Vacuum advance mandatory for a street car?
Here's an example right out of a Chrysler manual for a street hemi
Notice that under "vacuum" they are showing different vacuum readings, and the max shown is about 11 degrees at 15" Hg vacuum.
This once again is distributor degrees
Notice down at the bottom, an initial of 12.5 BTC
Notice mechanical (centrifugal) is 9.5 at 1400 RPM. Again, X2 for crank, or 19 degrees
So for this engine, a factory initial of 12.5 + 19* mechanical gives you a total of 31.5 degrees
Add to this the max. vacuum of 11 (22 at the crank) and this means that above 1400 RPM, at light throttle, this engine would see timing of 12.5 + 19 + 22 degrees or 53.5*
It was mentioned that such things as changing carbs can change port characteristics. This can be a problem. "Back when" (70's--80's) when I had a warmed over 340 in my Landcruiser, which weighsabout 4K, I had several different carbs on the engine, from Q J to AFB. I had some trouble with part throttle ping, evidently because one / some carbs had "too much" vacuum at the port too early