For Mattax

Dana,
Post #4. The VA unit has a spring & rubber diaphragm inside, just like a Holley p/valve. Adj VA units that an adjustable plate that compresses [ or releases the spring ] to counteract engine [ manifold ] vacuum. Vacuum is high at idle, so idea is that spring is fully compressed against the stop, adding extra timing to the static timing. Example, if static init is 6* & MVA adds 20*, engine will idle @ 26*. This is exactly how my GTO was delivered from the factory, idling with 26*. Most adj VA units can add up to 30*. You can buy a stop plate for GM dists that limits the total travel of the actuator arm; with Ch you have to fabricate a stop. The spring adjustment is to ensure steady timing at idle, & in gear. The #1 cause of comments like 'MVA didn't work for me' is not MVA, it is the setting up. Using a non-adj factory VA will have too stiff a spring & timing will be erratic; similarly, using an adj VA unit with the spring adjusted too stiff does the same. The car has to be placed in gear, at idle, to make sure the timing remains steady. MVA can add 1-3" of vacuum at idle because the extra timing burns the fuel more efficiently. Excellent example of this in post #5 where the engine runs cooler with more idle timing; engine is using the timing more efficiently, sees less load, runs cooler.

VA goes to zero @ WOT. Timing at WOT will be the addition of the static timing plus the centri curve inside the dist. Above example, 6*init + 30* centri, engine has 36* @ WOT.
Hope this explains it better.