GM is the same. Vac turns the bottom wheel thats normally only moved when you rotate the distrib.
This whole subject seems like Sasquatch to me. Everyone knows about, no ones seen it. With youtube and web images I'm surprised theres not 1 image of timing going above total. Not that I have found anyway.
I know normally after you set total mech advance you reconnect vac and go for a drive to listen for me. People tell me that the ping Is because vac is adding too much to total any going too far advance (above 50 for example). My impression is that the vac canister is wrong got that car and isnt 'removing' its idle advance soon enough and you got mech advance coming in at the same time. You might be at 30* partial advance when you only need 25*
For now I'm staying locked at 38* and letting the msd do the idle and advance curve