Originally Posted by ColoradoDriver
Since I live at 8500FT elevation, there is certainly less pressure and thus slower burning fuel mixture. My concern is that modern vehicles already adjust their timing (and fuel amounts) for the higher altitude. So the engine management system is still looking for 89 octane. I know it will "back-off" timing if it detects pre-ignition, but I would rather just feed it the 89 it expects. (Years ago vehicle manufactures sold cars with milled "Denver Heads" to increase internal pressures. Also the carbs came with leaner "Denver Jets." That would not work today because we all drive back down to sea level, and the EPA would not allow those mods.)
Also, several previous vehicles owned were turbo charged. So the altitude excuse to sell lower octane fuel really did not apply for turbo/supercharged engines. For those engines, 93 octane would be nice "up here," but they only sell 91.
A car doesn't know what octane fuel is in the tank. The recommended rating (89 for example) is what the vehicle was designed for during use at or near sea level. As altitude increases, the need for a fuel to resist pre detonation decreases by about 1 ON per 1000 feet. Roughly. With that in mind, 85-86 (R+M/2) is equivalent to 89 (R+M/2) at 6000 feet.
On turbo engines, this still applies. Boost ratio remains the same regardless of air density.
( atmosphere pressure + required boost) / atmosphere = p2/p1 ratio
Lower air density at the intake would result in less pressure on the exit of the spool. Of course, we are talking absolute pressures here.
Lower octane fuels generally deliver more energy per gallon. Ethanol is a common way to cheaply increase the octane rating of a fuel. Although ethanol contains less energy per unit of measure than does petrol.
I usually use 87 unless towing or it is really hot around here. FWIW, I am damn near at sea level.