Machine shop sunk the seats pretty deep in a set of heads that are supposed to be 71cc (LQ4, will be boosted, never done this before).
I re-shaped the combustion chambers to get the air flow back, then since I'm there, polished the chambers.
Chambers are now 72.5cc +/-0.1. That takes me from 9.4:1 to 9.25:1.
Is it going to be worth it to get the heads milled ~0.010" and get them back to 71cc?
Are you racing for money?
It's going to be boosted? If so I wouldn't worry about it.
I say heck no. First, that difference would honestly account for maybe a couple HP at best. Old school numbers used to say 15hp per full point (all other things being equal. This also assumes typical old-school displacements and power numbers of the day) The big thing about compression is that it matches cylinder pressures with cam events. Big compression + big cam = big power, but compression alone doesn't do very much. The other thing to realize is that you polished the chambers which means you could likely get away with a couple more degrees of advance and recoup that couple hp with timing.
Second, you're boosting it. Just boost it another 1 psi and you've made more power anyway. When I'm building a boost motor, I specifically go to great lengths to lower compression to match the boost I want without resorting to race gas.
My guess is you gave up 3 hp, but the extra 1psi of boost you'll put in will net you about 10 additional hp.
If you care about fuel economy, I'd tend to say chase as much compression as you can reasonably get or as your other goals will allow. But otherwise, for boost, don't worry about that small a difference. Just turn the boost up a hair and it'll even out in the end for the most part.
Lower compression means you can run more boost but you'll pay for it in spool. A high compression turbo engine may not have the same peak power potential, but can be a very fun little thing to drive.
0.25 point, though? Not worth thinking about.
6.0 to 6.25 might be worth it. Unless you are running a special class, meh.
As others have said with forced induction it is nothing to worry about. I would be much more concerned with cam selection and turbo sizing.
This is for my 6.0L LQ4 LS motor for my '61 Apache pickup.
My intention is stock cam and stock converter, for now. I don't intend to cam it up, but I know I will be bored with it within 5 years of completion.
I have a billet 7875 turbo for it, which apparently will light up pretty quick on the 6.0L.
In reply to SkinnyG :
Send it. Then start building another engine with stuipd horsepower to stab in there once you get board.
Maybe just get some of those offset crank bearings, .010 offset should bring the pistons up enough to get the compression back. 
Zip those heads on and let 'er eat. You'll be pulling the heads to do a cam and valve springs soon enough 
akylekoz said:Maybe just get some of those offset crank bearings, .010 offset should bring the pistons up enough to get the compression back.
I wouldn't on an LQ4. Normally good advice, but the pistons are already .025" above the deck at TDC. With the available head gaskets (especially ones that will hold boost) he'll kill quench and risks the pistons hitting the head.
In reply to Curtis :
Wait, what! I thought I just made that E36 M3 up as a joke, now I need to go and look for some. Maybe I'll have a crank ground .100 shorten my pistons and build a stroker. Don't listen to me, I only have bad ideas.
You have to make them by taking some .020" or .040" over bearings and machine them in a jig, but it was a flathead thing back in the day.
for what its worth, in my experience, 1 full comp point is 4% hp.
Thank you all for your comments, and especially Curtis, who has given me a good long toke of reality on this.
This vehicle is (theoretically) my last "Hurrah!" for a daily. It will never see a track, never race for pinks, never darken a drag strip. I just want to put together something reliable, but the "while I'm in here" and the "I have the technology" do pop their head up once in a while.
It's a stock LQ4, Gen4 rods and pistons, GM LS9 MLS head gaskets, ARP studs. Trying to keep it sensible, and sort of regretting buying the turbo kit. And here I am.
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