[c6ae]

I have played with this timing detail quite a bit, (428 w/ CJ specs)
Adding a vacuum advance and using the factory specs for a 390-4V as a benchmark to start tuning I increased my mileage from 11 to almost 14... that’s better than 25%!

The advantage of the vacuum set-up is that when you increase engine load the timing automatically retards. With a dial type electronic control if you forget to do this severe engine damage may result.

The stock specs (in distributor degrees) are;
Mechanical centrifugal advance 11-1/2° at 2000 with a vacuum advance of 10°. These are added to a static crankshaft-timing advance of 10° for a total of 51° crankshaft degrees at 4000 rpm

My set-up is like this- in crankshaft degrees;
Static 10°, plus 25° centrifugal all-in at 3000 plus 15° vacuum advance. Total is 50°

The vacuum control is connected at the timed port so that the vacuum advance comes in at about 1000 rpms (idle speed is 800)
My car has a “tall” gear so to be drivable at slow speeds (5 –10 mph) I need to keep the initial advance and idle speed low to avoid excessive clutch slipping. A different engine may want completely different timing specs