can be ap-plied to cylinderheads. When an intakevalve of a specific size is opened a fixedamount, it exposes a flow path for air calledthe curtain area. Through this open area,measured in square inches just like thehole in the wall, air/fuel mixture moves ata specific rate depending the pressure dropacross the valve (as you recall, the pres-sures are calculated by the simulation soft-ware at each degree of crank rotation). Ifthe valve and port were capable of perfectisentropic flow, the simulation equationscould calculate the precise mass flow thatentered the cylinder during this moment intime. But real cylinderheads and valves arefar from perfect, and its flow bench datathat tells the simulation how far fromperfect the real parts perform. By dividingthe isentropic flow by the flow-bench data(both at the same pressure drop) of acomparable head/port configuration at eachincrement of valve lift, the simulation soft-ware creates a discharge coefficient curvethat it can use as a correction factor forport flow at all other pressure drop levels.

The true power of this method lies inthe fact that the cylinderhead tested on theflow benchused to develop the discharge-

Theoretical Isentropic Flow

Actual Measured Flow 45% Of Isentropic Flow

The absolute maximumflow rate through an

orifice with no lossesfrom turbulence, heat, or

friction is called theisentropic flow. This is

the flow rate thatnature will never

exceed for the givenpressure drop and holesize. If we measure theactual flow through the

hole and divide it by thecalculated isentropic

flow, we will havedetermined the flow

efficiency or dischargecoefficient.


coefficient curvemay have been designedfor an entirely different engine and useddifferent valve sizes. Nevertheless, as longas the head configuration matches thesimulated cylinderhead, in other words, theports have similar flow efficiency, the dis-charge-coefficient curve will closely adjustisentropic flow to real-world corrected flow.So a specific choice in the Cylinderheadmenu, say Pocket Porting With LargeValves may accurately model a factoryperformance head for a smallblock Chevy,a cylinderhead on a 4-cylinder Toyota en-gine, or even a motorcycle head. With dis-charge coefficient corrections, its not theabsolute flow numbers that are important,its how well the valve and port flow fortheir size that really counts.

As you made selections from the Cylin-derhead menu, you may have wonderedwhether the flow data used to correct theisentropic flow in any of the menu itemswould match the published flow of cylin-derheads that you would like to simulate.Considering what you now know about therelationships between cylinderheads ofsimilar efficiencies and how flow data isused by the program, it becomes clear thatknowing the flow-bench data used in theprogram would probably not be helpful. Theflow may not have been obtained from thesame valve sizes or even the same brand

VALVEHEAD

VALVESEAT

CURTAIN AREA

When a valve of a specific size is openeda fixed amount, it exposes a flow pathcalled the curtain area. Through this openarea (measured in square inches) gassesmove at a specific rate depending on thepressure drop across the valve.

cylinderheads. On the other hand, it isentirely feasible for the simulation to sub-stitute user-entered flow data from testsconducted on a specific set of cylinder-heads. This data could then be used todevelop custom discharge-coefficientcurves that would, in turn, closely modelcylinderheads. While this is not currentlysupported, it will be incorporated in upcom-ing versions of Motion simulation software(if you haven't already, send in your regis-tration card; youll receive information onupgrades and new releases).

Ram Tuning AndPressure Waves

Up to this point our discussion has cen-tered around airflow, valve size, and dis-charge coefficients. The assumption hasbeen that reducing restriction and increas-ing flow efficiency will allow more air/fuelmixture to enter the cylinders and producemore horsepower. Initially this is true, butwhen port and valve sizes are increased,power begins to fall off at lower speeds,then at higher speeds, and finally very largepassages reduce power throughout theentire rpm range! At first, this may seemquite mysterious. If minimizing restrictionwas the key to improved airflow and power,this phenomenon would indeed be inexpli-cable. But as weve discovered, the ICengine does not function by simply direct-ing the flow of air and fuel as a hose pipedirects the flow of water. Powerful wavedynamics take control of how air, fuel, andexhaust gasses move within the inlet andexhaust passages. Because of these phe-nomena, there is an optimum size for theports and valves for any specific applica-tion, from street economy to all-out dragracing.

