compression ratio/unleaded fuel tuned 8 valve air cooled

Discussion in 'Motorcycle Racing' started by Brendan, May 12, 2004.

  1. Brendan

    Brendan Guest

    Have an 80's gpz550 engine , what sort of compression ratio could it
    increasd to for a tuned engine but using unleaded? 10:1 standard but
    up to 12:1 on avgas mix for racing.

    how effective is octane booster? can someone tell me what a 25% avgas
    mix octane rating is, and could this be rating fuel be made of
    unleaded/pro boost?
    Brendan, May 12, 2004
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  2. Brendan

    Bill Smith Guest

    Why mix avgas and pump gas when there are so many good racing
    gasolines to use? Quality control is much better than you can get
    mixing it up yourself.

    Compression ratio you can use depends on, apart from octane of the
    fuel, cam duration and overlap, and intake and exhaust port flow.

    When the NASCAR guys were forced to use restrictor plates under their
    carbs, they found that they could use compression ratios as high as 15
    to 1, something they could never get away with before.

    The short answer to your question is; it depends.

    Bill Smith
    Bill Smith, May 12, 2004
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  3. Brendan

    Brendan Guest

    ....Building a hot engine for road use and 1/4 mile, that I can run on
    pump fuel and octane booster
    ..... The cams are about 280 degrees duration and probably about 105
    lobe centres but .5 mm extra lift. Gas flowed and ported head and 2mm
    oversize stainless valves, 28mm smoothbore carbs and tuned 4-1 with
    dyna ignition limited at about 11 000 rpm.

    What I really want is a direct comparison between unleaded/avgas and
    unleaded/octane booster
    Brendan, May 13, 2004
  4. Brendan

    Doug Eleveld Guest

    When the NASCAR guys were forced to use restrictor plates under their
    Since you sound knowledgeable on this subject do you mind if I ask a
    semi-related question?

    Is the reason that they can run 15:1 compression ratios is that the
    restrictor plate acts like a partially-closed throttle and causes a
    partial vacuum during the intake stroke, even at full throttle. Then
    the effective compression ratio would be more in the normal range like
    12:1. Am I understanding this correctly?

    Just trying to increase my understanding of engines a bit...

    Doug Eleveld
    Doug Eleveld, May 13, 2004
  5. Brendan

    Bill Smith Guest

    Essentially yes, but you have to avoid expressions like "effective
    compression ratio" because it can mislead you. The compression ratio
    is always the cylinder volume plus the combustion chamber volume
    divided by the combustion chamber volume. But you're right, less fuel
    air mixture is getting in there so a smaller space is required to
    raise it to the desired pressure.

    Bill Smith
    Bill Smith, May 14, 2004
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