Raising compression by milling heads
 

How many thousanths milled from the head will it take to reduce one cc in combustion chamber volume? This is on open chamber bbc heads. Looking to raise compression from 8.1:1 to 8.5:1 (at least). Using aluminum edelbrock heads, so I don't think I can use the steel shim gaskets?? Have closed cooling though, so corrosion wouldn't be a problem if I could. Any ideas/input would be appreciated,

BT :cool:

I have a formula around somewhere for angle milling. Ill look for it at work tomorow. It should work for straight cut with a little massaging.

I thought about angle milling too, but afaid that would make my exhaust angle out too much? Any experience there? I have twins and the room between the risers in minimal already.

BT :cool:

Make sure that you mill the intake manifold after the heads are milled. This will make for a trouble free assembly.

Most performance machine shops know how much to mill to change compression.

Blue Thunder, I don't think you want to go to all the trouble of ANGLE milling your heads. Yes you will make more power with angle milled heads, but if the shop makes one teensy little mistake (and they may not always admit it by telling you) it will cause you a ton of headaches! Just ask Mr. Gadgets---it's not worth the headaches and all the grief. STRAIGHT milling the heads would be okay---but ANGLE milling you're just asking for potential problems. We wacked off .040" from my heads to gain about a 1/2 point in compression, but understand there are variables involved. Also, do NOT mill anything off the intake manifolds until you FIRST mock them up on the milled heads. We thought we had to mill my intake manifolds after milling .040" off my heads, but found out we didn't have too---they fit fine. Best Wishes. :)

Yea , I agree . Having done the angle thing, I wouldnt do it again. It makes everything custom. It didnt really affect my exhaust enough to notice.

Going only a 1/2 pt in compression..........How much will that really give you

Cost + Headace of fitting stuff may not even = noticeable gain ?

Ask some guys who have played around on the dyno what an expected gain will be before you go through the trouble.

Just a thought.

HEY GUYS THE RIGHT WAY TO DO IT IS ANGLE MILL THE HEADS SO ANY STOCK INTAKE WILL FIT WITH OUT CUTTING THE INTAKE ,,I DO THIS ALL THE TIME AND IT KEEPS EVERYTHING IN LINE
ONE HALF POINT DOES HELP ON A N/A MOTOR AND FOR THE 40.00 BUCKS TO CUT THE HEADS ITS CHEAP H.P. NOT ALL SHOPS KNOW HOW TO DO THIS BUT MOST GOOD ONES CAN :p :p :p

I had my 454 rect port heads milled .020. The machine shop that did the work said that for every .005, it would reduce the CC size by 1cc. When I calculated it, it increased comp by .25.
The machine shop also said that as long as you don't go more than .030, that there shouldn't be any problems with the intake.

Also take into consideration which head gaskets you are using and which ones you are going to use. Stock mercs are like .030 and the Fel pro Marines are .039. So if you mill .030 and go from the stock mercs to Fel pro's you gain .021 and about .25 on your CR.

Also as far as gaskets with the aluminum heads, the felpro #1037 which I used says its compatible with iron or aluminum heads. The #1027 says aluminum only. The 1037's have a SS core.

I believe I will get them surfaced for flatness (.008-.010). This will be the third time of doing that for a total of about .030 removed from virgin castings. If the 1cc/.005 is correct, I will end up with 112cc combustion chambers. This will put my ratio at about 8.37:1. Maybe I can sneak a little more out with head gaskets to get me nearer to 8.5:1.

Thanks fellas. As always, the input is much appreciated!

BT :cool:

BT;

While you are getting some good advice here, I would also want to be sure that the quench area in the motor is right. How far down the hole is the piston at T.D.C.? This measurement added to your headgaskets' compressed thickness will give you quench. To me this is very important to make good power. The quench area is what creates necessary turbulence in the combustion chamber. You want the quench to measure somewhere close to .040" but no less.

Checkmate... that was one thing I kind of gave up on when I figured out I cannot run shim gaskets with aluminum heads. I also am not rebuilding the rotating assemblies this time so cutting the decks is out. I haven't measured yet, but am guessing the piston to deck clearance is the standard .020. If I could locate a shim style gasket for use on aluminum I'd be in good shape. I have closed cooling so corrosion is not a concern. Just can't find the right gasket.

BT :cool:

just wondering why you run such low compression ?

8-1 is pretty much standard in stock marine engines (7.4 330 hp). with flat top pistons. You either cut the heads or change pistons.

AMBUSHER .. going with the edelbrocks I have for now. If you post those in the parts section I bet they move quick.

Griff... thanks for the tip on the gaskets. Ordered the 1027. The bird I get my stuff from ordered me some .051 compressed thickness gaskets and I about ****. Needless to say, he has four more of those on his shelf now and some of the1027 inbound. It was nice to be able to tell him the part# I wanted. 1027/1037 drop over a cc off the standard .039 gaskets. Still won't get the ideal compression or quench, but its all working in the right direction.

BT :cool: