# CAM BASICS, TERMS, & HOW TO CHOOSE ONE



## PontiacJim (Dec 29, 2012)

I put this lengthy piece together to make understanding the "camshaft" a little easier for everyone. I pulled a lot of this info from my collection of books, magazines, and the internet. *I am no cam expert.* I am not an engine expert. I don't own a machine shop, a flow bench, or engine dyno. I know a lot, but don't claim to be an expert in anything. I am a hobbyist like most of us, but good at researching & reading, and have a nice collection of Pontiac info, books, magazines, and DVD's I have collected over the past 40 years. Plus all kinds of other printed materials because I like all cars and big HP.

*Part 1 - Cams Explained*

The cam can be considered the heart or the mind of a performance engine where characteristics of the profile shape and its lift can be critical to performance. No single cam can do it all, so when building your engine, you have to decide the best RPM range you plan to use the most, or where you want the best power band to be. Cam selection can be matched to the transmission and rear gear ratio's so if the cam is weak at the bottom RPM range, gearing can pick up the difference and get the engine up into the cam's power band and then let the engine do its job.

This discussion is aimed at flat tappet cams but in general do apply to roller cams. I have never run a roller cam and am not against them, just prefer a flat tappet cam and don't put together an engine build where I feel I can benefit from them - I also build on a budget; roller cams and related parts are out of my budget. Roller cams/lifters can be spec's differently due to their faster opening/closing lobe contours and their ability to hold the valve open longer at peak lift - all of which can mean more power if we compared a flat tappet cam to a roller. The roller cams don't require a "break-in" period like flat tappet cams and are not subjected to the issues of "low ZDDP levels" in conventional oils - which doesn't seem to be the issue it used to be, and there are additives if you feel the need. Flat tappet cams require splash lubrication and I feel some may go overboard trying to maximize the thinking of "keeping oil off the crank provides more HP" and limiting oil flow to the "upper end" via restrictors in the lifter bores - all of which can cut down on splash oiling (BUT, you can use a tool to cut a groove in the lifter bore for more oiling to the cam, purchase Rhoades lifters with the "Super Lube Groove" cut in them, or get EDM lifters with the small hole laser cut into the base to provide oil to the lobe).

The down side of a roller cam set-up is the expense, the need for much higher spring pressures which generally mandate upgrading rocker arm studs/rocker arms, stronger timing chain & gears to avoid excessive stretching from the higher spring pressures, matching distributor gear for use with a roller cam, new shorter pushrods (which don't spin and may produce uneven wear at the pushrod cup with high mileage use (?)), roller/needle bearing failures, link bar failures, and the side loading of the lifter bores when some aggressive lifts are used - which can cause cracking/breaking of the lifter bores, and some brand lifters seem to be noisier than others.

Many factors within the engine come into play when selecting a cam, either flat tappet or roller - cubic inches, bore, stroke, rod ratio, compression, piston material, piston/deck height, RPM range, head type - closed/open chamber/iron/aluminum, head flow, intake, carb, exhaust, and ignition are some things to consider while matching the components to one another.

Be aware of piston-to-valve clearances, rocker arm geometry, and knowing the limits of the valve springs and rocker arm studs when going with a larger than stock cam. Broken parts or an engine is not what you want.


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## PontiacJim (Dec 29, 2012)

PontiacJim said:


> I put this lengthy piece together to make understanding the "camshaft" a little easier for everyone. I pulled a lot of this info from my collection of books, magazines, and the internet. *I am no cam expert.* I am not an engine expert. I don't own a machine shop, a flow bench, or engine dyno. I know a lot, but don't claim to be an expert in anything. I am a hobbyist like most of us, but good at researching & reading, and have a nice collection of Pontiac info, books, magazines, and DVD's I have collected over the past 40 years. Plus all kinds of other printed materials because I like all cars and big HP.
> 
> *Part 1 - Cams Explained*
> 
> ...


*Let's start out with CYLINDER PRESSURE:*

Today, the first component you should pick when you build your engine is compression ratio. Available octane ratings are more critical when building your engine due to the unleaded factor and ethanol blends. Once you have narrowed down a compression ratio choice, the camshaft should be your next selection based on the compression, head flow (CFM's), and the power band you are looking for, and then you can broaden that into horsepower & torque levels. With a cam choice in mind, you can then select your other engine parts, to include your drivetrain choices, that will compliment the cam and not fight it. If you purchase your engine parts first and they are not matched, they could turn out to be a very poor combination when selecting a cam and you may have to be willing to change some of these components or run the risk of having a disappointing engine. The best engine combination is when the desired torque & power band of all the components (camshaft, cylinder heads, intake manifold, carburetor size, compression ratio, headers and exhaust size) are matched to work in the same RPM range that you have chosen for your needs. A low RPM torque cam with a 2 x 4 tunnel ram intake is going to be a bad combination. A .550" lift with stock heads will be a bad choice. Selecting the wrong carb, intake, exhaust manifolds, or even the drivetrain components for your build may not be as obvious - until you get in the car and drive it.

Cylinder Pressure is an important aspect in selecting a compression ratio and a cam and something most do not consider when choosing their cam. The engines compression ratio (called static compression) creates cylinder pressure. The timing of the intake valve closing, coupled with the static compression, gives us a cylinder pressure called "Dynamic Compression" and will be covered later. The limits of cylinder pressure are based on the octane of the fuel you are using where the octane number indicates how much cylinder pressure the gasoline will tolerate without exploding. You want a very fast controlled burn and not an explosion. Exploding fuel is called detonation and it can destroy engines and parts if not corrected quickly.

Cylinder pressure is a result of 5 things, the final displacement of the engine, the compression ratio of your engine, cylinder head material whether iron or aluminum, the closing point of the intake valve, and the air density or altitude where you live.

You want to determine exactly what your actual compression ratio is or what it will be when building your engine. The higher your compression ratio, the higher the cylinder pressure. The lower your compression ratio, the lower the cylinder pressure. Using the factory or manufacturer's advertised compression ratio can be very misleading because the true compression may be actually lower. This is true in the Pontiac world as the factory advertised compression is actually lower in reality.

The engine's static compression ratio is determined by adding up the following numbers: the bore & stroke, the combustion chamber volume in cc's, the valve reliefs/dish/dome in the piston top in cc's, the piston's deck height or the space between the top of the piston at top dead center (TDC) and the top of the block's deck in cc's, and the head gasket bore & thickness in cc's. It seems that as a rule of thumb that an engine's compression should be between 9:1-9.5:1 to run on pump gas with iron heads, but of course this can vary depending on the build and your altitude.
Cylinder pressure equals heat and the more pressure the more the heat and the higher the octane you need to use. The type of material the cylinder heads are made of, such as iron or aluminum, will affect combustion heat and thus cylinder pressures. Aluminum heads draw out the heat from the combustion chamber much faster than the iron head and this in effect reduces the cylinder pressure. A general rule-of-thumb is that whatever the maximum compression is that you can use with the iron heads, raise it at about 1/1.5 points when switching to aluminum heads.
The compression stroke is what makes cylinder pressure. It begins right at the end of the exhaust stroke as the intake valve begins to open before top dead center(TDC), then draws in the air/fuel mixture on the intake stroke as the piston travels down the cylinder towards bottom dead center(BDC), and then begins to go up on the compression stroke. The compression stroke cannot start until the intake valve closes and seals off the air/fuel mixture which is at some point after bottom dead center (ABDC). The closing point of the intake valve after bottom dead center determines when the piston actually begins to build cylinder pressure as it travels up the cylinder on the compression stroke.

