I understand that it has to do with LSA and bigger cams, but exactly what is it that gives a bad cam that sound? I know that duration and overlap affect the dilution of the intake charge, but what makes it so choppy? Misfires, exhaust gasses firing in the intake or exhaust tracts, etc. - what makes the sound that gets a gearhead's full attention immediately???

the exhaust and intake valve are both open at the same time, so you have what amounts to a massive vacuum leak. the bigger the cam the more the overlap and the more the exaust pressure trys to force everything out the intake valve.

I'm no expert in this, but to me it just sounds like poor combustion at idle. I can get my truck, with a stock cam, to sound the same if it's too cold, or the mixture/timing is way off.

With the overlap, I think there is poor cylinder filling at idle RPMs.

I'm no expert either, but I think the reason more cam means more noise is because after the combustion occurs the exhaust valves opens sooner which is noiser either because the explosion is still happening or there is more of the sound wave to go out the exhaust. Cams are designed for certain RPM ranges, and a hi-perf. cam pretty much runs like crap at idle because of the overlap and reversion, the intake gases arent supposed to go back towards the carb! A big cam isnt doing a good job of moving air thru the motor at idle.

What makes that idle sound?

Several things--one of them related to overlap, the other to the exhaust lobe.
I said related to, not caused by. Two examples: 272 cam on 104 LSA has same overlap as 288 cam on 112 LSA, both at 64°. Do you really think they will idle alike? What about 304 cam on 106 LSA and 320 cam on 114 LSA. Both have 92° of overlap---Will they idle alike?
No, because the shorter durations on the tighter LSAs have much more of their area in the overlap cycle, and therefore produce a lower air velocity in the intake port, and giving poorer fuel metering. This produces the stumbling....
When and how the exhaust valve opens adds the sound we love, or not. Two cams with the identical exhaust opening point, but different exhaust opening velocities, produce very different sounds. I always open my exhaust valves fast, and this adds a real crackle to the sound.
Anyway, this is my thoughts on idle rumble...

UDHarold

I wish I had made sound clips of my 408 motor. With the hyd flat tappet cam 286/296, 112LSA at 110ICL the motor had a nice lope and a nice but mellow exhaust sound, almost as quiet as stock. With the roller [279/294, 113LSA installed at 108ICL], you would swear I dropped in another 100CI and raised the compression. Just as Harold desribes - It plain sounds nasty. No other changes made, running through 1-3/4" headers, 2-1/2" dual (no h or X), Dynomax 20" case turbos and 2-1/2" tails. Whereas before it was kind of muted through the entire exhaust system, it now has that rapsy sound right from the headers all the way back.

My motor was the same way. With the Comp Cam 292 I had (hyd. flat), it was a lopey mellow sound. With the roller cam, which has a 4* tighter LDA and 7* less advertised duration, it's a choppy and louder idle. Overlap is the same, but the amount of lift in that overlap area is much more with the roller.

Joe,

Is that thing running right now?

Running yes. Driveable no. Body is bare metal from roof back, no tailights. Also hood is off. Give me a yell next week if you want to hear it run though (leaving in 4 hours to go to Vegas for 4 days), any exucse to fire it up and rattle the neighors windows works for me.

I wanna hear it!

Have a good trip...and remember, what happens in Vegas... (I think thats how it goes).