Performance Camshafts EP10-2010!50!58

8/13/2019 Performance Camshafts EP10-2010!50!58

1/650 OCT-DEC 2010engine professional

The goal in rebuilding an engine is toreturn its performance and reliabilityto what it once had new. The goal inbuilding an engine is to increase itspower within the capabilities of thatengine without unduly ruining its otherperformance factors drivability,mileage, reliability, and perhaps smog-law compliance.

Building a performance engine is notjust a matter of tossing speed partslike a big cam into it, nor is building highperformance or racing engines anywherenear as simple as some people imagine.Change one thing and thats a differentengine. Change many things and youhave entered another profession, as anengine developer.

The package of professional skillsyou acquired as a rebuilder still applyto building high performance engines,whether for yourself or for customers.But a few advanced skills must also beacquired (or rented). So must further,up-to-date insights, for making effectivedecisions long before the first cuttingedge touches iron. Then knowledge ispower.

The topic of engine performance isenormous, and enormously complicated.So one article can lead only so far. Thisarticle goes beyond the basics of howengines operate across six workingcycles and how cam designs affect thosecycles and looks for a moderate but realincrease in power to a production enginethat must run well at reasonable rpm onpump gas.

Find the BalancedPackageBefore changing anything on an engine,closely examine it in detail. See how each

aspect of the engine balances against

the others. Maximum usable rpms arelimited by resistance to gas flow throughthe engine, and maximum piston speedis limited by stresses from the inertia ofmoving parts. Bring the weaker featuresof the engine up to the performance levelof its strong points.Focus your attention close to the action,in the combustion chamber and ports.The better the burn and freer the flowthrough those, the more power develops.The better the balance among features inthe combustion chamber and ports, andthe better the cam(s) choreograph theactivity there, the more power develops.But the further away you wander fromthe action, the less improvement comesfrom your efforts. Chroming the cat-back exhaust system looks pretty butaccomplishes nothing.

In the big 2-valve iron V8s most of usare most familiar with, the short blockis generally a pretty strong assembly,usually capable of handing quite a bitmore power than it now puts out in aregular passenger car. With the exceptionof its pistons and static compressionratio, build much the same short blockas you would for 100,000-mile servicelife. Power potential lies upstairs, in theheads, the intake and exhaust systems,and especially in the right performancecam to direct the action. Coming in the other door are manymodern, perhaps unfamiliar, alloy4-valve engines. Most of their headsalready flow air extremely well. Slightlydifferent camshafts can release furtherpotential from these heads and makemore power than the production lowerend can handle in one piece. So here yourfirst task becomes structural. Replacealuminum threads with steel. Fit a

permanent girdle around free-standing

tops of cylinder bores to encourage themto sit still for the honing head, and laterfor faster-moving pistons. Then fine-tunethe top end.

Rod RatioSomewhat surprisingly, the connectingrod affects intake flow. More specifically,the ratio of the center-to-center length ofthe connecting rod to the stroke of theengine termed the rod/stroke ratio orjust rod ratio has a significant effecton Volumetric Efficiency. More surprise:the effect of rod ratio differs for 2-valvevs. 4-valve engines.

Airflow in a normally-aspirated engineis driven into the cylinder only by thepressure difference between the 14.7psi of the atmosphere and whateverless pressure is inside the cylinder atthat instant. The greatest difference inpressure occurs shortly after the piston ismoving downward at its fastest. Pistonvelocity peaks when the rod and thecrank throw are at right angles to eachother.

The exact number of degrees ATDCfor maximum piston velocity can befound in any trigonometry table. TheTangent of the angle ATDC is twice therod ratio for that engine. Then add 2-3for time for that news to reach the intakevalve at the speed of sound and affectairflow there. The sum should comebetween 70 and 80 ATDC. The shorterthe rod ratio, the earlier that pistonvelocity peaks. Airflow in 2-valve heads begins slowly.So airflow through these heads respondsto a long rod ratio, close to 2:1, formaximum draw after 75 ATDC.By comparison, a 4-valve engine flowsa lot more air at the lower and mid lifts

through its smaller valves and ports.

