I am not dissagreeing with you in any way but if that is the case then why do we have vacuum advance on the distributer if the timing did not need to be advanced as RPM's get higher? I definetly want to know more about this.
Timing does need to be advanced as RPM's increase, but when you get near WOT and/or heavy load, the vacuum drops off. The mechanical advance is only reliant on RPM.
I agree with algmmech with everything except the part about the vacuum increase at the manifold port as the throttle just starts to open, then the vacuum decreases when throttle is increased. But then again, I drive with a heavy foot and have no reason not to believe him. I do a lot of tuning with a vac gauge as well.
Example: Your distributor advance needs ported vacuum. The ported vacuum pull advances your distributor timing as you accelerate. If by chance you tie into manifold vacuum by mistake (and properly adjust your timing) you will actually retard your timing as you accelerate.
This is BS. Vacuum is vacuum, the level of vacuum and how it plays out in relation to the throttle is the only thing different. There are people that run manifold a vacuum source instead of ported to the dizzy and have no ill effects. Manifold vacuum will not "retard your timing", you just will have a different amount of spark advance. I've done it and to be honest, didn't notice much in the way of performance gain or loss as a result. Some engine performance people will specifically tell you to run manifold vacuum. Try it.
You can look at it like this. The manifold port reads it's maximum at idle and will decrease under load or throttle opening at a somewhat proportionate rate (as long as load remains constant) until it reads next to nothing at WOT. Ported vacuum will read zero at idle (or it should unless the idle is too high). When the vacuum plates open but not a lot of load (read higher RPM with only partially open throttle plates), vacuum becomes present and is at a fairly substantial level. When load increase (read low RPM, open throttle plates) or the RPM's increase with no load, the vacuum level starts to drop off. At WOT, again, near zero vacuum.
When plotted out, the manifold source will start off at 100% and slowly taper to zero, fairly linear and predictable. The ported source will start at zero, rise to it's peak under partial throttle and no load, the taper back off again and will look more like a bell curve.