As might have been mentioned; Octane is a number indicating the burn rate of the fuel and it's resistance to detonation, and has nothing to do with the energy (BTU/h) of the fuel.

However, higher octane in higher compression engines will be able to burn more efficiently as the spark can be advanced further, allowing the engine to extract more of the energy from the same BTU/h of fuel.

So, octane doesn't mean more energy, but it can mean more power. Going higher than the car is able to utilize will do nothing however and at high altitudes high octane can actually be a disadvantage in a non-turbo engine.

As far as using an octane lower than the carmaker's specification: In the owner's manuals on my M104 cars it warns against using lower octane fuel, and has specific reduced speed and load requirements if lower octane fuel must be used. Mixing lower octane with higher octane is allowable to a certain percentage per the manual.

Using too low an octane can cause detonation, which can lead to burned valves and damaged pistons if the engine isn't designed to compensate enough. It also will cause the ignition timing to retard in most modern cars, which will lower the power output and efficiency, and increase tailpipe emissions.

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Gone to the dark side

- Jeff