The first thing to remember is the ONLY difference between manifold and ported vac is at idle
. Otherwise, both vac sources operate exactly the same.
The second thing to remember is a lean air/fuel mixture takes longer to burn than a rich one. I'll get back to that....
Don't confuse centrifugal advance and vacuum advance. They perform different functions. Centrifugal advance is a function of engine RPMs. As the compression stroke speeds up with higher RPMs, the centrifugal advance speeds up the timing of the spark, so that peak cylinder pressure remains just after TDC. Therefore, centrifugal advance has a direct affect on power and performance under load.
Vacuum advance, on the other hand, doesn't affect the power curve, at all. It is a function of engine vacuum pressure. When your engine is idling, or you are cruising with low load, vacuum pressure is high resulting in lean A/F mixtures. Since a lean mixture takes longer to burn, the vac advance speeds up the timing of the spark for a more complete burn under high vac pressure situations.
Here's where the major difference is. Manifold vac is present at idle, ported isn't. Ported vac advance retards timing at idle, which creates an incomplete burn in the combustion chamber. This does reduce NOX emissions, but it also significantly increases exhaust temps, which in-turn increases coolant and overall engine temps. With increased exhaust temps, engineers found they could inject fresh air (AIR pump) into the exhaust stream and increase the after-burn of hydrocarbons in the exhaust.
So, ported vac advance does reduce emissions, but it's at the expense of idle performance and efficiency. It takes away advance when your engine needs it most....under the high vacuum, lean mixture situation at idle. It increases engine temps, reduces fuel efficiency, and makes a weaker less stable idle.
You don't have to take my word for any of this. I, too, was skeptical until I started experimenting and recording my own results. Once I had a clear understanding of spark timing, the choice between manifold and ported was easy.