As a cam gets bigger(longer duration) the intake valve stays open longer and closes later to fill the cylinder with as much of the air/fuel mixture as possible. But, as the cam duration increases and keeps the intake valve open longer to maximize the air/fuel mixture, the piston goes past its lowest point at bottom dead center and begins to moves up higher in the cylinder bore on its compression stroke where at some point the intake valve is closed. This effectively reduces the compression stroke (or the squeezing of the air/fuel mixture with the intake valve closed) making the compression stroke shorter which in turn reduces the cylinder pressure. This is why cam manufacturers recommend a higher static compression ratio to counter the reduced cylinder pressure of the later closing intake valve on a long duration cam such as those cams having 280 degrees of duration or more. This type of long duration cam which reduces cylinder pressure may not work with an already low compression engine. Also, as cam duration increases, the power increases and the torque & power band also moves up in the RPM range.

The other side of the coin is that the shorter duration cams close the intake valve earlier after bottom dead center which puts the piston further down in the cylinder bore and closer to the bottom of the engine's stroke by the time the intake valve closes. This traps less of the entering air/fuel mixture in the cylinder but the earlier intake closing effectively makes the compression stroke (or the squeezing of the air/fuel mixture) longer and increases the cylinder pressure. This type of cam can build cylinder pressure and is better suited for the lower compression ratio's. The smaller the duration number, the lower the RPM that the torque & power band will be. Here it can be tailered for such uses as RVs, towing, stock engines, etc..

With a given static compression ratio, you will see a higher reading on a compression tester gauge when using a stock or low duration cam because the intake valve is closing earlier on the compression stroke. The resultant longer effective compression stroke always delivers a higher gauge reading. Install a longer duration cam and the intake valve closes later and you will see a lower reading on a compression tester gauge because of the shorter effective compression stroke. But, the longer duration cam will make for a higher compression effect at higher engine RPM's, but at the lower RPM speeds and especially at starter-cranking speeds, the effect will be a lower compression. So in doing a compression check of the engine's cylinders, it is not as important to know what the compression pressure is, but rather, the consistency or differences seen between cylinders.

In general, a big cam = lower cylinder pressure, and a small cam = higher cylinder pressure. One cam grind will not do everything so you may have to make a compromise based on what you want your engine/car combo to do. If you want to drag race with a cam that pulls at 6500 rpm or more, don't expect the engine to operate on the street very comfortably. If you want to tow a camper, don't expect the engine to spin 6,500 RPM's.

Dynamic Compression. As noted ealier, at BDC, the intake valve is still open as the piston is rising up the bore on its compression stroke with no actual compression occurring because of the open intake valve. Compression does not begin until the intake valve closes(IVC). Once the intake valve is closed, the air/fuel mixture starts to compress. The dynamic compression ratio(DCR) is expressed as the ratio between the volume of the cylinder area above the piston once the intake valve closes and the volume above the piston(the static compression ratio(SCR) at top dead center(TDC). The DCR is what the air/fuel mixture compression ratio actually is. It is often lower than the static compression of the engine. In short, the DCR is dependent upon the intake valve closing point along with the static compression ratio - cam specs have as much effect on the DCR as does the mechanical specifications of the motor.

DCR is not an absolute, just a tool to use in better selecting/matching your cam to your compression and vice versa. The characteristics of an engine combo running at high speed changes the engines volumetric efficiency which will have a major effect on cylinder pressure(as would nitrous, a supercharger, or turbo). The DCR is more applicable to street and street/strip cars where much of the daily driving is at lower engine RPM's. A good rule of thumb is to have the engines DCR in the range of 8-8.5:1 and that can be dependent on iron or aluminum heads and altitude (I feel a street engine with iron heads should be at 8:1 or lower). Higher than this, there may be detonation problems with pump gas. Engines with “small” cams may do better with a lower SCR to avoid detonation while engines with “big” cams that have a later IVC point may tolerate a higher SCR. When race fuel is used, much higher DCR (and static CR) may be used because of the detonation resistance of the fuel. Several Dynamic Compression Ratio calculators can be found on line if you want to play around with these numbers.

Air density, or altitude, can be overlooked and has an effect on cylinder pressure. It is not often considered when building an engine. Air is thinner the higher above sea level you go. Less air going into the cylinders means less cylinder pressure at top dead center when the spark plug fires. It’s a lot like lowering the compression ratio in the engine. Cylinder pressure starts to really become affected when the altitude begins to get around 1200’ – 1500’ above sea level and at these higher altitudes the engine may need additional compression to increase the cylinder pressures needed to take advantage of the 91 octane gas which would not be needed at and altitude of 1,000' or less. So knowing the altitude at which you live and deciding where the car might be driven could cause some adjustments in your engine's compression or camshaft selection to build more cylinder pressure. This may also explain why you read how one guy runs 10.5:1 with no problems and a similar build has "pinging" with 9:1.


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## PontiacJim (Dec 29, 2012)

PontiacJim said:


> *Let's start out with CYLINDER PRESSURE:*
> 
> Today, the first component you should pick when you build your engine is compression ratio. Available octane ratings are more critical when building your engine due to the unleaded factor and ethanol blends. Once you have narrowed down a compression ratio choice, the camshaft should be your next selection based on the compression, head flow (CFM's), and the power band you are looking for, and then you can broaden that into horsepower & torque levels. With a cam choice in mind, you can then select your other engine parts, to include your drivetrain choices, that will compliment the cam and not fight it. If you purchase your engine parts first and they are not matched, they could turn out to be a very poor combination when selecting a cam and you may have to be willing to change some of these components or run the risk of having a disappointing engine. The best engine combination is when the desired torque & power band of all the components (camshaft, cylinder heads, intake manifold, carburetor size, compression ratio, headers and exhaust size) are matched to work in the same RPM range that you have chosen for your needs. A low RPM torque cam with a 2 x 4 tunnel ram intake is going to be a bad combination. A .550" lift with stock heads will be a bad choice. Selecting the wrong carb, intake, exhaust manifolds, or even the drivetrain components for your build may not be as obvious - until you get in the car and drive it.
> 
> ...


*Part 2 - Cams Explained*

The RPM at which maximum horsepower and torque are developed increases as a cam’s duration is increased. The RPM at which maximum torque is developed in a factory Pontiac 400 increases 1,000 RPM from the 066 cam to the 041 cam. This means that the strong torque range is moved from the lower RPM range to the higher RPM range. As a result, there is very little torque left in the lower RPM range with the 041 and why 3.90 and 4.33 rear gearing was needed to accelerate the car. Engine vacuum drops, idle quality suffers and needs to be increased to keep the engine running, and both initial timing and the advance curve need to match the cam. But, install this cam in an engine with larger cubic inches and more CFM's, it can be a more docile cam and less of a "race" cam.

On a stock Pontiac 400CI, increasing valve lift up to about .470 inches will generally increases torque, but does not alter the power range providing other parameters remain the same.

Once a cam such as the 066 is installed in a 400 CI engine, the maximum torque increases very little. As the factory cams increase in duration/overlap, the maximum torque occurs higher up in the RPM range. 066 cam - 445TQ/2,900 RPM; 067 cam - 445TQ/3,000 RPM; 068 cam - 445TQ/3,600 RPM; 744 cam - 445TQ/3,800 RPM; 041 cam - 445TQ/3,900 RPM.

