increasing compression ratio
 

I was wondering how much power i could get from my 302 by increasing the compression ratio of my GT40 heads from 9:1 up to 10:1 or even 10.5:1?

How about a 200 straight six with 8:1 compression going up to 10:1 :D

Thanks,

note: i haven't installed my GT40 heads yet so any suggestions as to anything else i may want to do is greatly appreciated (i'm going to gasket match, don't think i want to port/or have them ported unless it makes a huge difference)
Is increasing the compression ratio worthwhile since i'm going to be using nitrous oxide - i don't want to blow holes in stuff.

General rule of thumb is a 3% to 5% increase in output for every full point of compression that you can raise the motor. The SBF responds extremely well to increases in compression, especially if you keep the port velocities high which you will with the small port volume of the GT-40. Go for it. I've raced my old P-headed motor on 93 octane gas at 13.0:1 compression so don't be afraid to go for as much compression as you can squeeze out of the combination.

Jeff - I've seen you quote those numbers before. 5% of 225HP is only 11HP for one full point. I've seen other sources quote that 10HP for .5 point of compression. Who's right??? You did say that that is a "rule of thumb" but how much does it vary?

It is only a rule of thumb, and a rule that only considers the change in compression....nothing else. I've often thought of it as being way to conservative myself, but who am I to argue? I think it seems pretty reasonable if you're considering a stock motor going from 9.0 to 10.0, but it would really seem to fall apart if you think of a Hot Street motor that is pushing 17.0:1. You'd never get to 700 hp just by taking a stock motor from 9.0 to that 17.0. More often then not, you're doing other things at the same time that you're raising the compression to better optimize the combination so the gain due to the change in compression gets washed out. Oh, but I digress. At any rate, I've got several engine building programs that seem to concurr with the 3 to 5% rule. I just ran the numbers for a stock 225hp motor going from 9.0 to 10.0 and it give a new peak hp rating of a little more than 232hp. That's only 7hp or a 3% change.

thanks for the replies, those numbers sound reasonable.

If i get the heads milled i have to mill the intake 1/2 of what the heads are right?
How well do the heads/intake line up to the intake gaskets after milling, would i have to elontage the gasket holes?

Usually, milling the heads up to .030"-.040" on the outside does not require intake milling. Just have to put the intake on the shaved heads and check runner alignment.

Just an FYI...my machine shop (Fowler's Engines of Columbus, OH) milled my TFS TW's .045" and milled the intake side of the heads as well in order to compensate for the head surface milling. The intake ports line up perfectly and I had just enough space at the bottom of the intake to the 'wavy' part of the block to seal it up with black RTV. This milling took my combustion chambers from 66cc to 58cc and resulted in overall compression change from 9.2:1 to 10.0:1.

Make sure you do all the calculations so that you end up with the proper compression that you're looking for based on chamber cc, etc.

Good luck,
E