jag1886

Can anyone give me a guess-estimate as to how much you have to mill from a set of #8 heads to loose 5 cc's.
Just trying to keep tabs on my machine shop.

coppercutlass

I think between .020 and .025 roughly.

jag1886

I used one of those compression calculators and that's what I came up with but I figured the combustion chamber is smaller than the head gasket diameter and it might be even more.
I can't wait for this new set of heads, I took an article about porting #8 heads and he is doing everything that was in the article, he even polished the combustion chambers which I don't think is necessary but he's paying for it so I don't care (he did screw up my first build and didn't give me the pocket porting I paid for the first time). I think I'm going to end up which an even better set of heads than my #5's where.

cutlassefi

What did you ever do on a cam?

Normally it's about .006 per cc, but the wider the chamber the less it is per cc.

coppercutlass

Mark I believe the .006 is for bbo heads. Or that's what was stated in the mondello book I think it may be less as you stated depending on cc width as .I don't know if there is a diffrence as far as width between the bbo and sbo heads as I have never had them side to side so I usually do .005 for a rough guestimate on sbo's .

cutlassefi

They're about the same width, so I guess you could use .005 or .006.

jag1886

I decided to use the same cam, that's why I'm lowering the compression to match my current cam.
I really appreciate all the help you where to me, I just couldn't talk myself into the size of cam it was going to take to make that 10-1 happy. The other reason is money and labor, it would have cost 5 times as much to upgrade the cam over just lowering the compression, I can just sway the heads instead of pulling the engine for a cam change and hopefully in the end I won't have to worry about every tank of gas I put in it.
I promise to not be a pain in your a$$ again.

Lars

the old rule of thumb is for every .005 of milling you gain a point of compression, or loose a point of cc

cutlassefi

Incorrect. Depending on the engine size one cc may or may not make a .1 point of compression difference.

Lars

my bad I said it WRONG...
old rule of thumb, per .005 of milling reduces your combustion chamber volume by 1cc, therefore raising your compression ratio by a point.. generally.. Am I wrong?

coppercutlass

I don't thing 1 cc will gain a full point. If it was that easy then guys with low comp 350s would shave 2 cc's off their heads and have 10 to 1 compression and we would not have issues with guys trying to rais compression with stock set up .

coppercutlass

I did some math and on a std. Bore with .018 head gaskets and the piston being .025 in the hole with 23 cc pistons and 72 cc heads which is avg. Of #6 #7 heads compression was 7.88 to 1. Then tkate the 23 cc pistons and do flat tops it's 9.81 to 1 so 1 point of compression is roughly 12 cc's . Now I'm not sure if there is a diffrence between head volume and piston volume. Also my numbers might be a lil off but no by much I did this on my phone and tried to remember the numbers asbest as I could.

Lars

generally; lets say you have a 9.0 comp motor, milling the heads 20 thou will raise the comp to 9.4 give or take, milling reduces the cc volume but the valve job adds volume. milling also increases lifter preload depending on head gasket used. polishing the ccombustion chambers adds volume

coppercutlass

which that's right . I was talking cc volume 12 cc is roughly one point in compression . and I did some math and .048 give or take on an sbo is roughly one point in compression. what I was talking about the difference in head piston volume is how it will affect compression but volume is volume so it shouldn't screw with the numbers say for example if you had a flat top motor with say 79 cc chambers and you took 20 cc's of the head it would matter if you took 20 from the head or added 20 to the piston just for example this is all theoretically speaking of course but numbers are numbers .