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Thread: Octane and altitude question

  1. #1
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    Octane and altitude question

    I've heard many times that it's difficult to find gas over 89 octane in the mountains out west. Do engines require lower octane levels at altitude? In other words, while crossing the Rockies enroute to the rally next summer will my fully loaded GSA sidecar rig knock or suffer a loss of performance? Should I carry octane booster to compensate?
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    What, me worry? GILLY's Avatar
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    I would think you don't need it, but not sure. Reasoning is this, only anecdotal, not scientific at all:
    When I went through Death Valley, (very low altitude) I was still running the low grade stuff, which was all I could find, on my K75S. It was very hot out (113 IIRC), and the engine was pinging on it very badly, had to run 3rd gear, 4th at the highest and only when I was on the level. So reasoning is that at just the opposite, high level, might only require lower octane? No idea otherwise why they wouldn't want to sell it, but yeah I noticed that too, I was used to 92 or 93 octane and they didn't sell it up there, nor did I really notice any perfomance decease.
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    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by glenfiddich View Post
    I've heard many times that it's difficult to find gas over 89 octane in the mountains out west. Do engines require lower octane levels at altitude? In other words, while crossing the Rockies enroute to the rally next summer will my fully loaded GSA sidecar rig knock or suffer a loss of performance? Should I carry octane booster to compensate?
    Yes - octane requirements are lower at altitude.

    For example, at the pump Kansas: regular = 87, mid-grade = 89, premium = 91

    Colorado at altitude: regular = 85 or 86, midgrade = 87 or 88, premium = 89 or 90.

    Bikes run fine - and ours always get better mileage. I don't know why.
    Paul Glaves - "Big Bend", Texas U.S.A
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    Administrator 20774's Avatar
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    I think the issue is that at higher altitudes, there is less air and thus less that needs to get compressed. Octane is there to prevent knocking for high compression engines. If the compression is "lowered" due to reduced density of air, no need for the additional octane. Same thing if you end up putting base gaskets on an Airhead...you've lowered the compression ratio and the need for the higher octane.
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    I don't know why.
    Increasing the octane level at the pump reduced the combustion ability of the gasoline. When you gain altitude, you are reducing the oxygen level and reducing the combustion ability.

    Ideally, you want the gasoline to "burn". When there is too much oxygen and/or too low of a level of octane, then you will get "explosions" instead of a slow steady burn. The pinging you hear is explosions of the gas. Increases of octane level will slow the burn rate down, and you will burn more of the gasoline, and hence, get better gas mileage.

    You can also adjust the burn rate slightly with the spark plug gap and the heat range of the spark plug.

    Also remember that gasoline is octane. Additives are put in the gas to change the "octane level". Prior to 1975 they used lead to retard the burn rate. If I recall correctly they used tetraethyl lead.

  6. #6
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    Thanks for the responses and explanations. I'll remove that worry bead from the necklace.
    '07 R1200GS for solo rides
    '10 R1200GSA with Hannigan dual sport sidecar for rides with Barley

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    actual experience- i run 87 octane in my R1100S, as well as my F800GS. i live at 5K ft, often ride in the 10K+ range. power is down a bit compared to sea level, but not bad at all. never have issues with pinging, even in 100+ temps. however- going over the mountains you will not experience 100+ temps, even on the very hottest days we see. probably never gets much above 80-85F or so at altitude. If you come thru RMNP and over Trail Ridge Rd (US34), you might hit a heat wave, and see 71 or so. (btw- if you;'ve never done RMNP and Trail Ridge- put it on your "to do" list. it's really something special.) oh yeah.. you will like the improved gas mileage you'll see at altitude.
    Ride Safe, Ride Lots

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    Registered User SeabeckS's Avatar
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    I think even more moderate changes in elevation can make a difference. When I'm visiting friends and family in Northern Nevada my S runs fine on mid grade rather than premium. Home roads are generally within a few hundred feet of sea level for most of my mileage, in Nevada it averages about 4500 ft for the areas I travel. MPG at sea level about 41-42, and about 45-46 in Nevada. Even during relatively higher speeds on Nevada open roads my mileage remains above 45 mpg, I never get that sort of mileage at sea level....

    Bill Johnston

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    On a Ride sfarson's Avatar
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    Quote Originally Posted by glenfiddich View Post
    Thanks for the responses and explanations. I'll remove that worry bead from the necklace.
    FWIW, at least in Colorado, 91 octane is available at the vast majority of service stations.

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    Northern and southern California maximum octane available is 91.

    Death Valley, I believe has stations carried 89 or 90 max, but not 91.

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    Quote Originally Posted by glenfiddich View Post
    I've heard many times that it's difficult to find gas over 89 octane in the mountains out west. Do engines require lower octane levels at altitude? In other words, while crossing the Rockies enroute to the rally next summer will my fully loaded GSA sidecar rig knock or suffer a loss of performance? Should I carry octane booster to compensate?
    Since hundreds if not thousands of GS riders use the lowest octane available without problems I'm sure you will be fine.
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    Quote Originally Posted by martyhill View Post
    Since hundreds if not thousands of GS riders use the lowest octane available without problems I'm sure you will be fine.
    Hence the second part of my question regarding octane booster, but others have answered that. I was curious because fully loaded for a two week trip with my dog in the sidecar, I once had to use 89 in Ontario and the bike knocked pretty badly unless I kept the RPMs up a grand higher than usual. But that was a much lower altitude.

    Thanks, all.
    '07 R1200GS for solo rides
    '10 R1200GSA with Hannigan dual sport sidecar for rides with Barley

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    Quote Originally Posted by glenfiddich View Post
    Hence the second part of my question regarding octane booster, but others have answered that. I was curious because fully loaded for a two week trip with my dog in the sidecar, I once had to use 89 in Ontario and the bike knocked pretty badly unless I kept the RPMs up a grand higher than usual. But that was a much lower altitude.

    Thanks, all.
    which leads to the question- what is your normal running rpm?
    Ride Safe, Ride Lots

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    Quote Originally Posted by PGlaves View Post
    Yes - octane requirements are lower at altitude.

    For example, at the pump Kansas: regular = 87, mid-grade = 89, premium = 91

    Colorado at altitude: regular = 85 or 86, midgrade = 87 or 88, premium = 89 or 90.

    Bikes run fine - and ours always get better mileage. I don't know why.
    I've done most of my riding west of the Mississippi River, and while what Paul says is absolutely correct, I have found LOTS of small towns with only a single fuel pump (maybe two at the most) and nothing available with higher than 85 octane (maybe that's all farmers need?). I used to carry a bottle of octane boost in my saddlebag, but I discovered the bike (owner's manuel said 91 or higher req'd) runs just fine on low octane fuel, so I have quit carrying the boost.
    Royce
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    Quote Originally Posted by PGlaves View Post
    Ye
    Bikes run fine - and ours always get better mileage. I don't know why.
    less O2 at altitude, so EFI computer adjusts (reduces) amount of fuel being delivered.
    it is the same dynamic mechanism that produces decreased power at altitude.
    Ride Safe, Ride Lots

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