Manifold vacuum is accessed from within the manifold plenum a direct and constant suck from the cylinders. It is what it is. Should remain constant until VERY high rpm under heavy load.
Ported vacuum is accessed from a carb port that leads to a opening behind one of the throttle blades. As the throttle blade opens and air rushs through the narrow opening between carb throat wall and blade it creates a low pressure area and this is where the ported vacuum comes from. As the throttle blade opens further from throttle bore sides the low pressure decreases and the ported vacuum strength drops.
The factory used either and or both vacuum sources for distributer advance over the years. Today either will work well but you must understand what you want from it before choosing and using.
At highway cruise speeds very high total advance is often seen and used to good advantage for example: 16 locked in initial, 20 additional from mechanical, 20 advance from vacuum. total 56º. Or some variation of those numbers. Every motor is an individual.
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