High altitude, and low octane

[DeWitt Harrison]

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
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