I'll show my ignorance. I have a 65 GTO 389 tri power. What's the purpose of the vacuum gauge? I'm not even sure the gauge that's in the vehicle is working and if it is I don't understand what it's telling me.
If you have a vacuum gauge that's telling
you something, get rid of that car because it's possessed by an evil spirit.
The vacuum gauge tells you how much manifold vacuum you have. When your car is idling-whether it's fuel injected or carbureted-the throttle plate or plates are restricting the amount of air the engine can breathe in. The pistons are attempting to "suck" the mixture past the throttle. (Of course, in reality, it is atmospheric pressure that is attempting to "push" air into the engine as the pistons travel downward on their intake strokes.) When throttle is closed, vacuum is high in the intake manifold, from the throttle plate(s) to the combustion chambers. By contrast, at wide open throttle there is relatively little restriction to outside air entering the intake manifold, so vacuum in the manifold is very low (which it should be most of the time when you correctly take command of the horsepower availability of a 389/tri-power.
When you floor the accelerator pedal, you can watch manifold pressure (another word for vacuum) swing from strongly negative to nearly zero (atmospheric pressure). When your engine is "on the overrun," like using engine braking down a steep hill at high RPM, you'll see really high vacuum readings. Your vacuum gauge is also a sort of "poor man's" fuel mileage indicator; when vacuum is low, you are burning more fuel. A high vacuum reading tends to indicate a healthy engine. Engines with performance camshafts tend to read lower vacuum.