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Octane & Altitude

Discussion in 'Warranty questions' started by LuigiE90, Mar 10, 2011.

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    LuigiE90

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    I live in Colorado and have been told that due to the low atmospheric pressure at high altitude the engine compression is less and the chance of pre-detontation is reduced; therefore the engine will run fine with a lower octane rating then recommended in the owners manual. Is this all true? If it is correct would 89 octane at 6000 ft., for exempla be equvilant to 91 octane at sea level? I know the standard answer is use the best gas with highest octane it worth the extra few cents; but I am interested in the techanical aspects of altitude on required octane levels.
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    Brian A

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    There is another aspect to the issue as well: the age of your car.

    I believe modern engines are extremely tolerant to changes in octane number and can operate through a wide range. The knock sensors and electronics adjust spark timing cylinder-by-cylinder and revolution-by-revolution preempting knocking. If you have a new car, it can probably run on any kind of pump gas at any altitude without a problem.

    Steve Dinan alluded to this at an Open House at Dinan in Morgan Hill last September. He was explaining it from the other side, where a modern BMW can exploit the combustion characteristics of very high octane fuel. As I recall, he said that a modern BMW could "reprogram" itself to exploit something like 108 octane fuel just by accelerating hard and running it up to red line a few times.

    I think I am correct in the above: hopefully, as you requested, someone with deeper technical knowledge will respond. I know you're interested in the technical aspects of octane vs. altitude, but I just wanted to raise another issue that impacts the performance at altitude.
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    floydarogers

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    First of all, I believe that stations in CO (and other high-altitude areas) sell 84 or 85 octane as regular, and 88/89 as premium, because of the altitude factor. So you should get the highest octane you can, in any case.

    Second, depending upon what engine you have, your engine may want the highest. For instance, turbo motors always use the full capabilities of the engine.

    It used to be that carburetted engines, and older fuel-injected motors that weren't able to detect the thin air had to be re-jetted/adjusted for high altitude to run properly. This hasn't been the case for about 20 years (at least with BMWs), which changed to mass-airflow meters (rather than air-velocity based meters.) So even an N52 motor will "do the right thing" with the highest-octane fuel you can buy.
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    LuigiE90

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    Thanks for the information; I do have a newer car (2008,328i).

    Maybe technical wasn't the right word; I was just wondering as I drove higher into the mountains if I could use a lower octane without loss of performance.
    My owners manual recommends premium gas but says I can go down to 87 octane wihout engine damage, but that is at sea level. I am curious what that number would be if BMW new I was driving around at a mile above sea level.

    Just for the record I have never used anything but premium in my car.

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