Performance

Camshafts

BY DIMITRI ELGIN


2/6engine professionalOCT-DEC 2010 51

This allows its rod ratio to be smaller

without hurting power, more like 1.55:1.

Check the rod ratio in a Honda. Airflow

demand in the 4-valve engine occurs

closer to 70 ATDC.

Another flow difference between

2-valve and 4-valve heads is the ratio

of exhaust flow to intake flow. Exhaust

valves and ports are always made

smaller that the intakes, because exhaust

flow gets pushed first by high cylinder

pressure, then by the piston on the way

up. In 2-valve engines the exhaust port

flows between 60% and 80% of the

intake. Exhaust flow in 4-valve engines

is very high, somewhere in the 80% to

90% region. Later well see how these

factors affect cam selection.

Compression RatiosIt is important to realize that the enginesees three different compression ratios.One is the static ratio which we are allfamiliar with: clearance volume + sweptvolume, divided by the clearance volume.A number like 9:1 is a common staticcompression ratio.

The second is the effective compressionratio, which the engine sees when theintake valve closes against the valve seat.A number like 7:1 is common. This isdetermined by the interactions of thestatic compression ratio, the rod ratio,and cam timing for closing the intakevalve. (Wrist-pin offset has an additionalbut minor effect.)

Third is the dynamic compression

ratio which is when the engine is in the

peak power range and the volumetricefficiency is above 100% then thecylinder pressure-compression whenthe intake valve closes is at its highest,

example above 8:1. Building an engine for moreperformance often means raising thestatic compression ratio close to10:1, but keeping the effectivecompression ratio not much over 7:1.Anything lower gives up power. Anythingmuch higher will not run at low speedwith WOT on pump gas withoutdetonating and destroying itself.

Head ExaminationBefore selecting a cam, long beforechanging anything, take benchmarks

to determine what a production head

LEFT: Intake Stroke valve overlap.

Figure out what the

engine wants to do.Give it the cam(s) thatmake that happen.

ABOVE: Maximum piston velocity



PERFORMANCE CAMSHAFTSBY DIMITRI ELGIN

already provides, feature by feature.Trust no published specifications.Measure everything. What diameter andwidth are the valve seats? What angle?How did the factory finish the top andthroat cuts? Is there a good radius behindthe seat to direct airflow through theopen valve curtain? Does the backsideradius of the valve complement that?

Which way does the port aim mixtureinto the cylinder?2-valve heads direct

flow around the circumference of thecylinder in a motion termed swirl.4-valve heads send flow in head-over-heels tumble down the bore. Too little ortoo much of either motion stalls flow. Isthe mixture aimed dead into the cylinder,around its circumference in a swirl, orsplat up against a cylinder wall? HelloBBC.

Is airflow shrouded after it passesthe valve by running into a side of thecombustion chamber?Was the chambercast with some intentional shrouding?Check a SBC Vortec chamber. GM did

something clever there for guiding flowpast the valves. Is the spark plug wellout of the way of the incoming air/fuelcharge, or does it look as if it will besoaked silly before it attempts to fire?Will that affect how plugs should later beindexed? Look deeper down the ports. Do themachined cuts blend smoothly intoas cast surfaces? Do lumps or ridgesprotrude where casting cores once almostaligned? Feel inside. How sharp is theturn to the critical short-side radius? Didcasting cores leave a sharp edge there to

turbulate and stall flow?

What is the overall shape of the port,both lengthwise and in cross-section?Does the port narrow gradually along itslength to accelerate airflow, or changesize suddenly? Does it address the portfrom a high angle and turn smoothly intothe bowl area, or does it send airflowalong a long flat trip across the head thendemand a sharp drop at the valve? (Hithere, Jaguar XK.)

