So my question is this. What octane fuel would be best to run at altitude. Because the air is thinner should I run 87 octane instead of 91 or higher that I usually run at lower altitudes when racing? The car will be tuned at about 1000ft with 91 octane and probably around 26-27 timing max.

When I get to utah I will have to adjust AFR's with my Motec, but should I continue running the same octane, or could I pick up more with 87 octane? and would I have to reduce timing because of it? I will be on the dyno next week, I should have some new figures then. I will not have time to dyno in Utah to optimize, but I have board AFR's for tuning. I have found that 13.7-13.9 is tuned for optimum HP for my Bridge.

Thanks for the advice.

 

I don't understand the question so I will just post some info that may help:

- The octane requirement (knock threshold) for an engine usually goes down by an average of about 0.88 octane number per 1,000 ft.

- The octane number is purely an anti-knock index number, and therefore it does not necessarily indicate the energy content or flame propogation speed of the fuel.

 

I've been told that lower octane fuels burn at a quicker rate and have lower flash points when compared to a higher octane. Maybe part of the reason the flame front burns at a quicker rate is because it ignites at a lower temperature. Anyway, I figured that with lower compression/combustion pressures due to the higher altitude, maybe I would actually get more hp by running 87 octane instead of 91.

 

As Evil Aviator said, the octane rating is purely an anti-knock index. The energy density of the fuel, flash points, flame front speeds, etc, are for the most part independent of the octane rating. Any variances of the chemical properties other than anti-knock between different octane ratings is coincidental and should be so minor as to make no difference.

The flash point is the lowest temperature that gasoline vapors can be ignited using an external ignition source. Typical gasoline flash points are about -40°. Not a big concern for us.

The energy density of the fuel can vary based on the blending. Higher fractions of ethanol will decrease the energy density and increase the octane. This is NOT to say that higher octane fuels have more ethanol and thus less energy. Octane is affected by more than just ethanol content.

Flame speed is going to be much more dependent on the tuning than the chemical composition of the fuel, and is independent of the octane rating anyway.