Some BS creeping into this thread, such as post #42.
[1] Engines need a LOT of timing at cruise because the mixture is throttled by the nearly closed t/blades; greater distance between air molecules, takes longer for COMPLETE burn. This timing can be anywhere from 45 to 60*. This is ONE function of vac adv, to provide this additional timing.
[2] WOT, when the mixture is very DENSE, needs much less timing. Hence the curve in the dist.
Try NOT using vac adv & running 60* at WOT & see what happens to your engine....
Vac adv is no more of an economy 'thing' than lowering the CR to stop detonation.
Vac adv is a tuning tool. It is alive & well in modern engines, albeit it is done electronically. The LS engines with their efficient chamber design only need about 27-28* at WOT. They idle with 22* BTDC [
not 4* ].
THIS IS STOCK FORM. Extrapolate the proportions to our old bangers that need 36-38* at WOT & you can see why these engines idle much better with more idle timing. And we haven't even talked about big cams yet with low vacuum......
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