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Recently I've seen a couple of posts where people have gotten manifold pressure and the carburetor's vacuum advance signal confused.
The two are both 'vacuum' but they aren't directly related. The signal used to control the distributor's advance is a reflection of the rate of air flow through the venturi of the carb. The more rpm, the wider the throttle, the stronger the signal.
Manifold pressure is created by the cylinders on their intake stroke, sucking air out of the manifold. At maximum rpm, with the throttle wide open, manifold pressure will approach atmospheric pressure (ie, virtually no 'vacuum' at all). But since the rate of flow through the venturi is now at its max, so too is the venturi-generated vacuum-advance signal.
No mystery here. It's in all the books.
For those who never understood the principle of vacuum advance to begin with, the matter gets even more confusing when they encounter a dual-canister distributor, which uses manifold pressure (ie, vacuum) to return the advance plate to its static timing point any time the throttle is closed, either immediately or after some small delay as determined by a control valve. (This is a smog control measure, intended to promote better combustion during the lean-out/deceleration that occurs when you lift your foot off the accelerator pedal.)
The basic reason for using vacuum advance for ignition is it's greater sensitivity to changes in the load placed on the engine, which are accurately reflected by the rate of air flow through the carb. Centrifugal advance mechanisms offer a wider dynamic range than vacuum advance systems since the amount of total advance and the rate of advancement may be 'programmed' by adjusting the mass of the bob-weights & springs. In the late 1920's Remy (later Delco-Remy) developed a composite advance mechanism combining the sensitivity of vacuum sensing to initiate the timing's advance, with a centrifugal mechanism to sustain the advance, having found such a system provided better driveability.
-Bob Hoover
-Mar 2001
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