This a video that Uncle Tony had the other day that left me puzzled for this reason.... Of course the timing drops back to initial when you accelerate with it hooked to direct, so what? Can someone explain why that matters? I have one car off ported and one car off direct. I don't see any difference in any normal driving conditions. The off manifold does idle cleaner as I would expect. I run about 12 degrees of initial.
Manifold vacuum allows you to run a little less initial mechanical timing and ramp up initial timing via vacuum for a better idle n heat control with a modified engine n crap gas. The key to it is adjusting the vacuum canister. Not all engine combos are the same so can't tell you where yours should be. As you come off idle and accelerate normally, vacuum will rise, ported or Manifold and the distributor will act the same either way. At higher rpms, highway cruising, you want a lot of timing. Vacuum will be very high and the same using either, thus pulling more vacuum advance and using mechanical/centrifugal advance. As soon as you stab the accelerator to pass, Manifold vacuum will drop instantly, reducing timing back down to max mechanical/centrifugal advance that engine makes without detonation. W ported, the venturi effect of the throttle ventures will momentarily increase vacuum at that full throttle burst. For a moment, you will advance timing a little more before vacuum falls back down. The cars that came with old fashioned electronic emissions controls would use a sensor and reduce the timing electronically during that hard initial full throttle. Manifold vacuum on a car with "no" electronic emissions control will never be wrong. Ported vacuum on a car w no electronic emissions control, depending on modifications, weather, driving conditions etc... "may" cause a detonation issue when going to a full throttle "stomp", not always.
