Thanks for a good answer. Cab was also pointing in the right direction.

So the answer to the first two questions is that higher octane fuel will change your Air/Fuel ratio either up or down, and it can make the mixture either richer or leaner.

Higher octane fuel either has more, less or the same energy per gallon.

Trace knock happens and can affect power output. Changing to a higher octane fuel with everything else being equal can cause
increase in power due to elimination of trace knock.

Gasoline DOES NOT have the same amount of energy per gallon regardless of octane. It can vary, as gasoline is a common name for a range of mixtures of molecules.

It is true that the specific gravity of the particular fuel governs the amount of fuel metered by the carburetor. While this will have an effect on the mass Air/Fuel ratio, the effect on whether the mixture is richer or leaner can go either way.

Ethanol has less energy content per gallon, but a higher octane number. My rule of thumb is that 10% ethanol results in a 5% loss in mileage. As I have been using oxygenated fuel since 1991, I have had no problems with it. For about 85-90% of the country's population, oxygenated fuel is all the EPA allows to be sold except for race gas which is generally for off-highway use. All the urinating and crying about alcohol gives me a headache. It's funny that most of you have no objection to putting ethanol into your bodies! I think I'll go have a beer.

Racing gas companies have had no problem using ethanol in their fuels. Indy cars run on pure ethanol, switched from methanol to look "green".

Truth is, every different fuel needs a specific tune, and that's why Oxygen sensors are so valuable. The post talking about Lambda is right.

Now on to detonation. The absence of detonation does not mean it was a cleaner burn. Not in the least. For example, one way to reduce detonation is to make the mixture rich. All the absence of detonation means is that whatever was in the cylinder didn't self-ignite. Octane ratings of fuels measure the knock resistance of a particular fuel in a specific test.

So it all boils down to fuel chemistry, and since Fuel is a product manufactured by different companies and facilities using highly variable feedstocks, there aren't any successful generalizations. As Cab alluded to, each fuel has its own best air/fuel ratio. We're lucky that what comes out of the pump is pretty consistent.

R.