M758

So how much head shaving increase compression

Shave Head = Cylinder Vol change?
.005 =
.010 =
.015 =
.020 =
.030 =
.040 =
.050 =
.060 =

Van

it's about 1 compression point per mm (which is about 0.040").

I'll let you do the math, but, for example, in your title, you use 0.010" -- if you start with 10:1, you'll end up with 10.25:1.

Mongo

You wouldn't even notice it. I had a 0.030" shave and that's not even noticeable.

MPD47

Of course, you'll need to adjust your timing to compensate (which I'm sure you'll already know) and on top of that the chamber changes as you get further into the head so there is no standard rule for .00x = X CR

M758

No fixed ratio yes, but is there an approixmate curve.

iloveporsches

Here's a quick spreadsheet
https://netfiles.uiuc.edu/brgriffi/s...c%20change.xls

I've just taken the volume lost as the volume of a cylinder, so its not 100% accurate. If you wanted to go back and estimate the volume lost as that of a cone, it might be a little better.

Van

I realized my info here isn't exactly correct -- the 1 point per mm is the volume at the full bore diameter. I.e. using a head gasket 0.5mm thinner than stock would make your compression ratio 10.5:1, and using a head gasket 0.5mm thicker than stock would make the ratio 9.5:1

The volume of the compustion chamber in the head is pretty minimal -- so I'd guess, as TheStig said, shaving a major amount off is pretty negligable. If anything, you'd want to make sure you still had proper valve piston clearance with a head that had a large ammount taken off.

M758

Well as I think about it the cylinder approximation only needs to be accurate for .060. Reason is that for my purposes this is the max shave. Looking at how small 0.060 relative to the bore I'd think it would be a reasonable approximation to use a cylinder.

Anyone have a really nice picture of a stock head showing the valves. I't like to "eyeball the taper" over the first .060.

Van

This head had about 0.040" taken off:

M758

Hmm,
The cylinder approimation works perfect for a thicker / thinner head gasket. Hard to tell from that picture however if the combustion chamber is a cylinder for the first .060. Seems like it is not.

So is the head so close to the pistons that in the pistons would hit the head if it were not for the thickness of the head gasket?

Van

My gut feeling (just a guess without any calculations to back it up) is that for the first 0.060" we might be talking about 60% of the cylinder approximation.

As for the valve clearance, I don't know that answer -- I did the putty test because I was playing with advancing the valve timing. Those indentations are from the camshaft being moved ahead 1 tooth.

I assume you're playing around with your race car -- trying to get more compression? If you're really doing a complete rebuild, you could always have the valve reliefs machined deeper into the pistons, shave the head, and use a thinner head gasket. You might be able to get up to 11:1. Do the same with euro pistons, and you might get up to 12:1. (I think they're 11:1 stock... but not positive).

M758

Nope.

944 spec rules have a max compression of 10.5:1 and I am in charge of rules enforcement in Arizona. There is a spec the factory manual calling out head thickness and this can be measured with the head installed. So I figure I can use this to help determine if the cars are running too much compression.
By rule only stock 9.5:1 or 10.2:1 pistons are legal. So I measure the distance from the head tab to the block compare to factory limits (including headgasket) and how much "shave" is ok and then bang... legal or not. No measy compression ratio tool or pulling a head. If I get the numbers right I should take my 5 mins to get a measurement and determine legality.

MPD47

Euro pistons are 10.6:1 stock.

Mongo

It's not possible to get 12:1 on an NA unless you get higher compression pistons. You can shave the head max 0.060" and only get about 0.3 points possibly.

ryoji

But one can shave the top of the block as well, if he choose to do, instead of a head.