Cam Theory 101

What do you think of this camshaft? I hear this all the time. On every performance board on the net youíll see this question. Iíve asked it myself. Why? Because itís a mystic art that only very few really understand. Iíll be the first to admit that I donít have it down.

Itís my intent to at least give the reader of this article a basic understanding of what a camshaft is, what it does, the effects it has on an engine and how it does it. You will at least understand cam theory and what all the cam lingo means so you talk cams with your buds. I will get into cam selection in a different article. This one will be long enough and quite boring to those of you who already know the basics.

What is a camshaft? Itís the brain of the engine. It regulates the amount of fuel/air mixture that the engine can pull in and push out. Itís that simple. The amount of fuel the engine can efectively and eficiently burn and get rid of, will dictate the power the engine will generate. Not only that, but the cam will dictate where the peak power happens and how flat the power curves will be. Thatís why itís so crucial to select the right cam for every engine built. The wrong cam will destroy the engineís potential for power no matter how much money you invest in the rest of the build.

Before I go any further on cams, we need to look at the piston as it pumps up and down and what the valves are doing.

  1. Power stroke. The piston is at top dead center, intake and exhaust valves are both closed and the spark plug has just fired. The expansion of the ignited fuel/air mixture forces the piston down. Before the piston reaches bottom dead center, the exhaust valve starts to open.
  2. Exhaust stroke. The piston is at bottom dead center and starts to come back up. The exhaust valve opens fully and starts to go closed. Before the piston reaches TDC, the intake valve starts to open and the exhaust valve is still partially open.
  3. Intake stroke. The piston is now at TDC, both the intake and exhaust valves are partially open. As the piston travels back down the cylinder, the exhaust valve goes fully shut and the intake valve goes fully open and starts to shut.
  4. Compression stroke. The piston is at BDC and starts to travel up the cylinder. The exhaust valve is still shut and the intake valve goes fully shut.

Youíll notice that during the piston travel, there is a time when both valves are open. This seems counter productive, but it is necessary for optimum performance. This is a term called "valve overlap". Iíll try to explain why this is necessary.

After the fuel/air mixture is ignited, the expansion of the burning gases will be complete prior to the piston reaching BDC, but there will still be pressure contained in the cylinder. When the exhaust valve starts to open prior to the piston reaching BDC, some of the pressure in the cylinder will flow past the exhaust valve into the headís exhaust port. As the piston starts itsí travel back up the cylinder, the piston forces the remaining gasses in the cylinder out through the exhaust port. The velocity of the exhaust gasses flowing past the valve into the port creates a negative pressure (vacuum) in the combustion chamber (itís the same principle as blowing across the top of a straw in a cup of water. Water will flow up the straw). Prior to the piston reaching TDC, the intake valve starts to open. The previously created vacuum in the combustion chamber will draw fresh air/fuel mixture into the combustion chamber and some will even escape into the exhaust port. This will ensure that all of the spent gasses are removed from the combustion chamber. This process is called "scavenging". When the piston reaches TDC and starts itsí travel back down the cylinder, the exhaust valve will go fully closed.

The point in the cycle where the intake valve opens is very important. If the intake valve opens too late in the cycle, the initial amount of fuel/air mixture draw into the combustion chamber is reduced and spent gasses will not be effectively flushed from the chamber. If the intake valve opens too early in the cycle, vacuum will be reduced and exhaust gasses will be forced into the intake manifold. When exhaust gasses are forced into the intake manifold, vacuum is adversely affected and the intake runners will soot up. This effect is called "reversion".

The point where the exhaust valve goes fully shut is also important. If the exhaust valve closes too late in the cycle, the combustion chamber will be "over-scavenged". This will cause an excessive amount of fuel/air mixture to escape into the exhaust port because the intake valve is still partially open. If the exhaust valve closes too soon, the scavenging effect will be reduced, trapping exhaust gasses in the combustion chamber.

As you can see, valve overlap is a touchy time frame in the pistonís travel. Cam grinders have spent countless hours of research trying to get it just right.

Continuing the pistonís trip through the overlap phase and back down the cylinder for the intake stroke, the intake valve will go fully open and start to shut. After the piston reaches BDC and starts itsí way back up the cylinder for the compression stroke, the intake valve will go closed. The point that the intake valve goes closed has a large effect on cylinder pressure. When the piston is traveling back up the cylinder, it will force some of the fuel/air mixture past the still open intake valve into the intake port. When the intake valve closes early in the cycle, more fuel/air mixture will be trapped in the cylinder and more cylinder pressure will be created. If the intake valve closes later in the cycle, some of the fuel/air mixture will be forced past the intake valve into the intake port, which will reduce cylinder pressure.

