[Frank Messina]

I realize I'm a little late joining this discussion but thought I would add my 2¢ anyway fwiw. In a performance application, there is no good reason to use ported vacuum. If you have an emission requirement that you have to deal with then you'll need ported vacuum advance. Ported vacuum came on the scene in 1970 to help automakers meet the ever tightening emmission standards and until computers took over, the cars ran awful. I have experimented over the years with both methods and without exception the engine always performed better overall when the vacuum advance mechanism is connected to a manifold vacuum source. Better off idle performance, crisper throttle, quicker part throttle acceleration and a clean shut down. I performed this exercise again most recently on my Cobra, starting off with mechanical advance only , adding ported vacuum advance and finally moving to manifold vacuum advance. Manifold vacuum advance wins the seat of the pants dyno test again. It is worth mentioning that I make this statement assuming that the engine has a distributor with a relativley stock type advance curve. In other words 10 - 14 ° inital advance at idle and 32 - 36° total advance at 2600 -3000RPMs. You can easily fine tune the the amount of vacuum advance you are adding with an adjustable cannister that comes standard now on most aftermarket distributors.
Disclaimer: If you have recurved your distributor to give higher advance at idle, say 18 - 22° initial, then you are in a completely different camp and probably shouldn't bother with any type of vacuum advance period. This is race type timing and should not have anything added to the curve.
For a vacuum advanced street engine though, if you aren't using a manifold vacuum source, you're leaving performance on the table.
Frank