Camshaft Theory - Page 2

"Degreeing" the Cam

Now it gets even more interesting! While the lobe shapes and valve overlap on a specific camshaft can't be changed, the relationship of the camshaft to the crankshaft can. Once a profile is chosen the camshaft can be custom "tuned" to the overall engine configuration. That is, the relationship of camshaft timing to the crankshaft timing can be changed by installing the camshaft either "on center," (as designed by the manufacturer) or either advanced or retarded from the timing marks on the cam and crankshaft chain (or gear or belt) drives.

What does this do? Well, if the camshaft is advanced, the intake valves close earlier in the cycle. This has the effect of increasing cylinder pressure because the exhaust valve closes earlier during the intake stroke. This results in greater low-end torque, but less efficiency at the high end of the rev range.

Retarding the camshaft does the reverse, of course. Doing so has the effect of increasing cylinder scavenging, allowing an engine to "breath" better as it revs up. In fact, it allows engines to rev higher. Engines with retarded cam timing make better power at the high end, but lower torque at the low end. Racing engines benefit best from this type of cam timing.

Before running out to the garage to degree your cam, take note that your camshaft was probably installed "on center." In practical terms you can only advance or retard about four degrees (crankshaft degrees) either way before you run into trouble, such as piston/valve clearance, pre-detonation or otherwise unacceptable engine performance.

What About High RPM?

As piston speed increases things get really hairy inside the engine. The allowable time a cylinder has to fill, provide power and exhaust is extremely short. We need to do something with the camshaft to compensate for this condition, otherwise the engine will max-out at some intermediate rpm, say around 4500. We can't increase the lobe separation angle because that would cause the valves to hit the pistons.

The only thing we can do is increase the time the valves are open in order to let in more fuel/air or let out exhaust. The only way to do this is increase duration and, to some extent, lift. The more aggressive a cam, the greater the duration and lift.

Lift is severely limited by the geometry of the engine, especially those that use domed pistons. Also, the lobes themselves must be shaped properly (spread out) or the lifters will not ride on them correctly.

Duration, on the other hand, is up for grabs. Cam grinders can create lobe shapes that give long durations, allowing engines to rev well into the 8-10,000 rpm range because they can "breath." Those same engines, however, exhibit very poor running characteristics at low rpm due to low cylinder pressures and vacuum. They "lope" at idle and frequently won't run at all at speeds lower than 1,000 rpm.

That leads us to making the right choice for our engine. Since we can't have it all, we have to decide how much performance we need and how much loss of driveability we are willing to give up. Camshafts that were specified by the manufacturer originally ("stock" cams) were chosen to give the best low-end torque and ease of starting and driveability over the widest range of conditions. Higher performance engines were fitted with more aggressive camshafts for throttle response and to create the higher power outputs that the inertia of high rpm generates. Buyers of those engines frequently complained about starting problems, high fuel consumption and having to slip clutches in order to accelerate smoothly from a standing start.

When building an engine it is generally wise to install a "mild" camshaft, one that is a step or two up in aggressivity from a stock grind. Such profiles allow crisp throttle response, greater-than-stock power, low end torque and smooth driving characteristics, all at the "cost" of high power output.

Going to high duration and lift camshafts will give you greater power (assuming all the other engine components are matched to the cam) but you will experience poor idle, annoying low-speed driving characteristics, vacuum accessory problems and premature wear of alternators, water pumps and air conditioning compressors.

All of this is the reason why modern engines utilize variable valve timing devices and why relatively small displacement motors pump out impressive amounts of power. Older engines can't benefit from this technology, so we are left with compromises.