Gas Octane and altitude

[Matias]

Does anybody know if it's true that at high altitude (6500+ft) it is ok to use 85 or 87 gas? I've been using 91 in my '94 540i which I assume is safe but it doesn't hurt to ask. I know this guy with a 2003 Audi A6 and he only uses 85...I always thought he was cheap but then I heard that thing about the altitude and the octane ratings.

[JohnC]

kinda make sense to me since theres less oxegen up there you would want something that would burn faster. not sure what it would do to the car though, we can wait for opinions from the more knowledgable guys.

[Bellicose Right Winger]

It's true that the octane requirement for an engine is reduced at high altitude. Whether it's reduced to 85 or 87 is a much more difficult question. Engine pinging is a result the AF mixture self igniting. When gasses are compressed to 1/10 their original volume their temperatures rise. The temperature of the AF mixture as the piston nears TDC on the compression stroke is a major factor in whether or not an engine will ping. If this "heat of compression" causes AF mixture to exceed the autoignition temperature, mixture will detonate and pinging will be heard. Two things contribute to raising this "heat of compression"...the temperature of air entering the engine and the density of the air. At high altitude, where temperatures are often lower and the air pressure is always lower, engines are less likely to ping because AF mixture just prior to ignition is cooler then it would be with the same engine at sea level and 105F. Someone can dig out their thermodynamics text and calculate the temp difference. It's easily over 100F.

The Audi likely has knock sensors so it's not clear if the octane requirement has indeed been reduced 85, or his knock sensors are keeping it under control.

I know my '83 533i and my '93 525i (w/knock sensors) will run ping-free on regular when temps are below 60F. Both need premium the rest of the year.

Paul Shovestul






Does anybody know if it's true that at high altitude (6500+ft) it is ok to use 85 or 87 gas? I've been using 91 in my '94 540i which I assume is safe but it doesn't hurt to ask. I know this guy with a 2003 Audi A6 and he only uses 85...I always thought he was cheap but then I heard that thing about the altitude and the octane ratings.