Miata Mailing List: July 1998, Message #6

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On 6/30/98 20:51, Jack Hamilton jhamlton@hal-pc.org wrote:

>During a recent trip from Texas to Colorado, I noticed the octane rating
>of regular unleaded gasoline changed from 87 in Texas, to 86 in New
>Mexico, to 85 in Colorado.
>
>Can anyone explain this to me?

Octane is a measure of how fast fuel burns. Higher octane = slower 
burning. However, lower cylinder pressure = slower burning, too. At 
higher elevations, fuel tends to burn slower, because at a given 
compression ratio, the absolute cylinder pressure is lower than at low 
elevations, where there's more ambient air pressure to start with.

In the Bad Old Days before fuel injection and ECUs, high-altitude cars 
used much leaner carburetor jets and maybe 10 degrees more spark advance 
than sea-level cars to compensate for this difference in atmospheric 
pressure. Nowadays, to preserve power without having to fool with the 
timing, they use lower-octane gas, otherwise the fuel will not be not 
completely combusted by the time the exhaust valve opens.

---
Lanny Chambers, St. Louis, USA
'94C

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