Looking back a bit in cam history, the "early" days of cam grinding and cam grinds used the familiar term "three-quarter cam" to describe a high performance cam grind. It is in essence a generic term that covers a cam grind that you would select from your favorite cam grinder/supplier. The cams were essentially broken down into 4 groups in generic terms and then you selected a cam grind spec that suited your build. It is not unlike the terms of today - street, street/strip, strip, and race.

"Since a cam cannot be be ground to give top performance at both ends, the rodder must choose from any one of four or more basic grinds the type of cam that will suit his needs best. He can choose anything from mild to a radical grind with the basic grinds being: semigrind, three-quarter grind, full-race grind, super full-race grind, and special track grind. Most cams are reground from stock contour."

*SEMIGRIND:* The most conservative grind. Good for coupes, sedans, and trucks. Increases the power and speed without sacrificing good idling quality and low speed performance. Especially ideal for passenger cars, the semigrind will give and increase in general performance and acceleration, and may be used with or without other speed equipment. Grinds are usually spec'd as intake valves open/close 15 degrees BTDC/40 degrees ATDC, exhaust valves 50/10. Generally provides 10% HP more over stock.

*THREE-QUARTER GRIND:* A cross between the semigrind and full-race grind (thus the term "three-quarter" cam), the three-quarter grind sacrifices a certain amount of low speed performance in order to give you top speed and better acceleration. Idling characteristics are not as good as those of the semigrind, but once revved up past the idling point, performance is excellent and smooth. For best results with this grind, the engine should have high compression and dual carburetion. Being a compromise between the semigrind and full-racer, it can be used in either street or competition cars. The valve specs typically on the intake valve is 25 BTDC/50 ATDC degrees, exhaust valve 55/15. Raises HP 15-20% over stock.
FULL-RACE GRIND: An all-out cam for moderately bored and stroked engines, peaks at approximately 5,200 rpm, is designed for high speed work in the 50 to 120 mph range. Best for use in competition engines having high compression heads and dual manifolds. Intake valve timing is 30 BTDC /65 ATDC, exhaust 65/25. At least 20% HP over stock.

*SUPER-FULL RACE GRIND:* Same as the full-race grind except that the duration of the valve opening is greater to take full advantage of the additional displacement in oversized bores and lengthened strokes. It idles rougher and has poor low speed torque. Strictly for competition. Intake valve timing is 35 BTDC/75 ATDC, Exhaust 75/30. 25-30% more HP due to cam alone - additional engine modifications can raise HP output. Peak torque comes in around 3,000 RPM's and is not recommended for street.

*SPECIAL TRACK GRIND:* A special grind developed for short track roadster and stock car racing, it has much faster action than the average V-8 cam. Has a high rate of acceleration with longer duration, broader power range and combines low speed torque with excellent high speed characteristics.


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## PontiacJim (Dec 29, 2012)

PontiacJim said:


> *Part 2 - Cams Explained*
> 
> The RPM at which maximum horsepower and torque are developed increases as a cam’s duration is increased. The RPM at which maximum torque is developed in a factory Pontiac 400 increases 1,000 RPM from the 066 cam to the 041 cam. This means that the strong torque range is moved from the lower RPM range to the higher RPM range. As a result, there is very little torque left in the lower RPM range with the 041 and why 3.90 and 4.33 rear gearing was needed to accelerate the car. Engine vacuum drops, idle quality suffers and needs to be increased to keep the engine running, and both initial timing and the advance curve need to match the cam. But, install this cam in an engine with larger cubic inches and more CFM's, it can be a more docile cam and less of a "race" cam.
> 
> ...



*Part 3 Cams Explained *

MODERN DAY CAMS and a look at the terms used.

*What is a dual-pattern cam? * A dual pattern cam's exhaust lobe is typically ground 6 to 12 degrees larger, may have more lift, or both, as compared to the intake lobe. But if the intake port and exhaust port CFM ratio is known, well known cam designer UD Harold states that "if the ratio is under 70% exhaust-to-intake ratio, I use an 12 degree bigger exhaust cam. If the ratio is around 75%, I use 8 degrees, and around 80%, only 4 degrees." The theory is that the additional duration or lift of the exhaust valve will allow the dual pattern cam to make similar high RPM horsepower compared to a single pattern cam that is somewhat larger. Poorly designed exhaust ports on many stock cylinder heads can be improved by simply adding more duration. This can improve exhaust port flow and the scavenging of exhaust gases which increases engine power. The smaller duration used on the intake side should give the engine better off idle response.

*What is a single pattern cam? * A single pattern camshaft has the same lift and duration on both the intake and exhaust lobes. If your heads have a good intake/exhaust flow ratio (which stock Pontiac heads do not), then utilizing a single pattern cam will make more torque in the lower to mid RPM range as opposed to a dual pattern. When the CFM ratio of the intake to exhaust flow gets around 85% or more, a single pattern cam can work well. The use of a single pattern cam can also depend on how much backpressure the exhaust system has. A good flowing exhaust system such as open headers might do well with a single pattern, but with a restrictive system it may be better to select a dual-pattern cam that has more exhaust duration to get the spent gases out.

Here are the _Camshaft Terms_, following along with the areas of the lobe as it rotates and lifts the hydraulic or solid lifter. This operation works for both the intake and exhaust valves.

*Base Circle:* This is the back side of the cam lobe opposite the highest point of the cam lobe. This is the rounded area of the lobe where the valve is closed and no lift is provide.

*Lobe Ramps *- This is the area of a camshaft lobe that actually starts the lifting and the descending movement of the lifter. Ramps include the clearnance/lash ramp, the opening ramp, and the closing ramp. Camshaft lobe ramps are ground to have different rates of lifter movement in terms of velocity and degrees of duration, as measured in degrees of crankshaft rotation.
Following the lifter around and off the base circle of the lobe, the "clearance ramp" or "lash ramp" of a camshaft lobe is the small area that starts to move the lifter off the base circle onto the curvature of the cam's lobe. The clearance ramp takes up the play/lash in the lifter.

Following the clearance ramp is the "opening ramp" of a camshaft lobe is the point where the lifter begins to raise up and move just after the lifter's play/lash has been taken up by the clearance ramp. The opening ramp continues right up to the cam lobes highest lift point, or "nose" of the cam.

*Nose of the Cam Lobe: *The Nose of the cam lobe is the top area/part of the lobe that holds the valve fully open at its maximum lift as it transitions from opening to closing. The designed shape of the nose is much different between a flat tappet cam and a roller cam. And the designed shape can be made in different configurations from a more "peaked" lobe to one that is seemingly "flat" across its nose. The nose provides the total lift based on the cam's lobe grind, not the lift seen at the valve because different factors can give different valve lifts such as a 1.5 rocker arm ratio versus a 1.8 ratio, a longer or shorter pushrod, and the hot/cold lash settings of a solid cam. Keep in mind that the engine sees valve lift, not lobe lift.

*Closing Ramp:* Once the lifter has reached its peak on the nose of the cam, which is also the highest opening point of the valve, that valve has to be closed with the lifter now traveling back down the side of the lobe that lowers the lifter. This is called the "closing ramp." The closing ramp returns the lifter back to the lobe's base circle where no lift occurs and the valve is completely closed. Then the cycle begins again, opening and closing the valve.

*Asymmetrical Lobe: * is a camshaft lobe profile/design where the opening and closing ramps are not exactly the same. The reason some camshafts are this way is to try to achieve an opening ramp profile that has a high velocity and a closing ramp profile that has a slower velocity so it does not slam the valve shut. In this way the valve can be set down more "gently" than the rate at which it was first opened. Some cylinder head configurations may favor a slow opening intake or may need a slower closing exhaust to optimize airflow. Almost all modern hydraulic profiles are asymmetric. Very few cam grinders believe that the opening and closing sides should be exactly the same.

