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What Octane to use above 5000 ft elevation

Old 01-22-2004, 08:35 PM
Prairie Ford
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What Octane to use above 5000 ft elevation

You guys touched on a topic a few threads ago, but nobody gave a answer to the question that I have had for years. What octane to use in the mountains? I live on the plains and use 87 in 2001 SD V10. Runs just fine. I go to the Black Hills or Rocky Mountains and they sell 85 or 85.5 and 87 and 89 or 91. Owners manual does not distinquish between different elevations; recommends 87 octane. Don't want to spend extra money buying expensive fuel. What do you recommend?
Old 01-22-2004, 11:57 PM
V10 junkie
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When I first got my truck, I used 85 octane, I had a check engine light come on, I read the owners manual, it said to ONLY use 87 octance, so I filled it up with 87...not had a problem since.

I live in Utah, elevation 4800 feet and right now 11 degrees :-)
Old 01-23-2004, 11:50 AM
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If you plan to run it exclusively at high elevation use the 85, I do and have zero problems and actually get better mileage. Bonus is the 85 here is pure gas, 87 and 91 are blends. But, remember Denver is mostly over 5,000 ft.

Altitude for some reason makes a lower octane gas burn like a higher octane. I came from the flat lands myself and was always leary of 85 thinking it was just cheap junk, tried it and have used it ever since.

One thing I do have to remember is to fill up with higher octane when I head out to the plains. Depending on the amt. left in the tank I'll add either 87 or 91 to make it higher.
Old 01-23-2004, 03:49 PM
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Higher octane fuels are used for higher compression engines. They are designed to burn slower because the higher compression raises the combustion temp and makes lower octanes burn to fast causing preignition or detonation. Just going to a higher altitude allows you to burn lower octane fuel in any engine because the lower oxygen content in the air makes the fuel burn slower and cooler. Thats the reason you loose power at altitude. Most good race engine tuners will use lower octane fuels at altitudes as low as 3000ft. than they use at sea level.
Old 01-26-2004, 10:28 PM
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I live in Denver and use 85 octane, never had a problem and truck runs smooth, never even tried anything else.?
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