Is the cross-section square with

dead corners of zero flow, or perfectlyround so flow spins this way and thatbut not in?Is the exhaust port just abig hole in the head? (Hey, Mopar B.Meet Ford Cleveland.) Where does thecross-sectional area of the port becomesmallest, and how small is that? A newskill at casting with latex can bringthat shape outside for more direct andinsightful analysis.

What does the finish of the chamberlook like? It does not have to be polished(although doing that does not hurt), butsurfaces should be reasonably smoothwith no sharp edges. More power makes

more heat. Heat seeks peaks. Gentlybevel sharp edges, or the engine will pre-ignite off its own internal glow plugs.

Go with the Flow BenchSo far we have examined heads much asany avid gearhead would on his garageworkbench. We now know what theylook like. But how do they flow? Takethem to a flow bench. A flow bench is the measurement toolto analyze airflow through the ports. Abench can now be used in three modes.First, to measure the airflow rate through

each port as a function of valve lift.

Then to probe inside ports to analyze thedetails of airflow. Finally and this isvery new attach a wet-flow adapter tothe bench and actually see the dyed flowleaving the port and entering the cylinder.All three procedures add up to help youdecide how to address these heads. Air flows through intake ports into anadapter exactly the same diameter asthe cylinder and two bore lengths long.Air flows from the chamber through

exhaust ports into a stub stack thesame diameter as the header pipe and atleast eight inches long. Part of the funof operating a flow bench is getting tofabricate and inventory an interestingassortment of adapters.

Take airflow measurements in cubicfeet per minute across the entire range ofvalve lift, at every increment of .050.Continually adjust the flow bench so itkeeps working at the same depression,the working pressure difference throughthe port, to keep your measurementsmeaningful. For later comparison with

other heads or your modificationsto these, always work with the samedepression.

The SAE recommends a depressionof 28 of water (about 1 psi). Someexperienced engine developers use less,maybe 16. A few use more. Careful.Depressions of much more than 40produce falsely optimistic flow numbers.Truly bad ports can be made to flowgreat numbers at unreasonably highvacuum. But later theyll die on theengine, and that customer will be veryunhappy about your services. As with a

dyno, never rig the system just to make

Critical choke areaof intake port.

2 valve 5.0L V8 vs. 4 valve 2.3L 4 cylinder




big numbers. Its pointless. Accuracy isan operator skill, painstakingly acquired.And findings vary from bench to bench,even among the same model, by as muchas 10%. Conversion factors for readingsat different depressions are not reliable.So dont bother racing somebody elses

flow numbers. Believe what you see.Measure every port. If one port flows

less than the rest, that cylinder willneed different spark timing from theothers, and total spark advance willbe compromised for the weak cylinderrather that set for best power fromthe good ones. Low to mid lift is veryimportant on the exhaust valve. Mid tohigh lift is more important to the intake.But measurements of intake flow at liftas low as .050 is also important. Thatgets flow going and also accepts thefinal pulse of mixture arriving by inertia

before the valve closes at high rpm.Findings should be tabulated for later

reference, but for analysis graph all theflow numbers as a function of lift. Trendsjump out at you from graphs. At whatlift does the rise in flow level out on theintake and exhaust sides? What is theratio between intake and exhaust flows?The ratio should remain quite steadyacross the range of lift. If not, there isan opportunity to find power. I preferto port most heads to achieve exhaustflow 75-80% of the intake. Exhaustflow above 90% may make power at the

drags, but a lot of intake charge goessideways out the exhaust valve. Fueleconomy suffers and so does torque.

Experiment. Go a bit beyond yourexpected maximum lift, to find outwhat happens there. Try a valve with adifferent shape to its backside radius.See how much a clean back-cut improvesflow. If you have a surplus (say, cracked)head to play with, try different seatangles, widths, and multiple top andthroat cuts. Notice how a 30 seat flowsgreat at low lift but dies at high lift.Radius the top edge of the exhaust valve

margin, and compare flow numberswith a square margin. If the customerdemands bigger valves, try just one firstto see where this program is headed. What the bare head tells you is abaseline. Now attach the intake manifoldand carburetor (or FI intake and throttlebody). Everything changes. I have seena loss of 10-60 CFM after the intakesystem was installed, on cylinderheads properly ported with valve jobcompleted. After the intake was installedI have also seen great variations betweenflow numbers among different intake

ports, as much as 20 CFM. Most CFM

numbers quoted are without the intakemanifold installed. This is mistakingthe mission. It is flow through the entiresystem that the engine sees. Correctionshere pay off big.

