Ported or manifold Vacuum to Dist.

Dana67Dart said

In a relative nut shell...
Manifold vacuum will be high at closed throttle.
Ported will be high at open throttle.

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

The CAP valve does some thing with both to slow the timming return to no vacuum, IIRC. There is a CAP factory manual on imperial club web site again IIRC

Click to expand...

Ported vacuum has the vacuum inlet above the throttle plate, so with the throttle closed it sees no vacuum.
Manifold vacuum has the vacuum inlet below the throttle plate, so with the throttle closed it sees full vacuum.
As the throttle plate opens ported and manifold vacuum are the same.
Full open throttle, ported and manifold vacuum will be near zero.
Manifold vacuum is used to add ignition advance at idle. Adding advance at idle is helpful to stabilize idle quality and to prevent misfires at idle on engines that have cams with overlap that results in reversion and exhaust gases getting back into the intake charge. That exhaust and fresh charge is more difficult to light off and burns slower when lit, so on some engines and some cams, it is useful to add advance to the ignition at idle.
Off idle, as the throttle is opened ported and manifold vacuum are the exact same.