Consider what happens after the intakevalve opens and the piston begins moving


down the bore on the intake stroke. Aspressure drops, air/fuel mixture enters thecylinder. During this portion of the induc-tion cycle, any restriction to inlet flow re-duces cylinder filling and power output. So,while the piston moves from TDC to BDC,the rule of thumb for the ports and valvesis the bigger the better. After the pistonreaches BDC and begins to travel back upthe bore on the early part of the compres-sion stroke, the intake valve remains openand the cylinder continues to fill with air/fuel mixture. This supercharging effect,caused by the momentum built up in themoving column of air and fuel in the portsand inlet runners, adds considerable chargeto the cylinder and boosts engine output.However, as soon as the piston begins tomove up the bore from BDC, it tries topush charge back out of the cylinder. Largerports and valves not only offer low restric-tion to incoming charge, but they make iteasier for the piston to reverse the flowand push charge back out of the cylinder.Furthermore, a low restriction inductionsystem has a large cross-sectional areaand allows the incoming charge to movemore slowly (in feet per second), so thecharge carries less momentum and, onceagain, is more easily forced back out ofthe cylinder. So the rule of thumb to opti-

When the piston movesfrom TDC to BDC on theintake stroke, the biggerthe better is the rule forports and valves. Afterthe piston reaches BDCand begins to travel backup the bore, it tries topush charge back out ofthe cylinder. Now the ruleof thumb for power issmaller ports and highairflow speeds. Acompromise must befound. This race enginefinds that critical balancewith large runners thatgenerate 700-ft/sec peakport velocity at very highengine speeds.

mize power for the period of time betweenBDC and intake-valve closing is smallerports that produce higher airflow speedsare better. Since its not possible to rap-idly change the size of the ports as theengine is running, a compromise must befound that minimizes restriction and opti-mizes charge momentum.

As it turns out, optimum port and valvesizes for performance applications at aspecific engine speed must be smallenough to allow the charge to reach speedsof about 700 feet per second during theintake stroke. Then, as the piston movesfrom BDC to the intake valve closing point,the momentum generated by these speedscontinues to force air and fuel into thecylinder, optimizing charge density. Typi-cally, the best port size and cam timing forperformance allows a slight reversion offresh charge just before the intake valvecloses.

Unfortunately, the smaller port cross-sectional areas required to generate opti-mum flow velocities create a restriction toairflow and increase pumping work. If portcross-sectional areas were smaller and flowvelocities increased much beyond 700 feet/second, the added restriction and increasedpumping work would reduce overall cylin-der filling and engine output would suffer.


The magic balance found at about 700feet per second between charge momen-tum, inlet restriction, and pumping workallows the cylinders to fill with the greatestmass of air/fuel mixture throughout thecomplete induction cycle from IVO (intakevalve opening) to IVC (intake valve clos-ing).

What is the best cross-sectional areafor any specific engine; the area that gen-erates about 700 feet/second peak flowvelocities? That is a very tough question toanswer. In fact, optimum port shapes andcross-sectional areas are so interrelatedwith other engine variables, like enginedisplacement and rpm, cam timing, exhaustsystem configuration, intake manifold de-sign, compression ratio, and more, thatengine simulation software is needed tosort through the complexity and find themagic combinations. Furthermore, a fullwave-action modeling program, like Dyno-

mation discussed in the complete Desk-Top Dynos book, is required for this tasksince its the dynamics of finite-amplitudewave motion that are responsible for therecorded changes in horsepower.

Despite these complexities, there aresome rules of thumb that can be helpfulin selecting a workable port for common

Minimum Port Cross-sectionalArea Equal 0.85 Times ValveArea For Street Applications

Minimum Port Cross-sectionalArea Equal 0.90 Times ValveArea For Race ApplicationsDespite the interrelated-

ness of engine speed,cam timing, exhaust

system configuration,and more, there are some

rules of thumb inselecting a workable portfor common applications.

