Octane ratings and high altitude driving

[JK-81SC]

I have a question about premium gas and high altitude driving. Why is the high octane gasoline not required at high altitudes?

I live at 7000 feet, and I've heard people say don't waste money on premium grades, for stock normally aspirated engines. Is this because of the decrease in oxygen available for combustion or what?

[Bill Verburg]

Less air pressure at altitude means less air in the cylinder, less air to compress and lower cylider pressures. ie less tendancy to knock.

Back in the old days when carbs were the norm a car tuned for sea level would have black smoke pouring out the exhasts from running overly rich in the high passes. If the engine stopped for any reason it coulld be very difficult to restart in the highest backcountry passes.

Ask me how I know.

[356&914&912E]

I wonder too.

I live at 8,500 and work at 4,200.

I've talked to a guy here at 4,200 that has at 64 Ford Galaxie with an 11:1 427. He says the altitude lets him run crappy California pump gas without pinging: the altitude effectively richens the mixture.

[JK-81SC]

>I live at 8,500 and work at 4,200.

Ah, so you're the guy on TV commercial who looks at the long winding road leading up to the house for sale and says "We'll take it".

Sounds like a fun drive!

[David]

Don't the gas stations at higher altitudes already sell lower octane grades than at lower altitudes? So a medium grade at low altitude is the same octane as a high grade at high altitude?

[banjomike]

Originally posted by 125shifter
Don't the gas stations at higher altitudes already sell lower octane grades than at lower altitudes? So a medium grade at low altitude is the same octane as a high grade at high altitude?

I grew up in Salt Lake City (~4600ft.), and premium there is 89 or 90, premium here in CA is usually 91... I think... I know that regular was 86.

So you should probably still buy premium. At least you don't have to pay CA prices. Here in the Bay area, premium ranges from $2.39 to $2.69 /gal!!!!

[911boost]

You can safely run lower octane gas at higher altitudes. I am in Fort Collins just a hair lower than Denver, andi usually put it 90 or 91 octane, whichever one they have and my 911 has run great the whole time I have had it.

Bill