[dwaite72lnghrn]

I always look at vacuum distributors as giving you timing retard. At idle you pull full vacuum 20" (If in full vacuum port and stock cam). As you step on the gas vacuum drops so does your advance, aka retard, once you get to speed the vacuum climbs back up giving you your advance again. Load the engine vacuum drops timing retards giving you your mechanical advance.
Ported vacuum gives no advance at idle (good emissions numbers) as you come off idle vacuum jumps to 18"-20"and as long as the throttle plates are open you will get vacuum just like full vacuum port, only it seems to drop faster than the full port and under heavy load vacuum will drop of to near 0" faster.
So in the past the suggestion has been to run ported on HEI and full on Points. It doesn't seem to matter much as with cruising speed both ports pull 15"-20" depending on condition of engine. If you were to coast down a hill though full will be around 20"-22" and ported will be at or near 0". just my 2 cents.