Cylinderheads willprobably provide good

performance if the areaof minimum port cross-

section is equal to 0.9times the valve area for

race applications and0.85 for street use.

700 Feet Per SecondMaximum Flow

Speed

The best cross-sectional area for anyspecific engine, one that generates about

700 feet/second peak flow velocity, isdifficult to find. Optimum port shapes and

cross-sectional areas are so interrelatedwith engine displacement and rpm, cam

timing, exhaust system configuration,intake manifold design, compression ratio,and more, that engine simulation software

is needed to sort through the complexityand find the magic combinations.


applications. If the area of minimum crosssection in the ports is equal to 0.9 timesthe areas of the valves for race applicationand 0.85 times the valve areas for streetuse, the heads will probably provide goodperformance. This rule applies to typicalautomotive engines, not to high-rpm mo-torcycles or low-rpm aircraft. However, fortypical powerbands between 5000 and8500rpm, this rule should be reasonablyclose, providing the valve diameters arenot too small or too large for the applica-tion.

These same relationships between portcross-sectional areas and valve diametersare used in Motion Softwares Filling AndEmptying simulation. As you move downthe Cylinderhead menu, choices withineach sectionsay within the smallblock orbig block categoriesthe minimum portcross-section increases from 0.85 to 0.9times the selected valve areas. However,since the current simulation does not in-corporate as robust a wave-action analy-sis as the Dynomation program (discussedin the complete DeskTop Dynos book), portcross-sectional areas are not tunable bythe user.

The phenomenon of cylinder filling bycharge momentum is often called ram tun-ing, and is commonly thought to be an

independent property of induction tuning,separate from the complex theory of wavedynamics that most professional racers onlyvaguely recognize. Because the conceptof ram tuning is easy to grasp, it has beenwidely applied for many years. Long in-jector stacks, extremely large ports andvalves, and other measures have beenused to facilitate the free flow of additionalcharge into the cylinder after BDC on theintake stroke. But now that we have un-covered the fine balance needed betweenport cross-sectional area, charge momen-tum, airflow velocity, pumping work, camtiming, and engine speed, it is more obvi-ous that large ports and valves, by them-selves, are not the answer. Unfortunately,the insatiable desire to have large portsand valves seems to drive the cylinder-head market. Most customers simply wantlarger ports and valves. So head manufac-turers and porters turn out droves of headswith ports that are too large for the appli-cation. While they reduce restriction duringthe intake stroke, low charge momentumduring the majority of the rpm range of theengine reduces overall cylinder filling andpower output.

One important thing to learn from thisbook should be to permanently discard thenotion that bigger is better when it comes

The phenomenon ofcylinder filling by chargemomentum is oftencalled ram tuning, andoptimizing this effectmeans finding a balancebetween port cross-sectional area, airflowvelocity, pumping work,cam timing, and enginespeed. This custommanifold was designedwith the help of simula-tions, including Dynoma-tion (discussed in thecomplete DeskTopDynos book) to find thatillusive balance onsmallblock Fords.


to ports and valves. The right combinationis the key.

Sorting Out CylinderheadMenu Choices

Now that we have covered some of thebasic theory behind the choices in theCylinderhead menus, heres some practi-cal advice that may help you determinethe appropriate selections for your applica-tion.

Low Performance CylinderheadsThere are three Low Performance cylin-derhead selections listed at the top of theHead/Port Design submenu. Each of thesechoices is intended to model cylinderheadsthat have unusually small ports and valvesrelative to engine displacement. Heads ofthis type were often designed for low-speed, economy applications, with littleconcern for high-speed performance. Early260 and 289 smallblock Ford and to a les-sor degree the early smallblock Chevycastings fall into this category. Thesechoices use the lowest discharge coeffi-cient of all the head configurations listed inthe menu. Minimum port cross-sectionalareas are 85% of the valve areas or some-what smaller and, if Auto Calculate Valve

Size has been selected (more on this fea-ture in the next section), relatively small(compared to the bore diameter) intake andexhaust valve diameters will be used.

