Re: Timing & Vacuum Advance 101
I found this info in my archives. I don't know who to give credit to, but it's good info:
GM hasn't made vacuum advance units for decades, but they still sell them, made in the aftermarket by Standard Products (and Echlin before that) and boxed by AC-Delco.
All of them provide 16* of advance, and the four calibrations that cover most usages in terms of when they start pulling and at what vacuum level they're fully-deployed are as follows:
VC-1802 (stamped B22) - starts at 8", done at 15"
VC-680 (stamped B1) - starts at 8", done at 16"
VC-1765 (stamped B20 or B26) - starts at 6", done at 12"
VC-1810 (stamped B28) - starts at 4", done at 8" (same as the original Delco "236-16", sold under GM #88924985 and AC-Delco #D1312C). I buy them in bulk for about six bucks each from GMPartsDirect.com. This is the most aggressive unit available, and is the only one that works properly with a "30-30" cam.