The Stovebolt Site Forums The Shop Area Engine Shop Timed with Vac, now won't start

Optimal would be "advanced until just below when it pings under load."
It is a seat-of-the-pants determination. Drive it up a hill under load and listen. Make an adjustment. Drive it up a hill. Wash, rinse, repeat until it pings them retard the timing and drive it up the hill again and listen.
These engines were designed back when you never knew what exact octane you would end up with at a gas station.
This issue was so prevalent that GM included an "Octane Selector" on the distributor so that the driver could compensate for the various octane gas one would run into.
The octane selector allowed for changing the timing to keep the engine from pinging without losing ones place so they could reset it back to normal by using the marks on the octane selector. No timing light needed.