I believe most pointer/ball relationships are around 5 degrees advanced before TDC, but there are several possible bellhousing/flywheel combinations that might not make that an absolute fact. It's fairly simple to establish a true TDC reference by using a probe that screws into the #1 spark plug hole and stops the piston travel slightly before the piston reaches the top of the stroke, marking that position on the flywheel, and turning the engine backwards until the stop point is reached again. Mark that position, then measure halfway in between the two marks. That's an accurate top dead center position. Once true TDC is known, a delay-flash timing light can be used to evaluate initial timing, centrifugal, and vacuum advance. Any other timing method is either a WAG, a SWAG, or maybe a T-LAR!
Dave, your Blazer has a "detonation sensor"- - -basically a piezoelectric crystal that generates a voltage when it hears a ping, and sends a signal to the computer to retard the timing 3 degrees and listen for a ping again. It keeps doing that until the detonation goes away. It's doing that dance 10 times a second- - -much faster than a human ear can accomplish, and it's hard to reach the distributor housing to tinker with the timing from the driver's seat! The way to test the det sensor operation is to run the engine at a steady 2500 RPM and tap on the exhaust manifold with a hammer, and watch with a strobe light as the timing retards. I've seen a noisy valve lifter drop the timing back to 5 degrees AFTER TDC!
Jerry
