Over the past 60-something years, I believe I can count the engines I've seen that were designed to use full manifold vacuum on the distributor on the fingers of one hand, with a couple left over. One example was the early Chevy 327 V8- - - -1962 to 65 or so. Practically all the others use ported vacuum, with no advance at idle other than the initial spark lead set at idle speed. Ported vacuum advance happens at part throttle cruise, and goes away during acceleration, trailer towing, passing, etc.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!