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Thread: Fuel question

  1. #1
    Regular Member Spikey©'s Avatar
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    Default Fuel question

    Does anybody know, if you put super unleaded (Optimax and the like) in a car that is not set up specifically to use it, would it be possible for you to see a decrease in performance or would it just not make any difference?

  2. #2
    [Ex]Admin Duncan's Avatar
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    I think it will just not make any difference. I believe the engine has to be set (timing etc.) to take advantage of the different burn characteristics of a higher octane fuel).

    That's what I believe (says he sat here waiting to be shot down by some experts! )

    D

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    Regular Member Gsi3.2's Avatar
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    It shouldn't run any better, but it shouldn't make it run any worse at all..

    to take advantage of the higher octane values, the ignition timing would have to be advanced.. but unless there was a lot of pre-igntion ( pinking) the engine would not attempt to alter the spark rate ( retard the timing) that could show as a decrease in performance.

    Clear as mud??

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    Regular Member Spikey©'s Avatar
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    That's what I thought initially, but then got to wondering, if the timing has to be advanced to take advantage, then if it's not will the fuel burn correctly? If it doesn't, surely this will create a problem with the car overfuelling to compensate? Would this then lead to problems with economy or even power reduction?

    I'll wait for a few more opinions before giving my reasons for the question.

  5. #5
    Regular Member SirGSi's Avatar
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    If you're concerned about this in the Veccy then don't worry m8 as they have Knock sensors to adjust accordingly without any manual intervention needed.

    HTH
    Grant.

  6. #6
    Regular Member Gsi3.2's Avatar
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    indeed, they detect the knock and automatically retard the spark,

    that is why you don't get pinking like you used to, the car will automatically adjust the spark rate.

    overfuelling will only occur if the fuelling map has been adjusted at any time, the cars are programmed to not run rich, this will only occur if the fuel map has changed


    Got a problem Spike?

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    Regular Member Swatty's Avatar
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    I remember on my cavalier (many moons ago) it had the yellow dongle that you could swap round between the 95 and 97 octane – which tweaked the timing to suit the fuel.

    Shame you can’t do something similar on the new cars, or maybe it does it automatically somehow now.

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    Ex-Staff Full Member Ian T's Avatar
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    I tried optimax last year, it seemed to run better, but not noticable enough to pay the price out all the time.

  9. #9
    Regular Member VectraV6CDTi's Avatar
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    But you get more miles to the tank, so its doesnt work out so bad.

  10. #10
    Regular Member Spikey©'s Avatar
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    Ok, so here's the reason for the question...

    Been running with high octane fuels for a good while now, since the end of last year nothing less than 97RON has gone in. Ran fine, thought i'd detected improvement at the time, then had the remap done and assumed that it would have been adjusted particularly to run on higher octane stuff. All has been fine, no problems whatsoever.

    Couple of days ago, I got caught short on fuel, got as far as the light actually flashing. Only garage I could find at the time didn't have any premium fuel (Morrisons) so ended up having to half fill the tank with standard 95RON fuel.

    After about 60 miles or so, the car started feeling more responsive and torquey. I'm about 150 miles into that fuel now and the car is a different beast. Have to be quite gentle with the throttle FFS!!!

    Nothing else has been touched on the car recently, so the fuel is the only variable that could have made the difference.

    I'm guessing that the remap was done without factoring in high octane fuel so it wasn't having the full effect of the map due to timing or other perameters?

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