[davepl]

My understanding of the matter is that the vacuum advance was moved to ported in order to retard timing and increase heat at idle; this serves to ensure a more complete burn and also assists cars with A.I.R. pumps, as a hot charge makes it easier to finish burning "unburned hydrocarbons" in the exhaust.

I've long been an advocate of setting it up the way the factory did, but the question is whether or not there's a downside to running manifold vacuum on a car designed to be set up ported. I don't have an AIR pump.

Is the mechanical advance curve from an early 60s (manifold) different from that in the early 70s (ported)?