*Duration: * Longer duration keeps the valves open longer allowing more air/fuel or exhaust gases to flow at higher engine RPM speeds. It works out that increasing the duration of the camshaft by 10 degrees moves the engine's power band up by about 500 rpm.

Short duration with a wide separation angle might be best for towing, producing a strong, smooth low-end torque curve.
Moderate duration with wide separation angle might be best suited for an all-around street performance engine, producing a longer, smoother torque band that can still breathe well at higher RPM.
Long duration with a short separation angle might be suited for high-rpm drag racing, with a high-end, sharp torque peak.

Longer duration cams generally need more static compression.

*Intake Valve Opening and Closing Points: *What affect does the opening and closing points of the intake lobe have on performance? The most important point in the four-stroke cycle is the intake closing point. The timing of intake opening and closing determines the cam's total duration. The intake closing point is a big determiner in where the engine makes power. The intake opening point is also critical to vacuum, throttle response, gas mileage, and emissions.

An early Intake Valve Closing (IVC) improves low-speed torque, but limits high-RPM power since it also limits time for cylinder filling. Early intake opening can allow exhaust gasses back into the intake manifold and hurting the intake pulse velocity and contaminate the air/fuel mixture charge. But as engine RPM's increase, exhaust scavenging increases and the air intake demand/flow becomes be greater. So high RPM engines want the intake valve to be opened earlier which provides more time for the intake charge to fill the cylinder. But this generally leads to a trade off in the loss of low RPM power and driveability. 

A later IVC allows more time for a cylinder to fill at high RPM, but limits low-end torque since cylinder pressure is pushed back through the intake port. The intake closing point starts the compression stroke. The earlier it closes, the greater the cranking compression because of increased cylinder pressures. Early intake closing is critical for low-end torque and responsiveness. As RPM increases, the intake charge momentum increases. This results in the intake charge continuing to flow into the combustion chamber against the onrushing piston far past bottom dead center. Hence, the higher the RPM of operation, the later the intake closing should be to ensure all the charge possible makes it to the combustion chamber. A later intake closing point improves top-end power. The optimum intake valve closing point would be:

1.) Just as the air stops flowing into the chamber.
2.) Getting the valve seated quickly but not so fast that the valve bounces off the seat which can allow the charge to escape back into the intake port and disturb the next charge.
3.) Close the valve quickly and not waste time in the low lift regions, where very little airflow can occur, and no compression is building in the cylinder.
4.) Design closing ramps for hydraulic lifters that are not so fast that they result in noisy operation that some people do not want.

*Exhaust Valve Opening and Closing Points:* What affect does the opening and closing points of the exhaust lobe have on performance? Early Exhaust Valve Opening (EVO) can limit low- and midrange power by allowing torque-creating cylinder pressure to escape, but helps high-RPM performance by creating more time for exhaust gas to be expelled. A later EVO helps low and midrange power, but hurts high-RPM performance.

Opening the exhaust valve too early will decrease torque by bleeding off the cylinder pressure from the combustion of the air/fuel mixture that pushes the piston down. The earlier the exhaust opens the more pronounced the resulting engine sound due to the continued and ongoing combustion of the passing air/fuel mixture out the exhaust system. But, the exhaust valve has to open soon enough so it provides enough time to properly scavenge the cylinder.

Closing the exhaust too late can lead to increased overlap and allow reversion into the intake system. Later exhaust closing can better scavenge the combustion chamber of the spent gasses and provides more signal to the intake system at high RPM. Late exhaust closing will allow some of the new intake charge to escape out the exhaust system which decreases fuel mileage and increases emissions.

*Over Scavenging* - Not generally an issue, just something to take note of.

The goal is to balance the intake side with the exhaust side, and vice-versa. If one side is out of balance, it can affect the other, especially with respects to the exhaust side and over scavenging. The symptoms of over scavenging are not always obvious. Over scavenging can be due to a longer exhaust duration (which is the primary offender), too much valve overlap, or too close of a lobe separation angle. Over scavenging can also be RPM specific where it may happen at lower RPM's, but is no longer an issue at higher RPM's. So each engine and how it is set-up to perform at its best, has bearing on exhaust gas scavenging.

A poorly matched intake system, or induction parts that don't flow enough CFM's for the engine being built, may act as a restriction and "choke off" the engine to the point where it will not run much past a certain RPM even though the head CFM flow and cam selection should be able to pull to 6500 RPM (if even that high) without dropping off sharply in power. If you are limited to a specific intake/carb and prefer not to make any changes or upgrades to them, then a cam having a wider LSA, less overlap, or less duration/lift on the exhaust side might be needed.

Exhaust flow can be overdone even when a good flowing intake system has been used because the exhaust ports may be ported to flow extremely well or the exhaust system is too large causing much of fresh air/fuel charge that should be drawn into the cylinder to be scavenged right out the exhaust. Most will agree that back pressure in an exhaust system can be restrictive, but the only thing that could be worse is a reduction of back pressure to the point where it is in effect pulling a vacuum and a good portion of the air/fuel mixture right out of the cylinders. So some back pressure is preferable to avoid "over scavenging" and keeping the air/fuel mixture in the cylinders where you want it. Things such as adding a larger ratio rocker arm to increase exhaust lift (and therefore hold the valve open longer), using too large of diameter headers or pipes, or in the case of open headers too short of a collector, are some of the things that can also cause over scavenging of the exhaust.

A stock or even mild engine can benefit from a cam having addtional duration, lift, or wider lobe separation angle on the exhaust valve - and this is what the Pontiac engineers did. But when modifying the engine for more TQ & HP using the aftermarket parts available to do so, the build should take into account that balance between intake flow and exhaust flow and not get too carried away on the exhaust side and create an "over scavenging" effect that hurts performance. Just another area where "bigger" is not always better.


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## PontiacJim (Dec 29, 2012)

PontiacJim said:


> *Part 3 Cams Explained *
> 
> MODERN DAY CAMS and a look at the terms used.
> 
> ...


*Part 4 - Cams Explained

Lobe Centerline:* Lobe Centerline (LC) is the highest point of lift on the lobe, either intake or exhaust. Centerlines are listed as degrees of crankshaft rotation and often fall between 102 and 122 crankshaft degrees. Many cam manufacturers build into their cams a 4-degree advance and is typically noted in the cam specs. Don't let this confuse you when installing your cam, just follow the cam card specs when degreeing the cam. Lobe Centerlines can be moved around depending on where you install the cam when you degree the cam. As an example, if the cam card says to install the intake lobe on a 112-degree Intake Centerline (ICL), you would use a dial indicator and cam degree wheel/pointer to check the intake valve's highest lift point with the pointer on the degree wheel lined up at exactly at 112-degrees on the degree wheel. This means the cam has been installed “Straight Up”. If you wanted to ADVANCE the cam 2-degrees from the cam card's 112 ICL, you would again use the dial indicator and a degree wheel/pointer to check the intake valve's highest lift point and the degree wheel/pointer should be lined up at exactly 110-degrees. If you wanted to RETARD the cam 2-degrees, use the same process and the degree wheel pointer should show an installed position of 114-degrees. The Intake Centerline (ICL) is After Top Dead Center (ATDC). The Exhaust Centerline (ECL) is Before Top Dead Center (BTDC).

