Originally Posted by Cousin_Joe
I was wondering if there is any difference (or preference) in running vacuum advance through manifold or ported in the two ways below. The various items I've read on manifold -vs- ported aren't perfectly clear to me, I have a 383 (vortec heads, 9.7:1, 268xe cam, 2000 stall OD) which likes more initial so I have a vested interest. Perhaps it's related to my advance can, but the manifold seems to fluctuate the advance* for me.
A) Running 10* initial w/ ~10* from manifold vac advance, and using 'full' mechanical adv. or
B) Running 20* initial w/ 0* from ported, and limiting the to mechanical advance.
Ported vacuum with-holds vacuum advance at idle, no vacuum will be applied till the throttle blade moves past the port thus exposing it to the pressure inside the manifold. As the throttle moves toward WOT at some point the manifold pressure and atmospheric pressure become nearly the same, thus no vacuum is present with a normally aspirated engine. So the vacuum advance amount becomes less and less as the throttle is opened.
Un-ported vacuum shows the advance mechanism manifold pressure from the moment the engine fires. To that end vacuum advance is present at idle but otherwise functions as outlined in the first paragraph. This can help a low compression engine or a long cammed engine to idle and transition to mid throttle more smoothly.
There really isn't a right or wrong here, it's more studying what the engine as it's installed in the vehicle likes best.