what causes cars that are cammed to sound choppier at idle?
In: Engineering
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.
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