In the "how to" section on Tom langdons site they cover this topic fairly well. They recomend aproximatly 20 degrees vacuum advance at idle/cruise. It would be impossible to run that timing under a load, the whole point is to retard timing when you nail it. That is also why big trucks, full race cars, and generators dont need it(they are always under a load). Some timed ports will give you zero vacuum at idle and all timed ports give more the faster you go. It seems easier to just work it out with the initial timing and mech.advance, than to use timed vacuum, because timed vacuum responds only to speed, (not load) and we already have our mech.advance to do that. I have noticed that on the C cab Fords from the late 70's and early 80's (all use Holleys with ported vacuum advance) you can pull of the vacuun advance line while running and the idle does not change. Sorry to beat a dead horse so hard I am just having trouble understanding the need for ported vacuum advance.
