Just to confuse things a bit more, factors like fuel mixture, combustion chamber design, the offset of the piston pin, and the length of the connecting rod can also have an effect on where the ignition timing needs to be set. In general, the flame front that travels across the combustion chamber needs to meet the top of the piston just as the crankshaft rocks over center and the piston starts down the cylinder. Since the flame speed doesn't change much, but piston speed does, the spark must happen sooner as engine speed increases. That's why most engines need 35-37 degrees of spark advance at cruise speed, and only 0-10 degrees at idle. Then there's vacuum advance- - - -the spark can be advanced another 8-10 crankshaft degrees at light throttle cruise for better fuel economy. This advance goes away as soon as the throttle is opened for hill pulling, towing, or passing, leaving only initial and centrifugal advance when the engine is working hard.
After 40-something years of doing dyno runs, I've discovered that even ONE degree of over-advance once maximum torque is achieved at full throttle and heavy load robs more power than 5 degrees of spark retard from the ideal setting. Always err on the side of "not enough" spark advance- - - - -firing the spark plugs too early is trying to run the engine backwards! By the time you can hear a "ping" on an uphill pull, you're WAY over-advanced on the spark!
Tom, the best (only) tool to determine optimum spark (and/or valve timing) on any engine is a dyno. Unfortunately, very few people have access to one. I'm thinking about equipping a trailer with a bellhousing dyno and the equipment to make it portable and attending a few events like Homecoming or the KC gathering with it. Of course, a chassis dyno would be the best setup, but a portable one of them gets into the $100K+ range!
Jerry