While valve overlap is ground into the cam and can not be changed, the point during the pistonís travel that the intake valve closes can be changed. This is called "valve timing" which is not to be confused with ignition timing. It is also referred as "cam phasing" or "degreeing". When you here the phrase "advance/retard the cam", it simply means to change the position of the intake valve closing point. To advance the cam, you are closing the intake valve earlier in the cycle and retarding the cam will shut the intake valve later in the cycle. Now, before you get all excited and fired up to advance your cam, you need to remember that the intake opening, exhaust opening and exhaust closing points will also be advanced. Iíll cover cam timing in more detail latter.

Lobe Separation Angle (LSA) is also called Lobe Center Angle (LCA). This term is often confused with Lobe Centerline, which Iíll address later. The best way that I can describe LSA is to imagine yourself holding a cam in front of you looking at either end of it. Now cut off the journal so you can look directly at the intake and exhaust lobes. You will notice that the bottom of the lobes closest to each other, actually overlap. Remember valve overlap that weíve already discussed? Now find the center of each lobe at their highest points. Draw a straight line from these points to the center of the cam. The angle these two lines create is the LSA. The angle is expressed in degrees of angle. If you move the lobes closer to each other, the LSA gets smaller/tighter and the overlap is increased. When looking at different cam profiles for an engine, you will always (almost always) see the LSA listed. While this is a very important consideration, the valve overlap is often forgotten. A profile with a tight LSA will also have more overlap and this is what you should be thinking about when picking a cam, but thatís for a different article.

I mentioned that the camís Lobe Centerline is often confused with LSA/LCA. Iíll try to explain LC now. Remember when I was talking about cam timing and the intake valveís closing point? This is the camís Lobe Centerline. It is the intake lobeís center (at itsí highest point) position in relation to the position of the piston at TDC of the intake stroke. The LC is expressed in a measurement of degrees like LSA is. It is usually with 4 degrees of the LSA designation, so it is often confused. When the piston is at TDC intake stroke, the intake lobe will be pushing the lifter up, opening the intake valve. The center of the intake lobe will be around 106 degrees before the piston is at TDC, or the pistonís position of 0 degrees. Iíll try to clarify that last sentence a little. For every two revolutions that the crankshaft makes, the cam will rotate once. All measurements of degrees are actually "crank degrees". One full crank revolution is 360 degrees. When the piston is at TDC, piston position is 0 crank degree and when it is at BDC, piston position is 180 crank degrees. When the piston is at approximately 106 degrees past TDC intake stroke, the intake lobe will be straight up and the intake valve will be fully open. Cams will come with a recommended centerline position from the manufacture. The one in this example is installed on a 106 Lobe Centerline. When a cam is advanced or retarded, the Lobe Centerline is changed. If we were to advance this cam 4 degrees, we would install it at 102 degree Lobe Centerline and 110 degree Lobe Centerline if we retarded the cam 4 degrees. I mentioned earlier that advancing the cam will increase cylinder pressure. It will to a point. When the cam is advanced, the intake valve will open earlier during the exhaust stroke and the exhaust valve will shut earlier during the intake stroke. If the cam is advanced too far, reversion will occur and the exhaust gasses will not be adequately scavenged. Four degrees advance is usually the most that you can safely advance a cam beyond the manufactureís recommended LC. When the cam is retarded, cylinder pressure will be reduced but the scavenging process is increased. If you are experiencing pre-detonation, retarding the cam will help. It also has a tendency to move peak hp to a higher rpm. Again, care should be taken when changing cam timing. Another consideration when playing with cam timing is piston to valve clearance. When you change the valve events (timing), the clearances will change and should be checked.

Since we talking degrees, I might as well cover duration. Duration is the amount of time that the valve is open in relation to crankshaft rotation. It is expressed in crankshaft degrees. If we have a cam with a duration of 300 degrees, the valve will be open for 300 degrees of crankshaft rotation. There are two methods used to describe duration. Seat-to-seat or Advertised duration and at .050" duration. The advertised duration is the measurement from the very beginning to the very end of the lobe ramps. It is difficult to get an accurate measurement using advertised duration. Theoretically, you should be able to find zero lift of the lobe ramps, but it is harder than it sounds. To simplify this method, cam grinders pick an arbitrary number unique to themselves. It could be anywhere from .002" lift to .008" lift. Because cam grinders wont get together and give us consistent advertised duration lift points, they came up with a standardized method of @.050" lift. When the lobe is at .050" lift, the duration starts and ends when the lobe is at .050" lift on the other side of the lobe. When comparing cam profiles, itís best to use the .050" duration numbers.

