Taylor wrote
You meant retarding the spark for gasoline with lower
anti-knock performance, not advancing it.

I meant dynamic control of the spark advance, such as is normal in
automotive engines. Spark advance refers to how far in advance of TDC
(top dead center) the spark fires. But yes, terminology aside, you
are right - I meant using less spark advance than is common now. And
yes, that will slightly reduce the power output as I mentioned.

By the way aviation gasoline performance numbers (e.g. 100/130, 100LL [which is
really 100/130LL], 80/87) are not directly comparable to automobile pump
anti-knock index (commonly called octane) in the US, which use the R+M/2 method.

This is correct. The reference engine is different. However, the
reference engine for aviation use is also not comparable to modern
aviation engines. In other words, while the numbers are not directly
comparable, they're pretty much equally useful.

Michael