There are 3 options for vacuum advance sources. Ported, manifold and Venturi.
Ported is zero at idle and goes up as engine rpm increases so dist advance increases with rpm, usually desired as long as detention /pinging doesn’t occur.
Manifold vacuum is high at idle, drops with rpm so all your advance is at idle and drops with speed. Better launch and low speed performance but not at highway speeds. A lot of cars we’re plumbed this way.
Venturi vacuum is similar to ported but slightly different rise. The thing with it is you have to use a vacuum amplifier as it is too weak a vacuum to actuate advance directly.
