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Post by Aurora40 on Mar 20, 2010 7:31:19 GMT -6
Just to add another wrinkle. I live at 7500 feet and have always heard at high altitude you can use a lower octane. I found this at popular Mechanics Q & A. Octane is the ability of a fuel to resist knock, and high-compression engines tend to knock more. The obverse of that is that lower-compression engines can run on lower-octane gas. Air is thinner the higher above sea level you go. Less air going into the cylinders means less pressure at top dead center when things go bang. It’s a lot like lowering the compression ratio in the engine, reducing the need for high octane. Cars will run just fine on lower-octane fuel when they’re well above sea level—and all of those states are. Hopefully, by the time you get back down to denser air, you’ve burned off most of the low-octane stuff, and can refill the tank with higher-grade fuel. cheers That's true. Unfortunately, gasoline suppliers are allowed to sell lower octane gas as a higher grade in high-altitude locations. So your "91" octane may really be 89-90. So you still need to buy the same grade of gas as at lower altitudes, because the gasoline supplier has already taken advantage of the altitude (and hopefully charged you less for it...)
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Post by ntrenn on Mar 20, 2010 14:47:24 GMT -6
The law says that the fuel that goes in the tank at the station must meet the octane number that is on sticker on the pump. They can't sell 89 as 91, but the octane requirement at higher elevations is less, so you can get by with a couple of points less and still have the same knock resistance.
If your car needs 91 at sea level, it should be fine at 7500 ft with 89. Last time I was driving out west, they were selling 85 as 'regular' and it ran fine.
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scottydl
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Post by scottydl on Mar 20, 2010 22:14:22 GMT -6
If your car needs 91 at sea level, it should be fine at 7500 ft with 89. Last time I was driving out west, they were selling 85 as 'regular' and it ran fine. I guess that's an example of where sub-87 octane is still sold. I was thinking after my last post that I remembered pumps at 85-87-89 as a kid, but haven't seen anything under 87 here in the Midwest in the ~16 years that I've been driving.
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Post by Jae Dee on Mar 25, 2010 11:55:06 GMT -6
I like how some mechanic said a worn motor will be okay with a lower octane. I guess he forgot about carbon buildup creating hotspots making it more likely for detonation to occur. If you drive your car like a grandma and don't ever get on it lower octane will get you by. If you drive like me and get on it a lot stay with the premium. The fuel mileage will only change when the car senses knock and starts delivering more fuel into the engine and retards timing. It will sense knock using a lower octane because of the relatively high compression but won't be enough to really harm the engine. The car will detune itself a bit you have less performance but probably not noticeable.
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Post by dodgeman on Mar 25, 2010 15:36:00 GMT -6
I agree, as an engine wears, if anything the octane requirment will go up a little. Also, a modern car won't run richer at high alt. there is an O2 sensor and the mixture is adjusted automatically. In an older car with a carb. they will run richer at higher elevations.
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