Originally Posted by jayzbird

Can you clarify on the risks associated with running higher octanes? My understanding has been that other than higher cost and the slightly lower energy content of higher octane fuel (thus making less power if the motor would run the same timing and boost without it), there is no real downside.

I have always been told NOT to run higher octane levels then what your engine has been designed for, both by car tuners and motorcycle engine builders. I'm not willing to push the envelope and potentially risk a $25k +/- engine rebuild just to say my car runs better. If you want your car to run stronger just pony up and pay for the proper tune. A properly tuned and modified vehicle will always out perform a stock vehicle on running on a higher octane.

"How stuff woks":

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.

My advice is run what your manufacturer, tuner, or engine builder recommends, else run at your own risk... therefore, no support when needed in the event of catastrophic failure.