Ported or manifold Vacuum to Dist.

pishta said
IIRC ported was for emissions.

A fallacy I believed for a long time. Then I started going through old FSM and looking at pre-smog engines.

TrailBeast said
Most racers use manifold vacuum. (if advance is not locked out)

LOL. Most racers I knew used no vacuum advance at all. For drag racing, its one more thing to deal with - and its not needed.

TrailBeast said
With ported vacuum full advance comes in as the throttle is opened and drops back as more throttle is given.

Good description!
Diagram showing how that's done from the same '69 MTSC page as linked above.

Dana67Dart said
Some older Ford's I have had had both going to the advance canister. And it kind of averaged the two.

The mid 80's AMC jeep engines also used an non-linear valve (NLVR) that mixed ported and manifold vacuum while the engine was warming up. At normal operating termperature, it used ported. If coolant temps got too high at idle, it would switch to manifold. These were cat equiped engines and initial timing on the 360s were 10 to 12* at 600 to 650 depending on which year you look at.