Initial timing is only 1/3 of the timing equation. You have to be concerned with the total timing advance, which is a combination of initial, centrifugal, and vacuum advance. Initial advance is what's set with a timing light, at idle speed, with the vacuum line disconnected and plugged. Then there's cewntrifugal, which is speed-dependent from a set of weights on the distributor shaft opposed by springs. The higher the speed, the more the advance, until the system reaches a mechanical lock. Then there's vacuum advance, which responds to ported vacuum from the carburetor throat. it's supposed to go away at idle, be high during part-throttle cruise, and go away again at full throttle.
The combination of initial and centrifugal advance that works best for small-block V-8's is 36 to 37 degrees at about 2,500 RPM and higher. The vacuum advance will add about 10 degrees for part-throttle cruise operation, so up to 46 or 47 degrees for an engine running at no load and 3,000 RPM is not uncommon. Disconnect the vacuum line, and it should drop to 36 or so. I like to set the advance to 36 at high cruise, with the vacuum disconnected, and let the initial setting seek its own level. As long as the engine starts without a kickback, and it doesn't detonate on a pull, you're good to go. This method requires a delay-flash timing light, however, to return the timing mark to zero from cruise RPM.
I also alter advance curves and total centrifugal advance on my race engines, but that requires a distributor tester and some filing, welding, spring changes, etc. Like the disclaimer on TV says, "Don't try this at home!"
Jerry
