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.

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.

 

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.

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.

 

 

Good stuff PJ!

 

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

 

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

 

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.

 

This needs to be a sticky!!!!

 

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.”

 

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.

 

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.

 

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