Intake and exhaust are supposed to overlap, otherwise you wouldn't be able to get any performance at all. It's not uncommon for there to be 30 or 40 degrees at least between the time the intake valve starts to open and the exhaust closes. On high performance cams, the overlap can be considerably more. As the idle speed increases, it's possible to run much more timing and lift. A stock cam will have approximately 240 to 250 degrees of duration, while a race-only cam might have over 300 degrees of duration and it would be totally unsuitable for street or low speed operation. As cam duration and lift increases, compression, port size, intake and exhaust manifolds, carburetion, and exhaust restriction must also be modified. The cam is in the middle of the flow- - - -both ends of the chain, intake and exhaust, have to match the center link, or no advantage is gained.
Jerry
Last edited by Hotrod Lincoln; 09/10/2009 2:21 AM.
