Xr100 compression ratio question

Discussion in 'Motorbike Technical Discussion' started by clay, Jul 26, 2006.

  1. clay

    clay Guest

    So I found a good complete head for my sons Xr100. Question is, I can
    flycut the head to increase the compression a bit. How much do I take
    off to run the bike on 89-92 octane?

    Or if someone can give me the the correct compression ratio on this bike
    for the above octane range, and the bore & stroke, I can figure the
    flycut depth from there.

    any other suggestions before I jump off this cliff?

    This was the bike that jumped the timing chain, and bent a valve. Do I
    need to replace the chain too? I am replacing everything in the top end
    but the timing chain, so far.

    clay, Jul 26, 2006
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  2. clay

    FB Guest

    Yes. Don't do it, because you don't even begin to understand the REAL
    reason for milling a head.

    You don't mill a head so you can use pump gasoline.

    And racers wouldn't be paying $7 or $8 a gallon for high octane race
    gas if they
    didn't absolutely have to burn that expen$ive stuff.

    Head milling is done to increase cranking compression while waiting for
    dynamic compression to take over when you have a longer duration

    If you mill a cylinder head and do nothing to change valve timing, you
    can wind up with an engine that actually produces LESS POWER than
    stock, because it can no longer rev up to the power peak.

    Don't ask how I know that. ;-)

    If you want to run an engine on a more volatile lower octane fuel, you
    want to retard the timing so it doesn't ping under heavy load

    If you want to run an engine on a less volatile higher octane fuel, you
    want advance the timing until it just pings under heavy load, then back
    off the timing.

    That's all there is to that.

    That has nothing to do with cylinder head milling. You can usually make
    it possible to change ignition timing several degrees by using a Swiss
    file to elongate the screw holes in the ignition pickup or points
    backing plate.

    No Cincinnati or Bridgeport expertise needed there.

    Back to the REAL reason for milling a head. Hotrodders used to do that
    back in the 1950's when they installed a camshaft with longer duration
    and perhaps more valve lift.

    They discovered that the cam did allow the engine to operate at higher
    RPM, they did get more power, but, how to get mid range power and
    flexibility on the street or on a race course, as opposed to running on
    a drag strip?

    They installed high compression pistons, or they milled a whole bunch
    of metal off the cylinder head.

    Of course, that affected valve timing, because the cylinder head was
    now a different thickness, the end of the rocker arm was now closer to
    the camshaft, and the valves were opening earlier.

    The same thing would happen to you on your XR100 project because the
    distance from the crankshaft sprocket to the cam sprocket would be
    changed, altering the valve

    And you never even thought of that, did you?

    The hodrodders would run out and buy a degree wheel and a dial
    indicator and spend endless hours "dialing in" their camshafts until
    they got the MIDRANGE power they wanted.

    Set the camshaft so the intake valve opened early, that meant the
    exhaust valve opened and CLOSED early, and they had a low end torque

    Set the camshaft so the intake valve opened later and the exhaust
    valves opened later and stayed open longer and they had an engine that
    had beaucoup top end power, but wouldn't pull the car out of a slower
    corner on a racetrack.

    I could tell you camshaft lobe center numbers in relation to crankshaft
    angle, but it wouldn't help you adjust your IGNITION timing to get the
    engine to run on pump gas.

    Ignition timing and the reason for selecting a certain firing point in
    relation to crankshaft motion is a whole other topic, and, if you
    milled your head as you considered, you would be back in here asking
    about what the new ignition timing would be...
    FB, Jul 26, 2006
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  3. clay

    clay Guest

    Actually I did think of that, but since flycutting is usually only a few
    thousandths, compared to the distance between the crank and overhead cam
    on the XR100, I would not expect it to have a signficant effect on the
    cam timing.

    clay, Jul 26, 2006
  4. clay

    Wudsracer Guest

    If you want to raise compression, get a higher compression piston.

    By all means, get a new cam chain. The old one being stretched is
    probably why it jumped the sprocket and got out of time, bending a

    Jim Cook
    GasGas DE300
    Team LAGNAF

    Wudsracer, Jul 26, 2006
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