*Lobe Center Angle: * Lobe Center Angle (LCA) is the distance in degrees between the centerlines of the lobes on the camshaft. To increase duration, cam makers grind the lobes wider on the base circle of the cam. This makes the lobes overlap each other more, increasing overlap. More duration = more overlap. To increase overlap without changing duration, cam makers will grind the lobes closer together, making a smaller lobe center angle. Less lobe center angle = more overlap.

*Lobe Separation Angle:* Lobe Separation Angle (LSA) is NOT the same as Lobe Centerlines (LC), although the two are directly connected. LSA is ground into the cam and it cannot be changed like the lobe centerline. To check your LSA you calculate it by adding the intake and exhaust Lobe Centerline figures together and dividing their sum by 2.

A wider LSA make an engine idle better, produce more manifold vacuum at both idle and cruise, give better fuel economy, and produce a more evenly and broader power band by causing torque to build more slowly and peak later. A wider LSA will slightly reduce lower RPM cylinder pressure and may produce less peak torque, more peak power, but have a flatter torque curve and feel like a somewhat lazier responding engine when compared to a tighter LSA. Since Pontiacs are fairly heavy cars and most used automatic transmissions, adequate lower-rpm response and a wider power band were considered more important than peak torque, so Pontiac used wide LSA's on most all factory cams with the "066" cam having a 111.5 LSA.

A tighter LSA tends to increase cylinder pressure. These can work well with low compression engines. A tighter LSA can result in faster revving engine, move the power range down in rpm and peak the power in a narrower range. So engine torque builds rapidly, peaks out, then falls off quickly.

The engine build itself, the components and intended use of the engine, play an important role in defining what is wide and what is narrow. Longer stroke engines may need the wider separation to maintain good power output at high rpm, while shorter stroke engines may respond better to tighter separations to accentuate the faster revving ability and add needed torque.

For street applications wider separations give smoother idle, more vacuum, and increased efficiency, while narrower separations give more midrange and faster acceleration. The best compromise may depend on the induction system. Typical dual plane manifold and carburetor equipped engines tend to want separations between 110 and 112 degrees. Bracket race cars with higher stall speed converters, high compression, single plane manifolds, and large carburetors usually want an LSA between 106 and 110 degrees. Engines equipped with superchargers, turbochargers, or are used primarily with nitrous typically will work best with wider separations in the 110 to 116 degree range.

A naturally aspirated engines needs a narrower LSA's at really high RPM. With a lot of valve overlap, that unburned mixture is blowing out the exhaust at lower RPM's, but it also PULLS (siphons) a bigger charge of burnable mixture back into the cylinder at higher RPM's and actually raises the cylinder pressure (and the "effective" compression). Race cams with lots of overlap and narrow lobe centers don't "come-in" until 5,500 or so RPM. They have a very narrow power range like 5,500 RPM to 8,500 RPM, but when the power comes in it is explosive. Below this power band the engine will be a slug if the engine is not spinning up in the RPM range where that scavenging effect of the cam comes into play. So a 750HP engine that doesn't begin to pull until 4,000 RPM's may be put to shame by a 400 HP engine at 2,500 RPM because a 400 HP engine will have a “smaller” cam/less overlap and will make more torque, which is what moves the car, at a lower RPMs.

The LSA (lobe separation angle) is often debated. Some will insist that you need a wide LSA for the best performance and others will argue that a narrow LSA will provide the best performance. The fact is both have proven to provide excellent performance in various applications.

If you had 2 cams with the same duration, the lobe separation angle and overlap are inversely proportional meaning - as the lobe separation angle is increased, valve overlap is decreased, and when the lobe separation angle is decreased, valve overlap is increased.


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## PontiacJim (Dec 29, 2012)

PontiacJim said:


> *Part 4 - Cams Explained
> 
> Lobe Centerline:* Lobe Centerline (LC) is the highest point of lift on the lobe, either intake or exhaust. Centerlines are listed as degrees of crankshaft rotation and often fall between 102 and 122 crankshaft degrees. Many cam manufacturers build into their cams a 4-degree advance and is typically noted in the cam specs. Don't let this confuse you when installing your cam, just follow the cam card specs when degreeing the cam. Lobe Centerlines can be moved around depending on where you install the cam when you degree the cam. As an example, if the cam card says to install the intake lobe on a 112-degree Intake Centerline (ICL), you would use a dial indicator and cam degree wheel/pointer to check the intake valve's highest lift point with the pointer on the degree wheel lined up at exactly at 112-degrees on the degree wheel. This means the cam has been installed “Straight Up”. If you wanted to ADVANCE the cam 2-degrees from the cam card's 112 ICL, you would again use the dial indicator and a degree wheel/pointer to check the intake valve's highest lift point and the degree wheel/pointer should be lined up at exactly 110-degrees. If you wanted to RETARD the cam 2-degrees, use the same process and the degree wheel pointer should show an installed position of 114-degrees. The Intake Centerline (ICL) is After Top Dead Center (ATDC). The Exhaust Centerline (ECL) is Before Top Dead Center (BTDC).
> 
> ...


*Valve Overlap: *Valve overlap is the degrees that both intake and exhaust valves are open at the same time. Intake Opening (IO) plays a big part in establishing overlap. An early IO increases overlap and can lead to a sluggish engine, since the intake charge is contaminated with exhaust gasses. A later IO reduces overlap, improves idle quality, and increases low-speed torque. Exhaust Closing (EC) also affects overlap. An early EC reduces overlap, improving idle but limiting midrange power. A late EC increases overlap, which hurts idle but helps high-rpm power.

_Engine vacuum is affected by cam overlap. _ A wider lobe separation angle of 112-116 degrees typically means less valve overlap and higher vacuum at idle, while a narrower lobe separation (102-108) provides just the opposite and can affect power accessories like brakes which need vacuum. As overlap increases and manifold vacuum decreases, idle speeds need to be increased just to keep the engine running. Fine tuning the engine can also become difficult. Those cars using manifold vacuum to shift the automatic transmission can find low manifold vacuum causing stalling when dropping the trans into gear and trying to get part throttle response under load may be difficult. A higher stall converter may be needed along with a manually controlled shift ability.

_Excessive overlap results in reduced torque and soft throttle response at low engine speeds _in some instances below 3,000 rpm. At idle the intake air charge is moving at a very slow speed and there is plenty of time during the overlap period for the exhaust gas to move back up the intake tract and dilute the incoming charge - producing that nice rough idle many like to hear. As the RPM's begin to climb into the upper power band, there is little time for the exhaust gas to do anything except exit the exhaust port. Add the inertia of high-speed air/fuel mixture entering the cylinder as the intake valve opens and overlap can be very useful for pulling in that column of air into the cylinder. The 41 degrees BTDC of intake opening that was a poor power performer at the lower RPM's now works well to fill the cylinder with a big charge of air/fuel mixture and now makes great power at 3,000 rpm and beyond. So if the induction system and exhaust system are correctly matched to work together, the result is more power at the higher engine speeds.

_More overlap decreases the low RPM vacuum and engine response as well as increases raw fuel escaping out the exhaust_ (gas smell & burning eyes syndrome), BUT, more valve overlap (both intake/exhaust valves open) will improve the the midrange power provided by the "pulling in" of the incoming intake air/fuel mixture by the fast moving exhaust to in. This action by the exhaust of pulling in the intake charge at mid/higher RPM's will better fill the cylinders and can result in the improvement in engine acceleration, which drivers often notice. Less overlap (less time both valves are open at the same time) will increase efficiency at the lower RPM's and improve the engine's low-end response due to less reversion of the exhaust gasses into the intake port.


