gt 40 head questions

mikeyzx6r · May 24, 2011

alright guys i have a set of gt 40 heads that were giving to me and ive been reading that when putting these on they drop the compression ratio which i do not want to do so being said how much can a shave off the heads to reach around 10:1 ratio.. and if i do shave the heads can i run my cam with 1.7 rocker arms?

Mustang5L5 · May 24, 2011

What's the casting number. There are a few different versions floating around.

Most are 62.5cc combustion chamber size...+/- a CC or two.

Gearbanger 101 · May 24, 2011

From what I remember, the GT40's had approx 65cc combustion chambers (give or take). The sock E7TE heads had anywhere from 62-66cc combustion chamber volume depending on casting variances...so they're essentially the same. Also, you can't achieve that kind of compression, but simply just milling the heads. You're going to need to change out the pistons if you want to bump the compression to that extent.

Also keep in mind that if you do try milling the heads down excessively, you are going to run into piston to valve clearance issues as well as intake fitment problems that will require you to mill down your lower manifold in order to bolt it down. That being said, if you get a piston with generous enough valve reliefs, running the 1.7 rocker ratio won't pose a problem and neither will fitting the manifold.

RangerJoe · May 24, 2011

What cam are you using? Even if its a stock cam, I agree that getting to 10:1 will be difficult on a stock bore stock piston short block. I think that when I looked into it in the past, I figured milling .040 would give me 9.6:1 compression, but it also put my valve clearance in the dangerous zone.

Try selling the GT40s and buy a set of GT40Ps. They have smaller chambers, about 59cc's, and milling them about .030 will get you close to 10:1 with stock short block. You must check PTV clearance though.

Good luck!

mikeyzx6r · May 24, 2011

im using the steeda 19 cam im in the process of rebuilding my block as we speak so could i come close to 10:1 with flat top pistons and milling my heads and intake?

Gearbanger 101 · May 24, 2011

Well, if you're planning to change the pistons anyway, you won't need to worry about milling your head and intake. Just go with a bit of a domed piston to make up the compression difference.

Keep in mind though...if you ever plan on swapping heads later on down the road to something better, you'll have to choose a piston that will accommodate the chamber design of many of the aftermarket castings...otherwise you might again run into piston to valve clearance issues.

RangerJoe · May 24, 2011

mikeyzx6r said:
im using the steeda 19 cam im in the process of rebuilding my block as we speak so could i come close to 10:1 with flat top pistons and milling my heads and intake?

Simply going to a flat top piston isn't going to net you that big of an increase in compression. A flat top with valve reliefs is usually a couple of cc's smaller, which may raise your compression 1 or 2 tenths. Do a google search and find a compression calculator...you can toy with your options there. Flat tops and a .030 mill may get you in the upper 9.X:1 ballpark...also, going with a .030 over bore will help a little.

I know that going with a .030 mill will not affect intake alignment, at least in my experiences with it. I'm not sure how much further you can go before it becomes a factor. If you are using an aftermarket piston with deep valve reliefs, then you can get away with a lot more milling. Like someone stated above, depending on how much you mill may affect your intake aligment and require having it cut slightly to fit.

I have Probe SRS inline pistons with a TFS Stage 1 and 1.7 RR. My GT40s were milled .015, just to make them straight. When I checked my PTV clearance, the valves barely dented the clay. PLENTY of room to grow...

Rick 91GT · May 24, 2011

you can acheive 10:1 but the chamber will be very small, I actually dont think you can mill that much off a gt40 and still have it sturdy.

gt 40 head questions

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