The first low-performance choice mod-els an unmodified production casting. Thesecond choice Low Performance/PocketPorting adds minor porting work performedbelow the valve seat and in the bowl areaunder the valve head. The port runnersare not modified. The final choice LowPerformance/Ported, Large Valves incor-porates the same modifications and in-cludes slightly larger intake and exhaustvalves. Valve size increases vary, but theyare always scaled to a size that will gen-erally install in production castings withoutextensive modifications.

The low-performance choices havesome ability to model flathead (L-head &H-head) and hybrid (F-head) engines.While the ports in these engines are con-siderably more restrictive than early Fordsmallblock engines, by choosing Low-Per-formance and manually entering the exactvalve sizes, the simulation will, at least,give you an approximate power output thatyou can use to evaluate changes in camtiming, induction flow, and other variables.Also, its not essential that your model pro-duce an absolutely accurate horsepower

The Low Performancecylinderhead choices are

intended to model cylinder-heads that have unusually

small ports and valves.Heads of this type wereoften designed for low-

speed, economy applica-tions, with little concern for

high-speed performance.Early 260 and 289 small-

block Ford and to a lessordegree the early smallblockChevy castings fall into this

category.


number. A great deal of useful informationcan be found by simply looking at thechanges in power that result from variouscombinations of parts.

Smallblock CylinderheadsThe small-block and big-block choices comprise thetwo main cylinderhead categories in theHead/Port Design submenu. Choices fromthese two groups apply to over 90% of allperformance engine applications from mildstreet use to all-out competition.

The basic smallblock selections modelheads that have ports and valves sizedwith performance in mind. Ports are notexcessively restrictive for high-speed op-eration, and overall port and valve-pocketdesign offers a good compromise betweenlow restriction and high flow velocity. Thestock and pocket-ported choices are suit-able for high-performance street to modestracing applications.

The next step is SmallBlock/FullyPorted, Large Valves and this cylinderheadmoves away from street applications. Thiscasting has improved discharge coeffi-cients, greater port cross-sectional areas,and increased valve sizes. Consider thishead to be an extensively modified, high-performance, factory-type casting. It doesnot incorporate exotic modifications, likeraised and/or welded ports that requirecustom-fabricated manifolds. SmallBlock/

Fully Ported, Large Valves heads are high-performance castings that have additionalmodifications to provide optimum flow forracing applications.

The last choice in the smallblock groupis SmallBlock/Race Porting And Mods.This selection is designed to model state-of-the-art, high-dollar, Pro-Stock type cyl-inderheads. These custom pieces are de-signed for one thing: Maximum power. Theyrequire hand-fabricated intake manifolds,have excellent valve discharge coefficients,and the ports have the largest cross-sec-tional areas in the smallblock group. Thishead develops sufficient airflow speeds forgood cylinder filling only at high engine rpm.

Big Block CylinderheadsAll big-block selections are modeled around headswith canted valves. That is, the valvestems are tilted toward the ports to im-prove the discharge coefficient and overallairflow. All ports have generous cross-sec-tional areas for excellent high-speed per-formance.

The first three choices are based on anoval-port design. These smaller cross-sec-tional area ports provide a good compro-mise between low restriction and high flowvelocity for larger displacement engines.The stock and pocket-ported selections aresuitable for high-performance street tomodest racing applications.

The SmallblockCylinderhead menuchoices model cylinder-heads that have portsand valves sized withperformance in mind,like the heads on thisLT1. The stock selec-tions are not exces-sively restrictive forhigh-speed operation,and overall port andvalve-pocket designoffers a good compro-mise between lowrestriction and high flowvelocity.