*General Overlap Chart *from the internet that can be used.

“APPROXIMATE” SOLID LIFTER ADVERTISED OVERLAP DEGREES PERFORMANCE REFERENCE CHART

400ci…………………......500+ci………....Typical usage
25 deg………………..…...40deg…………....towing
45 deg………………...…..60*…………....ordinary street
62.5 deg……………...…..75*……………..street performance
80 deg……………………...90*…………....street/strip
92.5 deg…………………..100*…………...race
105 deg…………………...115*…………...Pro race

The way to calculate your cam’s "Advertised Overlap" needed for the chart above:

1.) Add your intake and exhaust advertised duration, not the duration at .050 tappet lift.
2.) Divide that answer by 4
3.) Subtract the lobe separation angle (LSA) from that answer
4.) Multiply that answer by 2, and you have the CORRECT advertised overlap to use in the chart above

Note: The intake and exhaust advertised durations of a solid lifter cam are typically shown as a duration at .015 tappet lift - one reason comparing hydraulic cams with solid cams is apples to oranges. And, because a hydraulic cams don't have any lash settings like a solid cam, the cam's advertised duration is rated differently. To use the chart above for a hydraulic cam, REDUCE the hydraulic cam’s intake and exhaust advertised duration, which is typically shown at a duration at .006 tappet lift, by 8 degrees.

Why did the car manufacturers, and many cam grinders, use the .006" tappet lift number? The .006" is supposed to represent the lift when the slack is taken up by the lifter as it is compressed and the hydraulic lifter goes solid and starts opening the valve. By changing the acceleration rate of the ramp, you can change the point at which the lifter goes solid. A real fast lifting ramp, and the lifter will go solid sooner, and you may see .004" as the number. 


"068" Cam Specs - Example

Add 180° to the timing events of each valve to calculate the duration. Keep in mind that the crankshaft goes through 2 revolutions for 1 camshaft revolution.

*Cam Duration *= Opening° + Closing° + 180°
*Intake Duration *= 31+77+180 = 288°
*Exhaust Duration* = 90+32+180 = 302°

*Cam Centerlines* = Duration/2 - Intake Opening; Duration/2 - Exhaust Closing
Intake Centerline = ICL = (Duration/2) - IO
288/2 - 31 = 113°
Exhaust Centerline = ECL = (Duration/2) - EC
302/2 - 32 = 119°

*Lobe Separation Angle* = (ICL + ECL)/2
LSA = (113+119)/2 = 116°

*Cam Overlap *=
Add your intake and exhaust advertised duration.
Divide that answer by 4
Subtract the lobe separation angle (LSA) from that answer
Multiply that answer by 2
288 + 302 =590
590 - 4 = 147.5
147.5 - 116 = 31.5
31.5 x 2 = 63°

Remember, Solid lifter cams use advertised durations that are typically shown as a duration at .015 tappet lift, hydraulic lifter cams durations are typically shown at a duration at .006 tappet lift. So measuring a solid cam spec against a hydraulic cam spec will not give accurate measurements.

Here is one, and simple, example to quickly spec out a cam for your build:

Choose duration based on Compression and desired engine RPM
Choose valve lift based on head flow
Choose the cam's overlap based on engine's idle/vacuum/performance range
Then let the LSA fall where it may

Others will:

Choose the LSA according to engine size/compression/use
Choose duration based on Compression and desired engine RPM
Choose valve lift based on head flow
Compare a cam's overlap based on engine's idle/vacuum/compression/performance range

*A narrower LSA* will:
Narrows the power band
Increases maximum torque
Moves torque to a lower RPM
Decreases piston-to-valve clearance
Builds higher cylinder pressure
Increase effective (Dynamic) compression
Increase cranking compression
Increase chance of engine knock
Valve overlap Increases
Exhaust Gas Reversion effect increases
Idle vacuum is reduced
Idle quality suffers

*A wider LSA* will:
Broadens power band
Reduces maximum torque
Raise torque to a higher RPM
Increases piston-to-valve clearance
Reduce maximum cylinder pressure
Decrease effective (Dynamic) compression
Decrease cranking compression
Decrease chance of engine knock
Valve overlap decreases
Exhuast Gas Reversion effect is reduced
Idle vacuum is increased
Idle quality improves

Cam Duration Selection measured @ .050" for Hydraulic and Solid Cams

*200 Degrees and under*
Smooth Idle, good fuel economy. Use with stock or low CFM carb and stock or small diameter tube headers. Can recurve distributor or use aftermarket ignition. Stock rear gears.

*200 to 230 degrees (hydraulic), under 230 degrees (solid)*
Good idle, low RPM torque. Good for towing, RV, and off-road use. Works well with low CFM carb, torque producing intake, aftermarket ignition, and headers. Good in cars up to 4,000 pounds and moderate sized engines. Stock or mild gear changes are OK.

*215 to 230 degrees (hydraulic), 230 to 240 degrees (solid)*
Fair idle, good low and midrange torque and HP. Good in cars 2,500 pounds and heavier with small cubic inch engines, or cars up to 4,000 pounds with medium-sized engines. Good with low CFM carbs, aftermarket intakes, headers, and recurved ignition. Near stock converters with slightly more stall speed recommended. Moderate lower gearing is ok.

*230 to 245 degrees (hydraulic), 240 to 255 degrees (solid)*
Rough idle, good mid-to-high RPM torque and HP. Good in cars up to 4,000 pounds with big block sized engines. Must use higher than stock CFM carb, aftermarket intake, headers, and ignition should be matched to RPM range. Can use ported heads. Requires high RPM stall converter and up to 4.5:1 rear gearing.

*240 to 255 degrees (hydraulic), 250 to 265 degrees (solid)*
Rough idle, mid-to-high RPM torque and HP. Cars up to 4,000 pounds with medium to large engines. High CFM carb to include multiple carbs, single plane intake or tunnel ram, large tube headers, low restriction exhaust, hot ignition. Best with ported heads. Very high stall converters and 4.5:1 and up rear gearing.


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## Baaad65 (Aug 29, 2019)




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## GTO44 (Apr 11, 2016)

Good stuff PJ!


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## PontiacJim (Dec 29, 2012)

*FINDING T.D.C. - CAM DEGREEING'S FIRST STEP*

The purpose of checking or degreeing-in the camshaft in the engine block is to determine whether or not the camshaft is installed in the correct relationship or phasing with the crankshaft. However, the most important step in phasing a camshaft is finding absolute T.D.C. of the #1 cylinder piston. Trying to operate an engine without this vital marker is like trying to read a tachometer without an indicator needle. The T.D.C. marker is the all-important datum (tuning) point from which all ignition and valve timing is based. Quite often, we have observed racers at Bonneville, drag strips and circle tracks who neglected to provide themselves with a T.D.C. marker. All stock engines have a stationary pointer affixed to the block, and a T.D.C. marker on the crankshaft harmonic balancer. But, these racers lost the original pointer when they changed to an aluminum timing gear cover. Or, on supercharged engines, when they changed to a steel crankshaft drive hub, they lost the original T.D.C. marker. Now, here is their predicament: they now have no way of accurately setting their spark lead or valve timing. Had this engine been accurately calibrated for T.D.C. by utilizing the "Isky Positive Stop Method" while still on the bench, all doubts and frustrations would have been avoided. Thus, a possible winner became a loser.