Duration is probably the most important aspect of a camís profile to pin down when selecting a cam. Cubic inch displacement, cylinder head characteristics, EFI, NOS, aspiration, compression, drive train, vehicle application and weight, desired peak power, desired engine operating rpmÖÖ.etc are all factors to consider when picking a cam. Iíve found that itís usually a task best left to the cam grinder to make. Iím not going to get into cam selection in this article, but I should talk a bit about the effects that duration has on an engine.

LSA for a performance ground cam is typically between 106-114 degrees. Sometimes even less than 106 is ground for stroker engines. When duration is increased and LSA is constant, the valve overlap is increased. When overlap is increased, vacuum is lower, cylinder pressure is reduced and reversion is increased. These are all undesirable traits for low end and midrange torque. You need cylinder pressure and vacuum for low end torque. Unfortunately, we cant have our cake and eat it too. For high rpm power, duration must be increased but we cant widen the LSA or the valve events will be occurring during wrong points in the pistonís travel. As piston speed is increased, the time that the cylinder can adequately fill and evacuate is drastically reduced. To compensate for this, we must increase the time that the intake valve is open to admit more fuel/air mixture, and the exhaust valve must be open longer for exhaust gas evacuation. The only way to do this, is to increase duration and lift. We are limited to the amount of lift because the lobe flanks/ramps have to spread out or the lifter will not ride up and down the lobe properly. Roller lifters help because they will transverse up a much sharper lobe flank than a flat tappet lifter, but thereís still a limit for them as well. A very aggressive profile is also hard on the entire valve train and camshaft.

Lift is the total height of the lobe. It is a measurement that is described in inches. A lobe lift of .500" is Ĺ". To get the total valve lift, we simply multiply the lobe lift by the rocker arm ratio. A lobe lift of .500" and a rocker arm ratio of 1.5 would give us a total valve lift of .750". If we used rocker arms with a 1.6 ratio, our total valve lift would be .800". When looking at cam profiles, the lift listed is typically total valve lift using 1.5 rockers. If you want to know what it would be with 1.6 rockers, simply divide the lift by 1.5 then multiply the sum by 1.6. .750 / 1.5=.500 X 1.6=.800

Cams lobes are ground either with either a symmetrical or asymmetrical profile. A symmetrical profile is a lobe that has mirrored opening and closing ramps/flanks. If you were to cut the lobe in half, both halves would be identical to each other. An asymmetrical profile will have different opening and closing ramps/flanks. Depending on the grind, one ramp will be more aggressive than the other. Cam grinders have found that the speed in which the valve opens and closes can greatly affect performance. Typically, the closing ramp will not be as aggressive as the opening ramp on asymmetrical grinds. This will prevent the valve from bouncing off the valve seat when closing.

As the cam rotates and the lifter makes the transition from the camís base circle to the opening flank, a ramp is ground into the base of the lobe on better cam profiles. The ramp provides a gentle transition from base circle to the flank. Ramps were first used for mechanical lifters that ran with a lot of lash. Picture a lifter riding on the camís base circle with .012" of free play (lash). As the cam rotates and the lifter hits the flank, the lash it taken up immediately causing a shock to the lobe and a noticeable tap when the rocker arm hits the valve stem tip. The ramp will allow the lifter to ride up on the lobe flank gently. As the lifter is traveling down the closing side of the lobe, another ramp is used to have the same effect on the lifter prior to making the transition from flank to the base circle. What many people donít realize, is hydraulic lifters need this same gentle transition. When a hydraulic lifter makes the transition from the base circle to the flank, the initial shock will compress the spring in the lifter affecting total valve lift and duration. The opening and closing ramps reduce these initial and exiting shocks. Not all cams are ground with transition ramps and even fewer have closing ramps at all.

To aid the engine to effectively evacuate the exhaust gasses, dual pattern cams are used. A dual pattern cam will have a different lift and duration between the intake and exhaust lobes. Small Block Chevyís for example, have pore exhaust ports that needs a little help evacuating exhaust gasses. A little more duration and lift on the exhaust lobe will give the engine more time to expel the exhaust gasses.

If youíve made it this far, I hope your not more confused than before you started. I have a tendency to ramble when I get talking about hottrodding. Iím in the process of writing another article geared towards cam selection. Hopefully it wont be as boring as this one was for you.

Michael Drew, (AKA md)
 
 

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