One degree of movement of the distributor changes the ignition timing at the crankshaft two degrees, since the crank turns twice as fast as the distributor. It doesn't take much adjustment at the octane selector to make a big difference at the flywheel. Also, one degree of ignition timing over-advance once the best torque point under load is reached robs as much power as 5 degrees of retarded timing from the ideal setting. I've proved that repeatedly on dyno runs. Over advance is trying to make the engine run backwards. ALWAYS err on the side of "not enough" timing advance. That's why monitoring vacuum at idle while changing timing makes no sense at all.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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