Xr100 compression ratio question

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.

ca

 

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
camshaft.

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
timing.

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
monster.

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...

 

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.

ca

 

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
valve.

Jim Cook
GasGas DE300
Team LAGNAF


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