Next, probe the flow with a Pitotwand. Find pressure differences, flowvelocities, dead spaces, and intriguingmysteries. Does the smallest cross-sectional area of the port restrict the airflow, creating turbulence, or is the portso large there that very little velocity

develops down the port? Is the intakeport efficient all over its area, or is thefloor and one corner dead? When youfind a dead space, fill that with clayand see what happens next. If its good,epoxy the solution in permanently. Checkalong the floor of the port on the short-side radius. Does flow follow the curve,or is it breaking off? The very latest bench equipment cannow view wet flow, dynamically and indetail. This is an extremely promisingprocess. Airflow is one thing, but arunning engine flows air wet, mixed

with gasoline in a ratio around 14:1. Wetflow more closely approximates whathappens inside engines. And for the firsttime you can watch dyed mixture flowinto the cylinder or better videotapeand freeze-frame it across the range ofvalve lift.

Wet flow is a powerful tool foranalyzing mixture motion into cylinders.Testing has shown that the flow througheach intake tract must be individuallymatched as a partner to its port so thateach cylinder receives the same amountof mixture and close to the same degree

of mixture motion. If some cylinders are

weak from poor motion, spark timingamong cylinders could vary as muchas 6. Timing the engine for the poorcylinder loses power. Timing set for thebetter cylinders burns the piston in theweak cylinder. Wet benches map the flow vortexesinside the chamber. Even the big boysare learning from this. Chryslers newhead for Pro Stock couldnt win. A wetbench discovered a huge vortex soakingthe plug. Chrysler reshaped the chamber.Dart, the respected maker of aftermarkethigh-performance heads, now wet-flowstheir designs.

Engine Cyclesand Cam TimingOnce you have the particularperformance characteristics of the headscarefully mapped, use that informationto select the one cam that makes theengine make power like it wants to.Compare your test results against whatthe engine experiences while it goes

through all six cycles. Engine cycles are determined by thedirection the piston is traveling and thetiming of the openings and closings of thevalves collectively termed valve-timingevents. Timing these events becomescomplicated because a lot of compromisemust be made in order to balance out allof engine operating cycles. Ill walk you through each of the sixengine cycles of a four-stroke enginefrom the viewpoint of determining howchanging every valve-timing event affectsother cycles, and how balance builds

power.

CompressionStroke (Static,effective,and dynamiccompressionratios)




TDC to Exhaust Valve OpeningBy the time the crankshaft reaches 90ATDC, cylinder pressure has droppedgreatly and most of the power thatcan be recovered from it already hasbeen. So opening the exhaust valve well

before BDC loses less power from thepower cycle than it later gains across thefollowing cycles. The lower the rod ratio,the faster cylinder pressure drops.

Exhaust Valve Opening to BDC

The blowdown cycle relieves excess (butunrecoverable) cylinder pressure andbegins clearing exhaust gases off theenergy of their own pressure. Otherwisethe piston would have to push all theexhaust gases out of the cylinder on thenext up-stroke, lowering horsepower

from a pumping loss. The timing for Exhaust Opening is theleast important of the four valve events.It can be anywhere between 50 and 90BBDC, so its timing is easily adjusted tomatch the performance characteristics ofthat engine. With higher compression ratios theburn rate is faster, so the exhaust valvecan be opened earlier, which aids inthe cylinder blow-down. With lowercompression ratio (static 8:1 or lower)you want to delay the exhaust openingas late as possible in order to utilize thelast usable bit of pressure that is on topof the piston. But that hurts the top endhorsepower, because the blow-downperiod is no longer as effective.

