[jeffko]

A good starting place is as Manowar stated. It also depends on what the engine likes, compression, carb tuning and getting good manifold vacuum. Most aluminum head designs can take a lot of advance though. I have had mine as high as 22 experimenting with setups with 11.0:1 and AFR205 heads on 93 octane and no detonation. After 22 degrees you could tell then engine didn'y like it and idle suffered. Probably would have started detonating at that point. I even tried 110 low lead aviation fuel with the same settings which helped the engine run cooler but little else. My engine performs it's best at 14 initial and 38 total @ 3000 rpm.