Please excuse the "partial hijack" of the thread, but here's some real good information on "manifold vs ported" vacuum. I know I've scratched my head on this topic more than once!

As has been stated above, when an engine is idling or at a steady cruise (when the load is very low) intake manifold vacuum is high because the throttle butterflies are nearly-closed; under acceleration, the throttle butterflies open wider, and intake manifold vacuum drops; it is essentially zero at wide-open throttle. The strongest vacuum signal will always be found in the manifold at idle. As stated – it is consistent anywhere in the manifold +/- scant percentages.

In my opinion, ported vacuum should be avoided at all times. Ported vacuum was introduced by GM as a component of their Air Injector Reactor (remember AIR?) system. It pumped fresh directly into the exhaust manifolds to encourage an afterburner effect to incinerate pollutants in the exhaust manifolds. For this system work at maximum efficiency it was necessary to retarded spark at idle. With retarded idle spark timing, the combustion burn in the cylinder begins late, and is not quite complete when the exhaust valve opens. This accomplished two things which (in theory) would reduce emissions: 1) the incomplete burn reduced combustion chamber temperatures, which reduced the formation of oxides of nitrogen (NOX). 2) The very significant increase in exhaust gas temperature helped ignite and consume the hydrocarbons in the exhaust flow as the fresh oxygen-rich air was introduced from the air pump.

As many of us can remember, these engines ran poorly. The problems centered on the enormous amount of wasted “heat energy” that was transferred through the exhaust port walls into the coolant, causing these engines to run very hot at idle. This then caused cylinder pressure to fall, overall engine temperatures went up, combustion efficiency suffered greatly, and fuel economy went down with it.

“Ported Vacuum” was simple to implement – GM just moved the distributor vacuum port orifice in the carburetor from below the throttle plate (where it was exposed to full manifold vacuum) to above the throttle plate, where it was only exposed to manifold vacuum after the throttle plate opened. This meant that the vacuum advance was inoperative at idle - retarding idle spark timing from its optimum value. These engines had low initial timing settings; they were usually set at 4 degrees before TDC and some even had initial timing settings as much as 2 degrees after TDC. The vacuum advances still worked at highway speeds, but not at idle, which gave GM a deserved reputation real dogs for several years.

Ported Vacuum was introduced as a component in early emissions control strategy. Anyone who tells you that ported vacuum is a good thing for performance and “driveability” is mistaken– it’s not.

(Information above is excerpted from “TIMING AND VACUUM ADVANCE 101” by John Hinckley)