Originally Posted by oldbogie
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.
This is the best explanation I've seen here. The only difference between mainfold and ported vacuum is that the former is taken below the throttle blades and the latter is taken from a port (hence the term) just above the throttle blades. As soon as the throttle is opened enough to uncover the ports, manifold and ported vacuum are exactly the same. The only difference is when the throttle is closed. As pointed out, depending on how the engine and the distributor are set up, you want one or the other.
What hasn't been pointed out is that ported vac advance really came into favor in the late 1960s as an easy way to control NOx emissions at idle. By preventing the vac advance from working, the combustion was less efficient and NOx was reduced. This same concept is why cars of the late 60s and early 70s had Transmission Controlled Spark. The vac advance was disabled in all gears except top gear to further reduce NOx. Since this tended to make the car run hotter, these systems also had a temperature controlled valve that would bypass the TCS and run full manifold vacuum to the distributor in the event that the car was about to overheat. Note also that this disabling of the vac advance was the concept behind the "NOx device" that was required to be retrofitted to 66-70 cars in the state of California.