It is a common error to miss T.D.C. by a few degrees due to the piston dwell at top center. Inasmuch as this inaccuracy will substantially affect subsequent timing, the following procedure is suggested to correct this error.



Mount degree wheel on the front of the crankshaft. Now bolt a stationary pointer on the cylinder block (see illustration). Pointer can be made of metal strip or 1/4 inch steel rod.
Mount a dial indicator securely to the cylinder block. Now adjust dial so that at maximum piston rise the indicator sweep hand travels through approximately .300 of movement. The dial indicator contact point should rest on the center of the piston as shown in Fig. 6.
Now to turn crankshaft over, use a long-handle wrench or lever so as to get an even, steady movement and not a jerky motion. The crankshaft should always be rotated in the normal running direction.
Holding your thumb down on the No. 1 piston (to eliminate all lash), come up slowly to T.D.C. until you reach what you guess to be the middle of T.D.C. dwell. Set your degree wheel to read T.D.C. against the pointer.









Now rotate crankshaft one more revolution and this time on the way up to T.D.C., stop exactly .200 (dial indicator reading) below the maximum piston travel. Now read the degree wheel; if for example, it reads 40 degrees before T.D.C., continue rotating slowly on up to T.D.C., over the hump and down the other side, keeping thumb firmly on piston. Watch dial indicator closely, and when it reads exactly .200 down from T.D.C., stop and note reading on degree wheel. If you have a perfectly split overlap, it should read 40 degrees after T.D.C. If it doesn't, you have not found exact T.D.C., therefore you must try again.
*MAKING CORRECTIONS*

Split the difference (your error in degrees) by moving the degree wheel radially on the crankshaft. After you have made the adjustment, come around with the crankshaft as before, stopping .200 below each side of T.D.C. When you get exactly the same degree readings .200 inch below each side of T.D.C., you have found absolute Top Dead Center. NOTE: The exact travel of .100-inch below T.D.C. is not important. Any check point between .100 and .500 will give good results, as long as you check each side of T.D.C. equidistantly.

*POSITIVE STOP METHOD OF FINDING T.D.C.*

The most practical way of locating T.D.C. is known as the positive stop method. No dial indicator is required for this procedure. First, let's see how it's done, utilizing the degree wheel.



Fasten the degree wheel to the crank. Then, take a stiff 1/4-inch rod or similar material and sharpen one end to form a pointer. Attach this pointer so that it rests very close to the damper to eliminate parallax viewing error.
Obtain a stout strip of steel about seven inches long and drill three 1/2-inch holes in it (see Figs. 7 & 8 for position of holes). This strip is placed across the center of the No. 1 cylinder bore and bolted on each end to secure it to the block. Caution: Be sure that the strip of steel is rigid enough so that it will not be deflected when the piston contacts the center bolt stop. Incidentally, the positive stop should be adjusted so as to stop the piston's upward travel at approximately .200 to .800 below T.D.C.
 













 
Rotate the crankshaft in normal running direction (clockwise) until the piston crown lightly strikes the positive stop.
Now, radially adjust and lock the degree wheel to the crankshaft at 40 degrees before T.D.C. at the pointer.
Now rotate the crankshaft backwards to the positive stop. If the degree wheel reads' 40 degrees from T.D.C. you have hit Top Dead Center exactly, and the zero mark between the two 40-degree readings is absolute T.D.C..
However if your readings were unbalanced, you will have to split the difference (your errors in degrees) by moving the degree wheel radially on the crankshaft. Then, try again until you get exactly the same degree readings against the positive stop on either side of T.D.C. NOTE: The lower the positive stop is located below T.D.C., the greater the degree readings will be. But, the results will always be accurate. T.D.C. always lies equidistant between the two positive stop readings.
*FINDING T.D.C. ON YOUR HARMONIC DAMPER WITHOUT DEGREE WHEEL*

Even without the degree wheel, you can and always should calibrate the T.D.C. mark on your harmonic damper when building or assembling a new engine. By using Step No. 3 and No. 5, each time you contact the positive stop, rotating both forward and backward, scribe a mark on the damper in line with the pointer. T.D.C. will be exactly between the two scribed stop marks. Carefully measure and scribe a permanent T.D.C. marker between these two stop marks. Remember the T.D.C. marker is the important datum (tuning) point from which all ignition and valve timing is based.

*CAM DEGREEING PREPARATION*

Having determined T.D.C., using your 1/2" travel dial indicator and degree wheel you are now ready to degree-in your camshaft. The two most common frustrations that people experience in cam degreeing are: 1. Checking at the valve. 2. Checking the valve-seat-timing.

*CHECKING AT THE VALVE*

Checking valve timing at the valve is not recommended because production tolerances on stock rocker arms can confuse your readings at the valve, whereas the direct motion of the lifter on the cam lobe will be the same for each lifter in the block. Another reason for never checking at the valve is that a rocker arm's theoretical ratio, usually 1.5:1, is true only at approximately mid (1/2) valve lift. The ratio varies from slightly more to slightly less than 1.5:1 through the lifting cycle, because the rocker arm continually varies its point of contact on the valve stem.








*CHECKING VALVE SEAT TIMING - CLEARANCE RAMP ERROR*

Checking the cam at the lifter is much more accurate but can still cause confusion if you try to check the actual valve seat timing, which involves checking on the clearance ramps of the cam lobe. The clearance ramps are the slow lifting portions of the lobe which provide a smooth, transition between the base circle and the cam flank on both the opening and closing sides of the lobe. On the clearance ramps, the first .010" or .015" of lifter movement is usually at the slow rate of .0005' per cam degree. In addition to gradually taking up the valve lash (necessary because of valve expansion and small deflections of the valve gear components), the clearance ramp provides the initial, gentle acceleration of the valve off its seat.









An example of these clearance ramps is described in the cam lift curve of Figure 9. As indicated in Figure 9, only the end of the clearance ramp directly adjacent to the cam flank is actually used to open and seat the valve, while the remainder is used to take up the clearance and compensate for small deflections or runout in the valve gear.


Since the clearance ramp rate of lift (velocity) is .0005" per cam degree, a slight error on your part of say .001" in checking the valve seat timing at a certain point on these clearance ramps, could account for 2 cam degrees (4 crank degrees) of error in determining the timing point as exemplified in Figure 10. And it is very easy to accumulate .001" error if the dial indicator's stem is not running parallel to the lifter (cosine error) or if you view the dial indicator's calibrations from an angle (parallax error) or if the cam bearings or tappet bosses are worn slightly. Obviously then to properly determine the position of your camshaft in the engine, the cam timing must be checked at a lifter height off the base circle where the velocity (rate of cam rise) is high enough so that small checking height errors of .001" or so will not result in gross degree wheel reading error.








*ISKENDERIAN .050 LIFTER RISE METHOD*

Many years ago a standard height was sought after by ISKENDERIAN engineers where all racing camshafts could be timed to give accurate results and in 1958 it was decided and later published in our top tuner's manual, "Valve Timing for Maximum Output" that .050" lifter rise off the base circle would be the accepted standard for our camshafts. This figure was ideal because it was not far enough off the base circle to confuse the engine builder when timing the camshaft, and it was high enough to show effective valve timing (a point where the valve is far enough open to pass an effective air flow). Also, the velocity (rate of cam lift) of most camshafts is approximately .004" per cam degree at .050' lifter rise. Therefore, a .002" error in checking height would only affect the degree wheel reading about 1 crank degree as shown in Figure 11. The ISKENDERIAN .050" lifter rise check has now become a standard in the racing cam industry.






