High altitude, and low octane

[Robert Myers]

At 11:53 AM 6/1/01, DeWitt Harrison wrote:

>I've been waiting for someone to bring up the relevant fact that,
>as altitude increases, not only does the total air density decrease
>but the composition of the gas mixture changes. Importantly,
>O2 thins out more rapidly than N2. At high altitude, there is
>proportionately less oxygen in a kilo of air so that a fuel metering

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Ohno ohno ohno!  The amount of oxygen in a kilogram of air is (neglecting 
water vapor content) essentially constant regardless of altitude.  Oxygen 
does not "thin out" more than nitrogen as altitude increases.  Now if you 
had said something about the amount of oxygen in a liter of air your 
statement would be OK.  The relative partial pressures of O2 and N2 remains 
constant regardless of altitude.

>system based on intake of air mass will inherently run richer
>than at sea level. This effects carbureted and injected engines
>alike and explains way 91 octane is generally adequate for
>Colorado cars. This still doesn't explain why we don't have 93
>octane at the pump if 91 is more costly.
>
>DeWitt Harrison
>88 5kcstq
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Bob
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