BDC to Intake Valve Opening

The piston reaches maximum velocity atabout the same number of degrees BTDCas it did ATDC on the way down, ora degree or so sooner with offset wristpins. The exhaust valve must be opensufficiently by this time so that spentgases in a hurry meet little resistanceagainst being pushed out.

How far the valve must be open isknown from flow-bench data. The propercam meets that need from a combinationof timing, total lift, and its rate of lift (itsvelocity).

Opening to Exhaust Valve Closed

The scavenge cycle occurs during theoverlap period, when intake and exhaustvalves are both open at the same time.The intake valve is just opening. Theexhaust is closing but not yet seated.Overlap is what the cam and valves aredoing, dictated by the combination of

total cam duration and the locations

of lobe centers. Scavenging is what theengine is doing with that.

A good number of engine processes(and a few unsolved mysteries) aregoing on now simultaneously. The mostimportant are (1) scavenging the last ofthe exhaust gases as much as possiblefrom the clearance volume, where thepiston cannot reach to push them out,and (2) initiating intake flow into thecylinder without wasting very much of itout the open exhaust valve.

Overlap duration increases as totalduration increases, and it also increasesas the lobe center decreases. Increasingthe time for Overlap makes more timefor scavenging at high rpms. Residualexhaust gases kill power twice over:they displace their volume in incomingcharge, and later during combustion theyabsorb heat that should have gone intomaking power. At 5000 rpm an enginewith a high-performance cam carrying55 degrees of overlap must complete theentire scavenge cycle in less than twothousandths of a second.

In standard engines, valves are opentogether for only 15-30 degrees ofoverlap. In a race engine operatingbetween 5000 and 7000 rpm, the overlapperiod is more like 60-100 degrees. Thepenalty for so much overlap in a streetengine is very poor running at lowerrpms, when a lot of the intake chargehas time to sidetrack directly out theopen exhaust valve. Mileage goes South.Heads overheat from fuel burning inexhaust ports. The engine runs hot.The exhaust system gets fueled like ablowtorch. The tailpipe turns white.

Catalytic converters fry. The buyer

blames the cam grinder.Timing Exhaust Closing must bebalanced against flow through the intakeport. If the intake port flows poorly frombeing too small (or too large) then laterExhaust Closing might help to initiateintake flow. I consider this only as a lastresort for kick-starting a lazy intake port.It always carries some charge out theexhaust valve, wasting fuel and all that.Make the overlap period as short as willcomplete the job of scavenging. Factor in

the effects from the combustion chambersize and shape (including the shapeof the piston top) and shrouding nearvalves. Balance power goals with otherrequirements for the intended usage,such as idle quality, low-speed throttleresponse, fuel economy, and smog testcompliance.

to Intake Valve Closed

I consider Intake Valve Opening thesecond most important valve timingevent, because that does two important

jobs. (1) It initiates the Scavenge Cycleand (2) it begins lifting the intake valveout of the way of the incoming charge.The air/fuel mixture began entering thecylinder during the Scavenge Cycle,builds to a maximum, tapers off, thenpacks in a final gulp.

The intake valve is in a race with thatpressure differential at maximum pistonvelocity that drives intake flow. The valvealways loses this race, because max drawhappens between 70 to 80 ATDC, yetthe intake valve does not open fully untilit reaches centerline, down around 105

to 115 ATDC.

6 Engine Cyclesand Cam Events


6/658 OCT-DEC 2010 engine professional


When you cant win, do your best.Get the valve out of the way as far aspossible by giving it a fast rate of lift, ahigh velocity. Much the same could beaccomplished by more valve lift, but thenthe nose of the cam gets pointy and realstiff springs are needed for closing the

valve a combination not favorable tovery long service life.

The Intake Closed point when thevalve seals on the seat is the mostimportant valve-timing event. This eventgoverns both the engines rpm range andits effective compression ratio. Closingthe intake valve later optimizes intakeflow for high rpm and allows inertia topack in its last gasp of air. The drawbackto that is back-flow at low rpm. Butclosing the valve earlier shuts down rpm.Pick your operating range.

