what causes cars that are cammed to sound choppier at idle?



what causes cars that are cammed to sound choppier at idle?

In: Engineering

The cam can be thought of as the brain of the engine. It decides when valves open and close. By changing this timing, the engine can be made to run well and different speeds. From the factory, idle quality is important, so a cam is chosen that idles well, and makes as much power as possible within that (as well as emissions and fuel economy).

One way to increase power is to make the engine more efficient at higher speeds. Since power is essentially torque multiplied by speed, making the same amount of torque at a higher engine speed means more power. Incidentally, that’s why it’s hard to get a cam that increases torque. It just moves it around. If you have a cam that makes the engine run better at higher speeds, you sacrifice it’s efficiency at lower speeds. That’s why it runs rough at idle. Very aggressive cams usually require the idle speed to be increased, sometimes significantly.

The choppy sound you described is known as cam lope.
Cam or idle lope is a result of the intake and exhaust valves both being open at the same time. This is called overlap. Mild cams have little or no overlap at any valve opening that makes a difference. Hotter aftermarket type cams have a lot because it aids high RPM breathing. At higher RPM the gases are moving at high speed and so they know which way to go despite the confusing signals from the valves. We’re using the momentum of the gases to get better cylinder filling and more power at high RPM. Because the gases are moving at high speed, they keep moving the direction they were going despite the opening of the other valve that they are not supposed to go through.
It makes idling at normal idle RPM difficult, though, because the gases don’t know which way to go. So, you get unburned mixture going out the exhaust, exhaust fouling the intake charge, and the motor struggling to run at low RPM. You can smooth out the idle by raising the idle speed, but for a really nasty cam that might mean 1500-2000RPM.

Consider that everything is a trade off with engines. Your well running engine is quiet and idles smoothly at the cost of power. It also gets good fuel efficiency.

If you want to maximize power you tune the engine for the specific condition you wish to maximize (high RPM, high power output). The trade off is the engine doesn’t run well at low RPM, and needless to say, are NOT fuel efficient.

An engine designed for high flow of air is not going to run well at low air flow conditions.

Honda came out with a V-tech engine that uses 2 different cam profiles. The first is for efficiency and allows the engine to idle well and perform well at low settings such as cruising down the freeway. Once engine parameters meet computer defined conditions, it changes the cam profile to become tuned for high power output at the cost of low RPM performance. This isn’t an issue because the cam profile returns to normal at low power settings and conditions.