Quote:
Originally Posted by mantella87
Excellent post. Do you have any insight as to why most gas stations in the western states typically offer 91 as the highest octane rating? I've heard so many different reasons, ranging from altitude above sea level to emissions regulations.
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AFAIK, it's simply because of the altitude. "Octane" measures the resistance to pre-detonation. The higher the rating, the harder it is for the gasoline to self-detonate (due to high compression ) before the spark plugs fire. This is knock. So... at higher altitudes, decreased oxygen levels mean your engine can't develop as much compression as it can at sea level (right?), which means there's less chance that pre-detonation can occur. So you can safely use lower-octane gas so long as you're at altitude. At least in the case of normally aspirated cars, stock cars. Note, I wouldn't fill up with 85 octane at 6K feet then drive down to sea level. That's asking for trouble.
Not sure what happens if you have an aggressive tune. This might be a problem. Even more so if it's a turbocharged car which can (somewhat) compensate for the altitude.