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Old 02-01-2007, 08:17 PM
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Originally Posted by 406 ss monte
valve overlap

That's keeping it simple.

The shorter the overlap the better the idle and vacuum. Short duration cams can maintain decent idle quality and vacuum with tighter LSA's while boosting low and mid-range torque because they still have relatively short overlap. As duration increases the difference in overlap between otherwise identical cams increases with the tight LSA cam having more degreees of overlap which has a more pronounced affect on idle quality and vacuum and the RPM at which peak TQ occurs. To maintain good idle quality and vacuum you need to increase the LSA as duration increases.

A good example is the GM Marine roller cam which is also the cam they use in the 350 Ramjet crate engine. Usually EFI cams have 114-116 LSA with 112 being considered the lower limit but the Ramjet/Marine cam is ground on 109 LSA. It is a very short duration grind which keeps valve overlap short enough that it still works well with EFI and gives decent vacuum and a "sewing machine smooth" idle in a carb application. Start adding duration to that cam and the idle would start to get pretty choppy with 109 LSA.