High altitude, and low octane

[DeWitt Harrison]

Sorry old chap. The partial pressure of O2 decreases with altitude.
At sea level, O2 accounts for about 21% of the total pressure
while at 18,000 feet O2 only provides about 12 or 13% of the total
atmospheric pressure. If this was not so, then the reduction
in octane with altitude wouldn't be reasonable at all. Cheers,

DeWitt Harrison
88 5kcstq

At 12:51 PM 6/1/01 -0400, Robert Myers wrote:
>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
>
>^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
>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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