Ok guys, Im working on the head and im getting around to CCing the Combustion chamber, the volume on the Chambers suppoerd to be 43CC's I have 44.5cc's per combustion chamber.So that means that the compression is going to be lower than what i though.with all the math we have done, we figured the compression to be at 8.5:1. We need to bring it back up to around 8.7: 1.


The guestion would be -----How much do we mill the head to loose 1.5 cc's per chamber? That way I can tell the guy at the machine shop how much to mill the head.



Thanks in advance.

Anthony
Boost Solutions

 

Riiigghhhttt Thanks Guys !

 

To get mm you multiply by 0.999972.
1.5 x " " = 1.499958 mm

If you scrube that off the entirety of the head you will effectively loose 1.499958mm's (I would tell the machinist 1.5mm) meaning 1.5cc's per cylinder/combustion chamber.

Just make sure you takin into account when figureing your static CR the head gasket you will be using, your deck height and piston top clearance. GL!

 

You can look up the term "dynamic compression" to gain an understanding. I will note there really isn't such a thing as dynamic compression from what I have read, but it gives you a perspective on compression and boost. Basically, a lower compression motor at higher boost will make more power than a high compression motor at lower boost at an equivalent "dynamic compression."

My personal opinion is that it will make no difference.

 

I would try giving somebody like BR, AMS, or Cosworth a call to see if they have figures for you. I know back in the old days when I would CC a hemi head the good head shops would know how much needs to be taken off to get the cc volume were it had to be.

Problem is the head is not a nice little square or cylinder so trying to figure the volume of a odd shape with tapered walls is going to be almost impossible to do.

Do you have to have 8.7? As 240 said with a turbo car there is a lot of give in and take with cr vs. boost. Problem with running lower cr is it will induce a little more lag and less torque in off boost driving situations but it will allow you to run more boost. Why venture away from 9.0, if your at 8.5 now did you order lower compression pistions?

 

^ DO NOT TAKE 0.060" off your cylinder head.

Now i havent measured or milled a evo head & re-cc'd it, however on smaller bore engines that i used to tweak, ~20 years ago, my rule of thumb was to reduce a combustion chamber volume by 1cc, I would mill 0.008" or 0.20 mm off the head.

Now, you can go on-line, located a compression ratio calculator, plug in all the numbers & change your haed gasket thickness accordingly to how this will effect your clearance volume.

dont take 1.5mm off your cylinder head unless you want to increase your compression by about 1 point

 

One question:

Why would you need to, or even want to? This makes little sense, and is certainly not worth the trouble or expense. IMO, it's actually moving in the wrong direction.

 

why is it moving in the wrong direction? I dont get it


the head had to be decked ways to match the gasket, and the block, its wasnt perfect. AND it was already decked .12 i believe.

 

Let me ask it another way . . .

What do you feel you will gain by going through the trouble and expense to do this?

. . . I am referring to an effort to raise the compression 0.2 points.

 

Are you only ccing the chambers and milling the head? Or, has cylinder bore and/or stroke been changed as well? Are you using overbore pistons?

 

There's not going to be any noticable difference between 8.5:1 and 8.7:1. It's really not worth the trouble. If anything you can run a tad more boost with the lower ratio..