So I see that some shops are starting to do slightly higher compression on 4G63's (10.5:1).

My head has to get shaved so I was wondering.....number one is there a formula for how many thousandths the head gets shaved when put back together with a OEM head gasket how high the compression is (IE: Lets say its shaved forty thousandths would that make compression 10:1.......I'm no mechanic and even further from being an engineer so hopefully someone understands what I'm asking).

And two is that ok to do on a relatively stock block? I come from Honda's so I'm used to high compression and I always like how high compression high boosted motors performed but since this seems relatively new to the Evo world not sure if this wise.

 

A general rule of thumb is 1mm changes the compression ratio by 1.
0.004 in. is 0.101mm so it shouldn't raise the ratio that much.
I've never done it, but if I ever did do it I'd feel better with forged pistons, rods, and head studs.

 

Does 1mm change it by a ratio of 1 or .1?

If compression on an engine was 10:1, and you changed it by 1mm, are you saying the new compression would be 9:1 or 9.9 to one?

 

I thought decking the head (cutting it) makes compression goes up?

 

There is an easier way to achieve your desired compression ratio...

JE Pistons offers off-the-shelf pistons that provide a 10.0:1 compression ratio: http://www.jepistons.com/Catalogs/Sp...i/4g63_22.aspx

To answer your question, you'd have to calculate the volume loss in the chamber after machining the head .040". Then you can proceed with CR calculations as normal.

 

.040 thou would be immense decrease in chamber volume.

Easy way to calculate is to go here-

http://www.eaglerod.com/index.php?op...id=7&Itemid=25

change the headgasket thickness from .051 to .011 and see what happens

Aaron

 

Ran the numbers on an 8.8:1 motor and that would bump it 9.62:1