The piston compresses the air/fuelmixture to a high enough pressure andtemperature for it to be ignited efficientlyby the spark. The effective compressionratio must be high enough to compressand pre-heat the air/fuel mixture for afast, complete burn.

But too much heat and pressure kickoff the whole charge at once in thedestructive explosion of Detonation.When pistons taken from a blown engineshow ring lands melted as if by a cuttingtorch, that was by Detonation. (If a hole

has been blasted through the center ofthe piston crown, that came from a hotspot in the chamber pre-igniting themixture.)

Tweaking forIdosyncrasies,Nitrous, Supercharging,

TurbochargingPrepare the cylinder head and fit agood, small-diameter exhaust system.To increase the rpm band, increase thecompression ratio. I do not advocateextra high lift, long duration, or very

high compression in a street car. I usevelocity. I have never seen a normallyaspirated engine make more power bylifting beyond the flow capacity of thehead. Within limits, experienced cam makerscan juggle timing events to customizevalve action to the special requirementsof that particular engine. Careful here.Not all valve timing is equally important.Exhaust Opening may be re-timed withlittle impact elsewhere. But Intake Closeis tied closely to the static compressionratio, and cannot be re-timed very

far without upsetting the dynamic

compression ratio, cylinder pressure,resistance to Detonation, burn rate, rpmrange, and just about everything else thatmakes power. Intake Opening is slightlyless important, and Exhaust Close lessthan that. So juggle needs of the engineagainst the importance of timing events.

I find it amusing to see people treatthe 4-valve engine like 2-valve engineswhen they select cam durations andlifts. Timing and lift for the 4-valveengine must be made different, duea 4-valves quicker air flow velocityas well as its high ratio of exhaust tointake flow. Even with only moderatetotal cam duration, a cylinder under a4-valve head sees enough air flow by 75ATDC. For example, a high-performance2-valve engine for the street wouldneed 270-280 of total duration, buta 4-valve engine would require only250-260 total duration for equivalentperformance. Any more duration closesthe intake valves so late that the enginewould become very peaky, hardlysuitable for street driving. 280-290for the 4-valve engine would be the

equivalent to 310-320 on the 2-valveengine.

In a 4-valve engine the intake andexhaust cams can use the same durationuntil the intake cam gets into the 270-280 duration range. In some production4-valve engines, its good exhaust flowcan over-scavenge cylinders. Lessduration on the exhaust would help that.

Timing the Exhaust Opening eventshould be re-examined whenever anengine begins using a supercharger,turbocharger, or nitrous oxide. Be carefulwith a 2-valve supercharged engine. The

extreme pressure still in the cylinder

can bend the valves and pushrod if theexhaust valve tries to open too earlyagainst it. Turbocharging requires widerlobe centers to narrow the overlapperiod. Nitrous responds to slightlywider lobe centers and more duration sothe exhaust valve opens earlier to relievethe higher cylinder pressures nitrousgenerates. Pick you power goal. Balancethe package for it. I hope that this information will helpyou better understand this very complex

internal combustion engine. Happytuning!

For a glossary on camshaft terminology,visit www.elgincams.com.

Dimitri Dema Elgin began working in the area

of automotive technology in 1957 and opened

Elgins Machine Shop in 1960. In 1985, he sold

the shop to his employees so that he could de-

vote his attention to his camshaft business, now

called Elgin Cams. With the aid of computers,

Dema makes his own camshaft designs. Some

of his best customers include General Motors,

Ford Motor Company, Nissan Corporation,

Zakspeed International, Porsche Motor Sports,

and Winston Cup engine builders. In his spare

time, Dema travels the country giving speeches

on cams and camshafts. He also teaches at

DeAnza and Skyline Colleges, Automotive Tech-

nology Classes. For more information, call (707)

545-6115 or go online, www.elgincams.com.

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Performance Camshafts EP10-2010!50!58