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Old 12-18-2011, 04:53 AM
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Cranking PSI isn't a very good indicator of how an engine will perform. Wear and ring/valve sealing aside, a radical cam will tend to pump lower than a stock cam but will have much more top end power. A tactic that has long been used by some engine builders is to change the cam timing in relation to the crankshaft in order to get the highest cranking pressure. This isn't a cure for a poorly selected cam, but could help, regardless. I repeat could help- there are a LOT of things that enter the picture so this is not an absolute by any means.

One reason overlap/LSA is mentioned in regard to cylinder pressure is because the intake closing point is not a spec that is as 'advertised' by the cam makers in their ads or in brief catalog descriptions. But it stands to reason a cam w/long duration at 0.050" is going to have a later intake closing point than a shorter duration cam, all else being equal. Hence the connection between the two.

What a lot of overlap/tight LSA can do that causes a poor idle or "lope", is it tends to decrease the vacuum at idle and can add to reversion. Either/both of these things will cause a poor idle.
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