A retarding distributor cannot be timed to the bulb without manifold vacuum or it will be too retarded when the rpms increase.
Now that is a new one, I would like to learn more. You are saying my timing should advance up to a point and then retard under high output? And this would be accomplished in my 1972 distributor how? Here is another thread where someone mapped timing advance against rpm. No where do you see a retard at the end of the curve.You can optimize its operation on manifold vacuum by trial and error. However, it won’t do what the vacuum advance / retard is supposed to do: retard the ignition under WOT high output conditions where you have precognition / detonation / pinging.
Yes. The point of a vacuum advance (or retard) distributor is to retard the spark under high engine load and low RPM, like going up hill in high gear with the pedal to the metal.
This was accomplished in your ‘72 by the VSV applying vacuum to the distributor with a WOT And low speed.