From a Mopar Muscle Magazine article:

Quote:

While centrifugal advance varies timing strictly by RPM, 3,000 rpm at wide-open throttle on your local dragstrip's launch pad doesn't call for the same timing as 3,000 rpm on US Hwy 99.

For increased efficiency, vacuum advance is a way to vary the timing in accordance to load. Linked to the distributor's pickup (or breaker) plate, the vacuum advance uses a spring loaded vacuum diaphragm to pull the plate in against the reluctor’ s (distributor cam's) rotation to advance the timing.

On the dragstrip, with your right foot mashed to the firewall, the engine sees full load for the whole 1,320 feet. So, purpose built racecars and racing distributors don't need vacuum advance. However, on a streetcar it is a must for maximum efficiency, plug life and economy.



And this:

Quote:

VACUUM ADVANCE

Only once the centrifugal advance curve is sorted out should the vacuum advance be dialed in. There are two considerations with vacuum advance: How much total and how soon?

How much total is built into the unit. The arm off the diaphragm has a stop built into it, which limits how much travel is available to advance the timing.

There are a bunch of different units, and fortunately most are stamped with the amount of advance in distributor degrees, which is half of crank degrees. Eight and a half degrees will give you 17º at the damper at full advance.

Theoretically, grinding back the two stops could increase the advance “in the can”, but in practice they're very difficult to get to. Advance can be taken "out of the can" by epoxying tabs which bend over the stops, but not, as I've seen suggested, by welding/brazing the stops. That'll cook the diaphragm.

When the vacuum advance kicks in, it can have a significant effect. If it starts at a too low vacuum level, light part throttle acceleration can cause detonation problems. Try to adjust the spring tension on the diaphragm by inserting a 3/32 inch Allen wrench through the vacuum nipple.

Counter clockwise delays the action, while clockwise kicks it in at a lower vacuum level. Most, but not all, diaphragms have this provision, with one full turn typically changing the kick in point by 1 hg. Vacuum.

As with centrifugal advance, to tune vacuum advance requires its curve be mapped. Again, this can be done with the engine used as the distributor machine, only this time using a hand vacuum pump to check the rate of advance for various vacuum levels.

The procedure is simple enough: Pull up vacuum with the hand pump while checking the timing. Note the vacuum pump's gauge point where the advance just begins to kick in and record this.

Move the vacuum level up in 1 inch increments, recording the vacuum level and how much the timing has correspondingly been increased. There's a trick here: Lower the idle speed to well below the RPM at which the centrifugal advance kicks in each time the vacuum pump level is pulled up.

Otherwise, as the vacuum advance brings up the timing, the RPM will increase causing the centrifugal advance to come into play and invalidating your readings.