Choosing a Cam

There are no hard and fast rules for picking a cam for a specific application. However, some basic guidelines are worth talking about. Most performance cams are chosen for the elementary purpose of producing more power. It's important to remember that too much cam sometimes results in a reduction in power, not the increase that was expected.

Engines in street bikes will perform better with a milder increase in lift and duration than more radical grinds. While bigger cams may have a higher peak horsepower, a milder cam may feel much stronger to a street rider where low rpm torque is important. The message is, if in doubt, play it conservatively when choosing cams.

Also, large engines can take more cam than small engines. A 95 cubic inch motor can handle a bigger cam than one with 88 inches. More cubic inches can handle greater air flows from cams with higher lifts or longer durations.

There is one last point about picking a cam that applies especially to Harley-Davidson engines. This point relates to compression ratios and fuel detonation or "pinging" as it is often referred to. Lowering the compression ratio of any engine will have one very predictable effect: less power at all engine speeds. The desired result of eliminating pinging may be achieved but only by reducing power output.

If you want more performance and stop the pinging in your engine, changing cams is an easy way to accomplish this goal. A cam with a longer intake duration will reduce static compression pressure at low speed which in turn will reduce or eliminate pinging. But with a longer duration cam and the higher compression ratio, the power at middle and high speeds will be increased which is what you wanted all along. This is an extra benefit of performance cams which tends to get overlooked but it is true.

Keep in mind that most automotive engines have compression ratios of 9.0 or higher and they run on 87 octane fuel without any pinging or detonation.

The explanation for this is easy to understand. Reducing the compression ratio has the effect of lowering the amount of "squeeze" on the gas being compressed in a cylinder. This reduction in pressure has the net result of generating less compression heat. With less heat a fuel charge has less tendency to pre-ignite or "ping" before the spark plug fires.

By using a cam with a longer intake duration, the compression pressure will be similarly reduced because the actual closing of the intake valve will occur with the piston higher in the cylinder. The piston now has less distance to travel to the point of spark plug firing. Less distance to travel on a compression stroke means lower static compression pressure and no pinging. This effect will be true even with 9.0 or 9.5 compression pistons. The net result is no pinging at lower engine RPM (where it always occurs) but more power at middle and higher engine speeds.




WHY DO CAM GEARS CAUSE NOISE:
In the last few years there has been more interest in quieter engines. Since cam gear noise can sound like bad lifters, we thought the following section would be appropriate.

Whenever the roller follower on a cam lobe passes the maximum lift point, the forces on the cam drive gear tooth changes direction. If more than .002 inches (.005mm) backlash is present between the cam and pinion gear, this directional change of force will result in an audible "click" as the backlash moves from the back side of each gear tooth to the front side.

While some positive backlash is necessary to prevent localized gear tooth overloads, excess backlash (and "clicking") may sound annoying but won't hurt anything.

Tight fitting gears will cause very noticeable whining which is definitely a more serious problem. Gear tooth and bearing damage can result from running zero backlash. In this case, a smaller cam gear or pinion gear would be required.

Different size gears permit custom fitting for