Before ethanol:
A gallon of 87 octane regular contained 117,600 Btu/gallon
A gallon of 89 octane contained 117,600 Btu/gallon
A gallon of 91 octane contained 117,600 Btu/gallon
A gallon of 93 octane contained 117,600 Btu/gallon
A gallon of 100 octane avgas contained 117,600 Btu/gallon
Ethanol (according to the scientists, not the government) contains 67,000 Btu/gallon.
Ethanol is often used as an octane booster as the AKI octane of ethanol is 116.5.
Mixing gasoline and ethanol will ALWAYS result in less Btu/gallon than any grade of unadulterated gasoline.
As Jerry mentioned, the higher the octane, the SLOWER the fuel burns. Often, higher octane fuels will NOT completely burn in a low-compression engine, resulting in both a loss of power and fuel economy. This is certainly true of avgas.
If one does find the higher octane fuel (or avgas) is working better than one's current fuel in these lower compression engines, then one should try the same octane fuel as before FROM A DIFFERENT STATION! Possibly the current station's tank has some contaminents or???
Now if one has a 13.5~15:1 compression race engine, then the avgas should work well.
My shop truck (not a stovebolt) runs 10.28:1 (measured when we built the engine, NOT advertised) compression. Under normal conditions, I run 87 octane fuel with NO issues. When I pull the 16 foot trailer, I put 89 octane fuel in the tank.
Seriously guys (and gals), the lower compression engines should run WORSE on the higher octane fuel PLUS it is more costly.
Jon.
