[bobcowan]

Essentially, the purpose of "octane" is to prevent detonation. You need enough octane to prevent that. How much that is depends on a variety of facters.

The most important is probably dynamic compression. Dynamic compression includes factors like static compression, intake valve closing point, altitude, etc. There are a number of calculators on the web that will get you the right numbers. For 93 octane, you generally want less than 8.5.

As the intake charge temp goes up, so does the detonation.

Lean mixtures detonate more than rich one's. Partly because of the additional fuel, but also because it has a cooling effect. Yes, you'll spend a little more in fuel, but pistons sets are pretty darned expensive.

Too much timing will ignite the fuel too soon, and that causes a significant amount of detonation. Some people think you need to run 36-38* of timing to make good power in a small block Ford. But the truth is that many engines will make more power with 32-34*. Keep the timing conservative if you need to, and don't worry about the little bit if power you might be giving up.

And, finally, engine load is also a big factor. Not usually much of an issue with a big engine in a light weight car. But if your power band is 2,200-6,200 rpm's, pulling a big hill in 5th gear at 1,800 rpm's is putting quite a load on the engine.