Milling 318 Heads to raise CR

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1974DartSwinger

1974DartSwinger

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Hey, I'm going to drop my heads off at the machine shop this week and wanted to get some tips first. Heres what I'm working with:
1974 Dart Swinger, 318 with edelbrock performer and 600cfm 4bbl, A904 with trans-go stage II kit, 2,200-2,500 stall converter, 8 1/4 with 3.23's, comp 268 cam/lifters with MP springs. I want to have a nice fun driver that can get up n' go but still be streetable. I'm not in any hurry to crack into the bottom half of the motor (might as well do a 360 if I do that) so instead of tossing some KB pistons in, I'm just going to mill my heads to raist the compression into the area of 8.5 to 9.5. My heads are bone stock 1974 318 heads, with mild porting and gasket matching.
Now the question!!!
How much can I shave off the heads to still keep my rocker geometry (stock non-adjustable) in a happy place? I know Mancini's sells rocker post shims up to 30 thous so that is an option if need be. So, how much can I safely shave off and which head gasket is the best to use? Does anyone know the formula for how much milling reduces your cc's?
Sorry, a little scatter-brained this morning, if anyone has some tips I'd be very greatful! Thanks!
 
For every .0095 off the deck remove .0125 off the intake side of the heads. .005 will reduce the chamber 1 cc. This engine should be 9.0:1 from the factory, using a Mopar Performance thin gasket will give you about .5 in compression gain. .030 is the max you can remove before you have to run shims or adjustable rockers. So if you cut .030 from the deck then you will need to remove .038 from the intake side of the heads.

But what I found is that you can cut .020 and not take anything off the intake side and the intake will still bolt up and you won't have any issues. So I would suggest that you cut .030 and cut .020 from the intake side of the heads, this way you'll leave more material for the valve cover gasket to seat on.
 
Now that's an answer! I was thinking about .030 but could not remember the formula of the intake side of the head specs.
 
My 318 is rated at 8.5:1 from the factory. I've been told the actual compression was closer to 8.0:1. I'm just going to cc the heads today and see what I'm actually at. So I'm going to be safe milling 30 thous without worrying about the top end going boom? Hmmm, maybe I'll mill 50 thous and use the 30 thous rocker post shims from Mancini's. . . . I just want to get my ratio up to a solid 9.0:1 actual compression or its going to run bad with my comp 268 cam.
 
1974DartSwinger said:
My 318 is rated at 8.5:1 from the factory. I've been told the actual compression was closer to 8.0:1. I'm just going to cc the heads today and see what I'm actually at. So I'm going to be safe milling 30 thous without worrying about the top end going boom? Hmmm, maybe I'll mill 50 thous and use the 30 thous rocker post shims from Mancini's. . . . I just want to get my ratio up to a solid 9.0:1 actual compression or its going to run bad with my comp 268 cam.
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I would just put a thin gasket on it and call it bueno. The compression is not too bad, if just left as is. I am running a Comp XE268 with stock heads with a 3 angle, close to stock thickness gaskets, Hooker headers, Holley 650, and a tall K&N 4" air cleaner with stock (for now converter), and 3.21 gears. It is a bit soft launching hard out of the hole below 2,000, because of the stock converter, but pulls very hard for a 318 above that. Around town, it has great low end power and behaves well, even though the cam will have a light to moderate lope. Its a good sounding motor as well. I shouldnt have an issue running in the mid 14s with this at all, which is good for a hwy geared cruiser.

With your converter, it should run even better, plus being basically stock internally, besides the cam, the 318 should last forever and take lots of abuse, with proper maintanance.
 
1974DartSwinger said:
Hey, I'm going to drop my heads off at the machine shop this week and wanted to get some tips first. Heres what I'm working with:
1974 Dart Swinger, 318 with edelbrock performer and 600cfm 4bbl, A904 with trans-go stage II kit, 2,200-2,500 stall converter, 8 1/4 with 3.23's, comp 268 cam/lifters with MP springs. I want to have a nice fun driver that can get up n' go but still be streetable. I'm not in any hurry to crack into the bottom half of the motor (might as well do a 360 if I do that) so instead of tossing some KB pistons in, I'm just going to mill my heads to raist the compression into the area of 8.5 to 9.5. My heads are bone stock 1974 318 heads, with mild porting and gasket matching.
Now the question!!!
How much can I shave off the heads to still keep my rocker geometry (stock non-adjustable) in a happy place? I know Mancini's sells rocker post shims up to 30 thous so that is an option if need be. So, how much can I safely shave off and which head gasket is the best to use? Does anyone know the formula for how much milling reduces your cc's?
Sorry, a little scatter-brained this morning, if anyone has some tips I'd be very greatful! Thanks!
Click to expand...
I was just thinking............what about nitrous for your low compression motor? I am sure that there are a number of members who are well versed on the subject.
 
wedgie said:
I was just thinking............what about nitrous for your low compression motor? I am sure that there are a number of members who are well versed on the subject.
Click to expand...

True, but its not the best thing for cast pistons.....
 
Hmm . . .no NOS on my 'teen. I enjoy being able to use my pistons as ash trays when I decide to upgrade. I'm just keeping it an all motor car . . . maybe turbo? lol Untill I find a '65 or '66 Dart this'll do me just fine.

Has anyone calculated their actual compression with a stock 318?
I think I'll just mill them 30 thous, toss in some 30 thous rocker shaft shims, mill the intake 20 thous and call it good.
I'll keep you guys updated. If I can run mid 13's once I throw in a posi and 3.55's I'll be happy.
 
1974DartSwinger said:
Mopar J. How do you like the hookers? Are they regular Comps or Super Comps?
Click to expand...

They are Comps and work ok; they are just a pain to work around, like any a body header setup.

With 3.55s or numerically higher, high 13s are possible, maybe a bit quicker than that actually.
 
I think with my 3.55's I had a best of 13.74 that was also 12 years ago when the motor was fresh. with 3.91's I ran a best of 13.55
 
BJR Racing said:
For every .0095 off the deck remove .0125 off the intake side of the heads. .005 will reduce the chamber 1 cc. This engine should be 9.0:1 from the factory, using a Mopar Performance thin gasket will give you about .5 in compression gain. .030 is the max you can remove before you have to run shims or adjustable rockers. So if you cut .030 from the deck then you will need to remove .038 from the intake side of the heads.

But what I found is that you can cut .020 and not take anything off the intake side and the intake will still bolt up and you won't have any issues. So I would suggest that you cut .030 and cut .020 from the intake side of the heads, this way you'll leave more material for the valve cover gasket to seat on.
Click to expand...
Isn't it .0095 off the intake surface for each .010 off the head gasket surface?
 
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