[Excaliber]

Vac and mech advance units, totally separate, one has no influence on the other. As engine rpm increases it NEEDS and can handle more advance. If your existing vac advance is to sensitive, either comes into play to quickly or advances to much, yes, you might have to disconnect it entirely and run only mech advance. Adjusting the vac advance (beyond the allen screw limit which SOME have, others do not), or controlling it precisely is very difficult and often unpredictable because of flucuating vacuum signals. Which is one reason many simply choose to disconnect it.

More advance is required with higher rpm because the burn rate of the fuel/air needs a "head start" to fully burn before the combustion cycle is completed. At low to mid rpm the mechanical advance may not come into play, very much or at all. In such a case (low rpm, light load, cruise speed) the engine vacuum signal is strong enough to compensate for the inaction of mechanical advance by utilizing the vacuum advance instead.

So low rpm, light load, you get vacuum advance. High rpm, the mechanical advance comes into play. Generally speaking, you need one or the other, not usually BOTH working at the same time. How soon the advance (from either source) comes into and how much advance it offers is called the "advance curve" in the tuning process. While the mechanical advance can absolutely be perfectly dialed in based strictly on engine rpm, the vacuum advance will always remain a bit of a "wild card". That wild card could enhance the overall performance, or hinder it if it's not setup correctly for YOUR cam profile, combustion chamber shape, quench or no quench, rich/lean conditions, engine rpm and load. It can get nasty trying to tune the vac advance.