How much power am I leaving on the table?

[KMcCullah]

My C4AE-G heads have valves with a concave on the combustion chamber side. This raised my combustion chamber volume to an average of 80.625 cc's for all 8 cylinders which puts my static CR at 10.37 and DCR at 7.25. This is with a .02 steel shim head gastket by the way. I was figuring on 74 cc's, which would have put my static CR at 11.16 and DCR at 7.78. I'm trying to keep this 390 pump gas friendly.
Thanks for the opinions guys.

[fe66comet]

The heads are what is going to hurt you without port work on the intake and exhaust pockets and reducing your valve stem diameter to 5/16.  This area is the bottleneck in the stock head.....Jon

[KMcCullah]

Thanks for the reply. I should have been more clear. These are the heads I just got back from Les Schmader. So they have been ported and flow pretty good. Probably too good for a 390. The valves are Manley for a LR 427 and are kinda a semi-tulip shape, which made the combustion chamber side of the valves dished or concave.
I'm just trying to figure out how much HP I may loose by being down nearly one full point of CR.
My other wonder is, how much would I need to mill off to get my combustion chamber volume down to 74cc approx? I've seen some posts that say .005=1cc and I've seen .007=1/10 of CR. So milling .07 would raise my CR one full point.
Anyhow..... confused here too. 

[plovett]

1 point of compression generally is worth around 3-4% power.  Going from 8:1 to 9:1 might be worth 4%.   Going from 10:1 to 11:1 might be worth 3% maybe 3-1/2% at most.  You're talking about a 0.79:1 drop in compression which I think would equate to a drop of roughly 2-1/2%.

So if you're engine is around 350 hp you might have lost around 8-9hp.  If your engine is around 400 hp you might have lost 10 hp.  If it's in the 450 hp range you might lose 11-12 hp, etc.   I can't tell you exactly, but it'll be somehere in that area.

paulie

[KMcCullah]

Wow Paulie! Maybe I'm not as bad off as I thought. I may just put this mess back together with some steel shim head gaskets and drive it like it's rented.
In the end, I hope the additional flow of the heads will more than make up for the power I'm losing to lower CR.

I'm seeing as I deal with this issue, the heads are where I should have started. It would have been a lot easier to start with heads and a known combustion chamber volume. Then pistons and cam to application. I'm learning the hard way. Dad swears I got my first piece standing up in a canoe. 

[jayb]

If I recall correctly you lose about 7.5cc of chamber volume by cutting .040" off an Edelbrock CJ head.  Your heads are probably close to that, maybe a little more loss of the volume with the cut.

If you want to compensate for lower compression you can always advance the cam to bump your DCR back up; that will help, provided you have the piston to valve clearance you will need.

FWIW, if it was me I'd cut the heads to get your compression back - Jay

[plovett]

What's the engine combo and what car, tranny, rear gear, intended use, etc?   That may make a difference.

paulie

[fe66comet]

On pump gas I wouldn't go past 10.0-1 anyhow. You will be retarding the timing quite a bit after that and loosing gains to compensate for poor fuel octane.

[Heo]

Any one know how much you can mill
a C4G head?

[fe66comet]

Barry or Jay could answer that I am sure , on the Edelbrock every .005 is 1cc with a max of .060. Comedic will make a .027 composite BTW then you can get a custom bore also......Jon

[KMcCullah]

The more I think about it..... I like Jays milling idea.
Before I get all slap happy with .02 steel shim head gaskets, I better check a few things. I think the flat part of my pistons are only .006 in the hole. .026 quench is too close for comfort.
I also need to try some clay across the pistons and check valve clearance. My ICL is advanced to 104 deg. The card calls for 108.
Paulie- This is the motor that I'm trying to get one more year out of in my highboy mud racer. (Was gonna be a 445) 4 speed with 4.56 axles. It's good for pushing buttons with the high dollar diesel guys too.