My reply looks reasonable until i hit submit and then it is turned into a ridiculous sea of words, without spaces.

"What IS ported or manifold vacuum? Manifold is a connection directly at the manifold.

Ported is a little hole into the carb, just on the carb side of the throttle plate. At idle, the hole is pretty well sealed off from the manifold, so no vacuum. As the throttle opens, this hole becomes more connected to the manifold, so vacuum at the port increases. It actually increases above manifold vacuum, which is dropping since the throttle is opening more, but there is high velocity air passing the port, causing more vacuum. Once the throttle is more than about 1/3 to half open, the manifold and carb are both seeing low vacuum, but there is still high airflow past the hole, so it sees a bit more. In practice, the port vacuum will be a bit above manifold from above idle to about 1/3 throttle."

"Manifold ported systems tend to have vac advance calibrated to give a fair advance at idle, say 5 static plus 10 vac, and they hold enough advance as the throttle opens to let the engine get going fast enough that the majority of the mech adv comes in before the vac advance disappears at wide throttle openings. If they don't do this they bog horribly. At part throttle cruise, both ported and manifold tend to give similar vac advance, for good economy. ?IOW, the port/manifold thing doesn't matter to the distributor per se, but it sure does to the engine, and the distributor is calibrated to account for this.?

A ported outlet will have no or very low vac at idle, and rise rapidly as you open the throttle off idle, then drop as you get to very wide throttle openings.?Ported systems typically have no significant vac advance until the throttle is well above idle, so you have the static setting, say 5deg, plus a bit of mech advance, say another 4 deg at 800 rpm, but zero vac adv, for a total of 9 at idle. "

How did they decide between the two options?

"The emissions setup changes things, they are so lean at idle that they need a lot of advance to run at all. " This is why MG Midgets running 1275 engines in the early '70s changed from ported to manifold vac for their US spec (strangled) emissions engines. They kept ported for their UK spec (optimized running) engines.

This is why I changed mine from manifold to ported vac, and had Jeff re-curve my distributor and replace the vac advance to suit it. Basically I have my 1275 tuned as per 1967 spec (the one they used on 1275 Midgets in the UK from '67 ~ '74).

Norm

If the port is located on the manifold, that is manifold vacuum (vac rises to max at idle).

Both types see about the same amount of vacuum everywhere else in the throttle position/RPM range, it is the difference at idle, I think, that is the most different.

Jeff Schlemmer at Advanced Distributors, or Joe Curto could explain it better. Joe helped me to sort it out on my car.

N