91 Octane

[BYprodriver]

Originally Posted by blue2000s

It's a matter of air density. Because there are fewer molecules of air and fuel to rub against each other in a combustion chamber at altitude, the tendency to auto-ignite is lower. Thus the lower octane number because it's correct for the altitude where it is sold, and the lower octane requirement.

The 91 fuel at the pump in Colorado IS the same fuel as the 93 octane at seal level.

If fuel rated at 93 octane at sea level is used at high elevation (2,500 ft & higher) it may increase the octane rating, but it definitely will not lower the octane rating.

[blue2000s]

Retracted statement

[shadrach74]

Originally Posted by BYprodriver

If fuel rated at 93 octane at sea level is used at high elevation (2,500 ft & higher) it may increase the octane rating, but it definitely will not lower the octane rating.

It really should not do anything to octane rating. The only reason some engines are fine to run lower octane at higher altitude is due to reduced manifold pressure. A turbo charged engine that needs 93 octane at sea level would do well to have it in Denver as well. You could likely get away with running lower octane in a turbo car at altitude, but not without some performance degradation due to the timing being retarded.