Good idea, bad? Raising the compression up a whole bunch? How much to take off before things go bad...etc etc etc...
List of precautions, things I need to do. Help me out.

i also want to shave my cyl head...i wouldn't raise the compression more then 10.5:1 but don't know how much to shave to get from 9:1 to 10.5:1

How much would have to be shaved off the head to pick up a half point of compression?  Maybe .020"?  That's removing A LOT of metal.  I'm familiar with the claims here but I remain skeptical.

Why are you skeptical? And what are the claims?

I only shave my head in the summer.  :mrgreen:

Since bore X stroke is 2.913" x 2.228, displacement = 29.7 cubic in, and compression ratio (CR) is 9:1, total volume is displacement plus combustion chamber (CC), so:

Displacement + CC
---------------------- = 9   ==>  D + CC = 9 CC
          CC

D = 9CC - CC  = 8 CC    ==>   D / 8 = CC = 29.7 / 8

                                              pi stroke2 x depth
CC = 3.7125 cubic in, but CC = ----------------------
                                                         4

                         4 CC
so depth (d) = ------------ = 0.557 in
                     pi stroke2

Now if CR = 10.5 to 1 and skipping all that

CC = 29.7 / 9.5 = 3.126 cubic in

d = (4 x 3.126) / (pi stroke2) = 0.469

so finally reduction (R) = 0.557 - 0.469 = 0.088 in

Theoretically you need to grind 0.088 in, 11/128 = 0.086 in, but of course, I dont know about valve groves clearances or gaskets thicknesses.

/\---Saved me the math! I was just getting started too.
Hmmm, i wonder what kind of power increase it would be?
I think valve clearances would be alright. Id stick to the same gasket thinckness I ahve now. I think I might try it.
Thanks for the Math again!

Quote from: Rich500Thanks for the Math again!

You're welcome, just let us know how it works...

QuoteTheoretically you need to grind 0.088 in, 11/128 = 0.086 in, but of course, I dont know about valve groves clearances or gaskets thicknesses.

Man, what are you grinding?  The top of the cylinder?  Remember, you're grinding off a hemispherical chamber with a 3 or 4mm squish band.  If all that is factored into that equation, you're good.  You're good anyway.

I suppose the next question is how much does the extra point and a half CR multiply the force?  There must be an explosion formula in the text books, right?

.088 is alot to MILL off the head. Don't forget, if you mill the head at all, you will have to get slotted cam sprockets to get the cam timing right after you mill.

Quote from: GisserMan, what are you grinding?  The top of the cylinder?

My engine will remain stock, I just made some figures for Rich, considering the combustion chamber a cylinder... and, I should say MILL instead of GRIND, sorry...

Quote from: GisserRemember, you're grinding off a hemispherical chamber with a 3 or 4mm squish band.  If all that is factored into that equation, you're good.  You're good anyway.

I guess the chamber isn't a perfect hemisphere, for what I remember from Clymers manual, but will check. Compression ratios have to do with geometrics. And 0.088 is 2.24 mm. On the other hand I can't tell if is actually possible to get 10.5:1 compression ratio with the actual head.

Quote from: GisserI suppose the next question is how much does the extra point and a half CR multiply the force?  There must be an explosion formula in the text books, right?

Again, theoretically, should calculate the termodynamical efect of heating the air in the MILLED chamber with the value of energy for the fuel, but that would take some more time and the figures could be very offset due to heat loses, pressure loses, non-burnt fuel, etc. However assuming same conditions for both original and MILLED chamber the figures would give us an idea of how much more power could we expect.

Quote from: reknelbDon't forget, if you mill the head at all, you will have to get slotted cam sprockets to get the cam timing right after you mill.

Yes, as timing has to do with engine's geometry too, something sould be done to fix the 0.088 in gap between the cranck and the camshafts.

I've had a head milled 0.020", but the power increase is negligible.

80 thou is a lot. I suspect you will run into piston and/or valve clearance issues. And you would definitley need to re-time the cams. 20 thou doesn't make much difference to the cam timing, and the cam chain tensioner can take up the slack without a problem.

You'd probably be better to remove material from the cylinders, rather than the head. Less change in the combustion chamber shape.

High compression pistons are a better way to go.

If you get over 10:1, you will have to run high octane fuel, which has less power than the low octane stuff and costs more. 11:1+ on an air cooled engine and you are into race gas at $4 Can. a liter...

Quote from: KevinCI've had a head milled 0.020", but the power increase is negligible.

what is your compresion at right now?

The last couple of engines I was racing were 11.5:1 with high compression pistons, running on race gas.

The 0.020" planed head was probably only at 9.25:1 or something - never calculated it.

is there pistons we can buy to raise compresion but to run it on unleaded gas?

i think JE makes pistons for the GS but that's all i know

JE can make pistons for your situation. But you will have to buy a minimum of 4 and it will cost about $800.00
Maybe you could use JBWELD to make a bigger dome on the stock pistons :lol:  :lol:

dont laugh, i've seen is used to reshape valve ports

I've used it on intake ports. But it wouldn't take the heat of the combustion chamber or exhaust port. :mrgreen:

For a second I thought you were serious!
Can higher compresion pistons be bought at a cheaper price then $800.00.

Quote from: BriggsFor a second I thought you were serious!
Can higher compresion pistons be bought at a cheaper price then $800.00.

I'm not sure if anybody sells less than 4 at a time. Maybe you can split a set with someone. :dunno: