My old iron headed 414 with tight quench and a Ross dual quench style piston @ 12.1:1 on 110 race gas liked 30* the best. Bumping down from 34 to 30 at a T&T netted over a tenth and a half gain and nearly 2 mph. Never really found the time to try it lower. Interestingly, when I freshened the motor just before I sold the car, we went with a diff cam, but that was the only change and it HATED 30* . Set it at 34 and picked up almost 2 tenths, go figure. I only ran it that one weekend then sold it, so I never got to really fine tune it. I can only guess that the cam choice sacrificed efficiency in the combustion process.