Cam Degreeing







www.iskycams.com


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## PontiacJim (Dec 29, 2012)

Great video from Comp Cams that explains some of the above terms so you can see the cam/engine working.


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## PontiacJim (Dec 29, 2012)

Here is a Comp Cams recommended break-in procedure for FLAT TAPPET cams. It talks about "heat cycling" which is what my machinist does when breaking in his engines on a test stand. he "heat cycles" the engine 3 times. But, he says he gets the engine up to operating temps and then shuts the engine down to allow it to cool back down to room temps. Then repeats. I don't know how long that takes to bring to temp, and of course you would need a temp gauge. So Comp Cams 10 minute run-in may be the same.


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## Boomstick (Sep 13, 2021)

This needs to be a sticky!!!!


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## PontiacJim (Dec 29, 2012)

Put together a little info on roller cams. I know some will swear by them, and yes, they can produce more power. I wanted to point out what I see as the rollers greatest draw back in a stock Pontiac block. The acceleration/ramp rates are important as this is where the problem can come in. Not such an issue on mild roller cams, but could be a problem if too aggressive. I feel an aftermarket lifter brace, available from most Pontiac engine builders, is a must even if you use a mild roller profile. The problem is on the passenger side of the lifter bores, not the driver's side. There isn't a lot of support as the cam rotates clockwise and applies a lot of pressure on the walls of the lifter bores that don't have the additional cross bracing. You may never have a problem, or you may crack/break the lifter bore and that spells disaster. So this is just my opinion - if I ever go with a roller set-up, it'll have the lifter bore brace.

In the first diagram is a cam lobe and roller lifter. The technical terms are explained as:

The* base circle* is the smallest circle drawn on the cam profile and measured from the cam's center. The cam size is dependent on the established size of the base circle.
The *trace point* is the roller lifter's wheel center point as measured from the cam's center.
The *prime circle* is the circle drawn using cam's center and the roller lifter's trace point center.
The* pitch curve*, or pitch profile, is the path that the trace point/roller lifter wheel follows. In cam layout, this curve is often determined first and the cam profile is then established by tangents to the roller or flat-faced follower surfaces.
The *pitch point* is a point along the cam's pitch curve indicating the maximum pressure angle, am.
The* pitch circle* is the circle drawn as measured from the cam's center and passing through the cam's pitch point.

The *pressure angle* (see second diagram) is the angle in degrees using the centerline axis of the roller lifter's body/roller to the face of the roller cam lobe. Drawing a line through the lifter axis and off the vertical off the face of the lobe along any point on the pitch curve from the point where lift of the lifter first begins and the calculated "normal point" results in an angle that can be measured to determine the steepness of the cam profile. If the steepness/lift of the lobe is too great, and angle too much, it can increase side loading pressures that can affect the smoothness of the action or damage parts. The angles taken are from the trace point when the lifter is on the base circle and no lift is taking place, to a point calculated along the pitch curve where maximum pressure angle is reached.

The *transition point* or crossover point is the position of maximum velocity where the acceleration changes from positive to negative (valves opening/closing) and the inertia force of the lifter's roller changes direction accordingly.

In the second diagram are 2 cam lobes; Roller lobe/lifter on the left and Flat Tappet lobe/lifter on the right. The roller cam lobe on the left is shown having a maximum pressure angle of approximately 60 degrees - due in part to the lobe shape. There is a strong possibility that clockwise rotation of the cam would not cause the roller lifter to rise, but instead, to jam against the lifter bore with the probability of breaking the cast iron bore. Designers often limit the maximum pressure angle to 30 degrees or less for a smooth cam-lifter action. However, if the engine lifter bores are made more rigid by design or an add-on lifter brace is incorporated, and the roller bearings are strong, and the cam's lifter body overhang below the lifter bore is small, then the maximum pressure angle may be increased to more than 30 degrees.

In the flat-faced cam/lifter diagram on the right, the jamming action that a roller cam may experience does not exist. The lifter's flat face has to follow the sloping curve of the lobe and the pressure angle is constant and low so no excessive side loading or binding exists.

From the internet, "Reynolds continues: 'In roller lifters the prominent issue seems to be bearing failure. This is normally caused by one of two things: in an engine with high spring loads that spends too much time at low rpm (driving on the street), the lifter will overheat causing the bearing to fail. Or, valve bounce at high rpm, will fracture the rollers and cause bearing failure due to the shock load.' ”

“Oil contamination is by far the major cause of lifter complaints. The hydraulic lifter is the most sensitive oil filter in an engine. The internal clearance between the plunger OD and body ID is only about .0002?. Any solid contaminants that enter a lifter are likely to muck it up. When lifters are returned to us with a complaint of ‘noisy’ or ‘won’t pump up’ we find solid contaminants inside the lifter about 99.9 percent of the time.”

Reynolds from Scorpion agrees that debris is an issue: “The main problem we see with hydraulic lifters is ‘bleeding down’. Most of the time this is caused by debris getting into the hydraulic unit causing sticking or getting into the check valve causing ‘bleed down’. Some of the types of debris we’ve found include metal from engine machining, material from a wear issue in the engine, gasket material and gasket sealer.”


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## Baaad65 (Aug 29, 2019)

Good stuff but now I get concerned because I don't have lifter bore bracing, so does adding 1.65 rockers like I did increase the lifter bore side load? The cam has a .510/ .521 lift, .230/236 duration before the 1.65 rockers. Maybe Butler thought the cam wasn't that radical for the bracing, I'd sure hate to pull the intake and valley pan but I would hate it more if I grenade the motor.


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## PontiacJim (Dec 29, 2012)

Baaad65 said:


> Good stuff but now I get concerned because I don't have lifter bore bracing, so does adding 1.65 rockers like I did increase the lifter bore side load? The cam has a .510/ .521 lift, .230/236 duration before the 1.65 rockers. Maybe Butler thought the cam wasn't that radical for the bracing, I'd sure hate to pull the intake and valley pan but I would hate it more if I grenade the motor.


Yes, 1.65's compress the valve springs more, and require a little more effort to raise them due to the location of the pushrod cup moved inward closer to the rocker arm stud, so more pressure goes back onto the roller lifters which means the cam has to overcome more pressure to raise the lifters and that can translate to added stress on the lifter bore wall. I have read at the PY site that the critical number was a cam having 200 degrees duration at .200" lift - if I am recalling that correctly. Butler typically sells cams where you should not have to worry, and I am sure they inspect/mike the lifter bore to make sure they are sound, BUT, don't know if they would guarantee a roller cam not to have a lifter bore crack or break using one of their roller cams. If they do, then I would not think twice about it. Just personally, my take and no one else's, I don't want to take a chance with the costs that go into these engines to have "that engine" with core shift and a thin lifter bore side wall. To me, its like keeping cast connecting rods and not choosing forged rods. It's done all the time and works out fine for the most part. But every now and then..........

Email Butler and see what they say, that would set your mind at ease.


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## Baaad65 (Aug 29, 2019)

Ok, I talked to a Butler tech about installing the rockers and he didn't mention that I would need the braces but it could of gotten overlooked in the conversation.


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## PontiacJim (Dec 29, 2012)

Another post/info that ties in with this post.









Flat Tappet vs Roller - BS


I kinda get tired of reading all the negative viewpoints against the "traditional" flat tappet cam that has been, and still are, used in many engine builds. I really get insulted when the flat tappet cam choice is associated with my age and the generation that I grew up in - as if roller cams...




www.gtoforum.com


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