The ported vacuum is there to help give an initial advance "bump" just as you start to open the throttle, to help burn the sudden rush of air and fuel. It peaks just as the butterfly cracks open a little. By 1/8 - 1/4 throttle it has weakened significantly. If you push the pedal deeper, ported vacuum DIMINISHES and eventually goes away almost completely if not 100, so there is NO vacuum advance at WOT or high throttle openings. It is there only when it is needed: when you just crack the throttle open to help you get going, AND at cruising speeds when the throttle remains barely cracked open, and the load is low. This gives you better throttle response in the first scenario, and better fuel mileage while cruising, second scenario. This is how, and why, it was used in the vast majority of engines that used vacuum advance distributors. They were designed to run this way.
Using intake manifold vacuum for advance is (excuse me) retarded, to me, because you have high vacuum at idle, and as you crack the throttle open, it starts diminishing, and only keeps going away the more throttle you give it. I'm tempted to say it does not help at any point... but not yet.
It allows you to run more initial timing at idle, and I guess that in theory at least helps you get moving if you open the throttle slowly, and the vacuum diminishes slower.. but then the engine falls on it's face when you need it to pick up speed and react to throttle. But, there are engines that were designed to run it this way, and they would not (necessarily) run well with ported vacuum, all else being stock.
One does not work in another, necessarily. I would assume it would require many changes in jetting and advance curves etc. to change an engine designed for manifold vacuum, to start running ported vacuum. Which is kind of what I am after. I have built countless engines based on ported vacuum, stock, street and race. But I have only dabbled in manifold vacuum engines. Mainly because they seem to be crude and unsophisticated choked-to death iron lumps with low IQ and low power for the CC's
I wouldn't mind if I could find a fully adjustable mechanical only distributor for this engine for cheap. Done that too on many ported-vacuum engines with BIG cams. Anything is adjustable though, just needs more work when you have to start with a stock distributor and experiment...
