All correct but one more thing I would add is due to the original goal of reducing emissions, the OEM vacuum advance may not be the best fit for performance, especially if there has been a cam change which results in an idle vacuum reduction. Here's a pretty good read on the subject. Very detailed: https://www.crankshaftcoalition.com/wiki/images/e/e4/Vacuum_Advance_Specs.pdf Excerpts from the .pdf: One of the key statements is the following: (VAC is short for vacuum advance canister) The basic rule for vacuum advance control (VAC) selection (henceforth referred to as THE RULE):THE VAC SHOULD PROVIDE FULL ADVANCE AT NOT LESS THAN 2" LESS THAN PREVAILING IDLE VACUUM AT NORMAL IDLE SPEED WITH APPROXIMATELY 24-32 DEGREES TOTAL IDLE TIMING. One other issue. Some think that ported vacuum advance is "correct", but it is NOT on pre-emission controlled engines.Ported vacuum advance is an emission control technique to increase EGT, which promotes oxidation reaction in the exhaust, but it also increases operating temperatures, increases the tendency to detonate and run-on at shutdown, and increases fuel consumption. With a handful of exceptions, all GM pre-emission engines equipped with vacuum advance used full time manifold vacuum.For some inexplicable reason, the '63 FI engine used ported vacuum advance- the first year vacuum advance was used on Duntov-cammed engines. Maybe GM thought that idle quality (always a problem on FI engines) would be better with ported vacuum advance, but it wasn't, and the '64-'64 FI engines got full manifold vacuum advance.L-72/71 have ported vacuum advance to meet CA emissions since there was only one version of this engine for all 50 states.If your experience with Corvette engines does not go back to pre-emission engines, then all you've ever seen is ported vacuum advance on emission controlled engines.Duke To do it right, you need a vacuum gauge and a Mityvac to test the canister, which many times is bad.