The final two selections simulate exten-sively modified rectangular-port heads.These choices are principally all-out, big-block Chevy heads, however, they closelymodel other extremely aggressive high-performance racing designs, like theChrysler Hemi head. As with the smallblockcategory, the Big Block/Fully Ported, LargeValves heads are not suitable for moststreet applications. These castings havehigh discharge coefficients, large portcross-sectional areas, and increased valvesizes. This head is basically a factory-typecasting but extensively improved. However,it does not incorporate exotic modifica-tions, like raised and/or welded ports thatrequire custom-fabricated manifolds.

The last choice in the big-block group isBig Block/Race Porting And Mods. Thisselection is designed to model state-of-the-art, high-dollar, Pro-Stock cylinderheads.These custom pieces, like their smallblockcounterparts, are designed for maximumpower. They require hand-fabricated intakemanifolds, have optimum valve dischargecoefficients, and the ports have the largestcross-sectional areas in the entire Head/Port Design submenu, except for 4-valveheads (discussed next). These speciallyfabricated cylinderheads only develop suf-ficient airflow for good cylinder filling withlarge displacement engines at very highengine speeds.

4-Valve CylinderheadsThe last threeselections in the Head/Port Designsubmenu model 4-valve cylinderheads.These are very interesting choices sincethey simulate the effects of very low-re-striction ports and valves in stock andperformance applications. The individualports in 4-valve heads begin as single, largeopenings, then neck down to two Siamesedports, each having a small (relatively) valveat the combustion chamber interface. Sincethere are two intake and two exhaust valvesper cylinder, valve curtain area is consid-erably larger than with the largest single-valve-per-port designs. In fact, 4-valveheads can offer more than 1.5 times thecurtain area of the largest 2-valve heads.This large area, combined with high-flow,low-restriction ports greatly improves airand fuel flow into the cylinders at highengine speeds. Unfortunately, the portsoffer an equally low restriction to reverseflow (reversion) that occurs at low enginespeeds when the piston moves up thecylinder from BDC to Intake Valve Closing(IVC) on the final portion of the intakestroke. For this reason, 4-valve heads, evenwhen fitted with more conservative portsand valves, can be a poor choice for small-displacement, low-speed engines. On theother hand, the outstanding flow charac-teristics of the 4-valve head put it in an-

The Big-BlockCylinderhead selectionsare modeled aroundheads with canted valves.Ports have generouscross-sectional areas.The first three menuchoices model oval-portdesigns. The final twoselections simulatemodified rectangular-portheads. The appropriateselection for this L29 big-block Chevy wouldprobably be the secondor third menu choicethefourth menu choicemodels a head with flowcapacity beyond thestock L29.


other league when it comes to high horse-power potential on large engines at highengine speeds.

The first choice in the 4-valve group is4-Valve Head/Stock Ports And Valves.This simulates a 4-valve cylinderhead thatwould be standard equipment on a fac-tory high-performance or sports-car engine.These mild heads offer power comparableto high-performance 2-valve castingsequipped with large valves and pocketporting. However, because they still haverelatively small ports, reasonably high portvelocities, and good low-lift flow character-istics, they often show a boost in low-speedpower over comparable 2-valve heads.

The next choice, 4-Valve Head/PortedWith Large Valves represents a mild re-work of stock type 4-valve heads. Largervalves have been installed and both theintake and exhaust flow has been improvedby pocket porting. However, care has beentaken not to increase the minimum cross-sectional area of the ports. These changesprovide a significant increase in power withonly slightly slower port velocities. Rever-sion has increased, but overall, these headsshould show a power increase throughoutthe rpm range on medium to large displace-ment engines.

The final choice, 4-Valve Head/Race

Porting And Mods, like the other RacePorting And Mod choices in the Head/PortDesign submenu, models an all-out racingcylinderhead. This selection has the great-est power potential of all. The ports areconsiderably larger than the other choices,the valves are larger, and the dischargecoefficients are the highest possible. Theseheads suffer from the greatest reversioneffects, especially at lower engine speedson small displacement engines. (Theseheads, like all head choices, are scaledto engine size, so that smaller engines au-tomatically use appropriately smallervalvesproviding the Auto Calculate ValveSize option is selectedand smaller ports.)If you would like to know what hiddenpower is possible

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