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Best rate of acceleration is lowest manifold vacuum in highest gear that will produce the desired rate of acceleration. If acceleration is too slow use next lower gear, while maintaining low manifold vacuum.
Any throttle position below the point of full load enrichment is fine and it can range quite a bit from 20%to 80%. This is because throttle position does not determine load. Load is a function of manifold vacuum. Once manifold vacuum reaches 100%, additional throttle will not increase power unless you go to full load enrichment, or in the case of my VX you loose lean burn, which is another stage of enrichment.
In an auto transmission you want the lowest manifold vacuum you can maintain without causing the transmission to hold the lower gears any longer than its earliest shift point.
Every engine is more efficient when it has the maximum amount of air inducted into the cylinders at the bottom of the intake stroke. RPM too low you loose the harmonics of induction resonance and that benefit. RPM too high and you are loosing energy to friction and reciprocation losses, which are due to the accelerating and decelerating masses in all reciprocating engines. Good intake harmonics can actually increase the volume of air slightly on the intake stroke with the inertia of the incoming air contributing a sight "supercharging" effect.
I think it is not well understood generally, that very low throttle positions can virtually eliminate manifold vacuum in almost any throttled engine as long as the load is sufficient, which is accomplished by choosing higher gears than most drivers use for acceleration.
Think of it this way. I call it pulse accounting. Every combustion pulse is much more energy efficient when it uses all of the engines available compression, which requires the manifold restriction to be the lowest possible amount. When manifold vacuum is 0 the maximum amount of air is in the cylinder to be compressed and that will produce the maximum amount of power per unit of fuel consumed.
Without wide open throttle enrichment.
If you produce that amount of manifold vacuum at the RPM where your engines BSFC is highest, you have duplicated that BSFC, and that is as good as it can get.
regards
Mech
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