Originally Posted by oldbogie
All ported vacuum does is withhold exposure to manifold vacuum from the vacuum advance at idle. It exposes the cannister to vacuum as the throttle blades move into the fuel transition slot. From that point on the vacuum advance functions the same whether ported or direct. Low compression or highly cammed engines can benefit from more idle advance, one way to get this without running the base timing ahead to where the engine is a difficult load on the starter is to connect the advance directly to manifold vacuum.
Vacuum advance is not a part of "maximum performance" if by that you mean most horsepower at WOT. Vacuum advance is there to increase performance and efficiency at part throttle cruise where the mixture density in the cylinder is low. This causes the burn to proceed to slowly and the exhaust valves open before max temp and pressure is achieved. This throws unburnt fuel out the exhaust. The vacuum advance is intended to provide a longer burn time so combustion can be completed before the e-valve opens.
Thanks for the reply Bogie. What do you consider a highly cammed engine?
And do you hook your advance to ported or manifold vacuum?