[68 P.O.S.]

Originally Posted by dmjlambert

It's not pinging.

I have 7 BTDC initial plus 23 from the can = 30 at idle and add mechanical advance through most range of engine use. If the vacuum drops on WOT to remove the vacuum advance, I would then have 7 BTDC initial plus mechanical advance.

If I change from a 23 degree can to 15 degree can:
I could have 15 BTDC initial plus 15 from the can = 30 at idle and add mechanical advance through most range of engine use. Same amount of total advance as above with the 23 degree can. If the vacuum drops on WOT to remove the vacuum advance, I would then have 15 BTDC initial plus mechanical advance.

So it seems if anything I would have more pinging with a 15 degree can, because it would retard the timing less in WOT. I may need to bring the initial down to 12 BTDC to keep it from pinging under load, which would then make my total timing less than I was getting with a 23 degree can.

What I'm trying to do is figure out why one amount of vacuum advance may be better than another. In what circumstances is less vacuum advance advantageous?

I take it you didn’t read the links I posted. Vacuum advance is only meant to be supplemental for when it is needed (in low load situations like idle and cruise so it can burn the lean mixture more efficiently, keep the engine cooler, and improve mileage and streetability), not used as your primary means of advance. That’s what the mechanical advance is for. What distributor do you have and how many degrees of mechanical advance does it have? How much vacuum are you pulling at idle? The crate engine heads you have on it, are they vortec or standard heads?