Too much engine work at once
I have heard it before, but don't see how more overlap boosts pressure
It doesn't......usually.
The pressure boost comes from tightening up the LSA,advancing the cam, and therefore having an earlier closing intake.
Heres an example; take the 268/110 cam and install it at split overlap. this would be at 108; and the ICA would be 62*
Now order same 268 but on a 104LSA and install it at split overlap.This would be 102, for an ICA of 56*.........................6 degrees earlier, and that is where the pressure boost comes from, slamming the valve shut earlier, on the mixture trying to go back up into the intake, being pushed in that direction by the rising piston..
Now order a 268/114 automatic cam and install that at split overlap. that would be an install of 112* and an ICA of 66*. This is 4* later than the 110 cam, and 10 later than the 104, and so a pressure loss compared to them. 10 degrees is a lot. A change in ICA of 3.5* is about one cam size, when installed at split overlap..In other words tightening up the LSA 10 degrees, allows you to run a cam about three sizes bigger with little to no pressure loss..... because the ICA remains or can remain, the same.
With an automatic trans and a hi-stall, this gets lost in the shuffle because as the rpm rises, there is less time for this pressure to be lost, and somewhere around 2200 or a bit this way or that, reversion is down to a minimum and nobody with a higher than 2200TC cares about it.
But with a manual trans,and street-gears like 3.55s, the engine is stuck down in that sub 2200 zone for a lot of the time,either in first; and especially in second this really sucks.