I stopped at a restaurant for breakfast in North Carolina recently and, after eating, wandered over to a dragstrip next door. In the parking lot, I saw a couple of guys working on a souped-up Chevelle and noticed that when they fired up the ignition, the car sounded really weird. Not merely loud, but the timing seemed really asynchronous, and made an odd popping noise.
I asked the guys what was causing this and the driver yelled over the racket, “cam.”
“Of course,” I yelled back, “it’s the cam!”
I had no idea what the devil he was talking about. What does a “cam” have to do with this rough idling sound–something that most of the others cars emitted as well. Don’t all cars have cams?
The timing of the valves is controlled by the camshaft. The valve timing on high performance engines is really extreme and is optimized for high rpm. For example, the intake valves open early and stay open for quite a while after the piston has started the compression stroke. Likewise with the exhaust timing. The object is to get as much mixture in as possible and reduce back pressure in the exhaust as much as possible.
This results in uneven idle and the necessity for a relatively fast idle in order to keep thing running at all.
David is correct. Also, the odd sound is often called a “lope” or “lopey idle.” A car that idles as rough as the ones you heard are set up pretty much for track-only use. But many people think that a street car with a slightly lopey idle sounds pretty cool—it tells people that you’ve actually modified your engine, as opposed to simply attaching a loud muffler.
Also, the reason your hearing the “POPPING” is that the car has a low restriction exhaust, or may be running open headers. The actuall “POP” is the hot exhaust gas leaving the engine one cylinder at a time…pop, pop,pop, pop. These cars also have more compression built into the engines which exaggerates the “POP” and makes the engine even louder, and in most cases faster.
All of the very lumpy idle can be attributed to the valve or lobe overlap of the cam. That is how long the intake and exhaust valves are open at the same time.
