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Thread: higher octane?

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    higher octane?

    Im running premix and currently use 87 octane and I am curious to as if using a higher octane such as 91 from the local chevron will cause too long of a stroke for the piston and throw off timing or will it be ok


  2. #2

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    Higher octane should be fine. Although, I don't know if its necessary.

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    Take the time to smile sirbreaksalot's Avatar
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    Quote Originally Posted by watervirgin View Post
    Im running premix and currently use 87 octane and I am curious to as if using a higher octane such as 91 from the local chevron will cause too long of a stroke for the piston and throw off timing or will it be ok
    What ski would be a good start, and higher octane burns faster giving theoretical shorter burn time & the
    Timing is set via mechanical means any wayz, so fuel wont affect that

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    ST750

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    Quote Originally Posted by sirbreaksalot View Post
    and higher octane burns faster giving theoretical shorter burn time & the
    Timing is set via mechanical means any wayz, so fuel wont affect that
    Uh, no.

    Higher octane fuel burns SLOWER, which prevents detonation in high compression engines. 91 shouldn't hurt anything in a 750, but I wouldn't waste the extra money on a 750. You won't see any difference in performance.

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    Ok cool. Thank you

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    Take the time to smile sirbreaksalot's Avatar
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    Octane actually has NOTHING to do with burn time actually
    Its only its abillity to resist knock....

    So running an engine on higher octane than what the engine is designed or tune for is just wasting fuel / money

    Most dumb fuel system (carbies , openloop with no fuel sensors ) tuned for low grade fuel get the same or worse mpg when using higher grade fuels ....that indicate one thing in my book....not slower thats for sure

    I saud theoretical because I could see the OP has obviously NO mechanical knowledge .

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    No knock let it rock.

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    How would you know if it's knocking?

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    Quote Originally Posted by sirbreaksalot View Post
    Octane actually has NOTHING to do with burn time actually
    Its only its ability to resist knock....
    It has EVERYTHING to do with burn time. A fuel that burns too fast doesn't burn 'smoothly'. A flame front that moves erratically causes the pistons to rattle, which is what causes the sound you hear when the engine knocks.

    Some engines use knock sensors (vibration sensors) to retard timing to prevent damage. More sophisticated engines (Caterpillar 3600 natural gas engines) actually measure the time it takes for the flame front to travel across the cylinder head. This is done with a second spark plug that isn't fired. Instead, it has a voltage applied to it and when the flame front reaches it, it conducts. The engines computer measures the time from when the first spark plug sparks until when the second one conducts, and timing is adjusted accordingly.

    If you were to try running a very high octane fuel in your 'Ski, you'd find that it won't run fast. (I've tried it in a motorcycle) Aviation fuels are available in relatively high octanes, 100/130. It's very rare, but there was even a grade of 130/145 octane. That would really make a high speed engine run badly.

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