High altitude - is high octane better....
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Thread: High altitude - is high octane better....

  1. #1
    Senior Member Donzo's Avatar
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    High altitude - is high octane better....

    I know that low compression engines are better with low octane fuel - faster burning fuel is better than slower burning fuel unless you are dealing with high compression motors -- it;s better to have as low an octane fuel that will burn well in your particular motor - better combustion and less carbon build up. But....I was curious if at very high altitude since air is thinner is it better to have slower burning fuel? The answer is no - higher altitude actually has the effect of lowering your compression more than at sea level - anyhoo here is the source I found for anyone else that is interested. I'm going to try and buy 85 octane in NV on the way to Sweet Water.

    https://mechanics.stackexchange.com/...high-elevation
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    Senior Member SportsterDoc's Avatar
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    Careful with the ethanol % on 85 octane.
    I have run 87 octane regularly up to 8,000 feet elevation, with satisfaction.
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    Senior Member CS1983's Avatar
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    I run 91, but not because it's 91. Rather, because it doesn't have corn syrup in it. Haven't had any issues.
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    Senior Member Donzo's Avatar
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    good point...have not factored in the alcohol.... I think there's a spot on 395 by a Walmart that has non ethanol for a little more $ - I'll check it out.

    Thanks
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    Senior Member Tweaker's Avatar
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    Thanks for answering this I was going to ask the same question.
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    Senior Member Darth's Avatar
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    List of non-ethanol gas stations & other stuff in the U.S.
    There are others, but I think this one is the best.

    https://www.pure-gas.org/index.jsp

    Don't forget...Google is your friend!
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    Senior Member rurlndum's Avatar
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    Unless you get your TW airborne you are not at a high altitude but at a high ground elevation.

    The higher the elevation you travel will result in lower engine compression and you will have less need for a higher octane fuel.
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    Senior Member Fred's Avatar
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    Spoken like a true pilot!
    Now a month ago I rode maybe 9,960 ft max elevation burning 91 octane atop a 10.25 Wiseco piston and had no noticable ill effects. Maybe it was the full moon.
    However today’s comments are by those returning with me for for the next full moon and an intended brief excision over 11,300 summit and possibly 3 days of extended 9,000ft plus riding. Thus our questions have a practical application.
    Perhaps I should dump all my bike and spare 91-test fuel into the 4-wheel mother ship and instead load up on 87 octane while also directing Adam-in-Nevada to fill my 5-gallon can with the cheap stuff...after all, nothings too cheap for my dear TW friends.
    Last edited by Fred; 08-02-2017 at 07:57 PM.
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    Senior Member SportsterDoc's Avatar
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    I visit higher elevations frequently.
    The only changes I have made for rides/drives above 5,000 feet is to further advance timing, when that is an option.
    On the 350 ci V-8 in my Datsun, I left the distributor clamp loose enough to twist the cap.
    Not that a PL620 pickup needed more than 300+ HP, but it was more fuel efficient.
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  11. #10
    Senior Member Donzo's Avatar
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    Quote Originally Posted by rurlndum View Post
    Unless you get your TW airborne you are not at a high altitude but at a high ground elevation.

    The higher the elevation you travel will result in lower engine compression and you will have less need for a higher octane fuel.
    My attitude gets better at higher altitudes....I mean elevations! Thanks for the reminder - but, if both my tires are off the ground (seldom happens) at 10,000 feet elevation ----- while I am defying gravity...I am at 10,000+ feet Altitude, I say this with the best of attitudes.

    Sorry I have a coffee buzz Donzo
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