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Can someone please explain camshaft overlap? Specifically, how does it effect timing, idle mixture and, the rumbly sound it produces? I believe I have a good understanding and want to get it all.
Thanks.

Generally speaking, overlap is the number of degrees that both the intake valve and exhaust valve stay of the seats. The lobe separation is the main culprit here. A wider separation has less overlap. This provides better idling and less choppiness, builds more vacuum at idle and increases compression pressure. It also brings the power band down to a lower RPM. Narrower lobe separation will have the opposite effect.