Raising compression 400?
 

I have a 1975 dodge dart sport. It has a 1978 400 from a rv in it bone stock. the 400 had 37k miles on it when pulled, i have also got a new cam kit for it and much more. I want to get its compression around 9.5-1. Factory is anywhere from 7.7-1 to 8.2-1 from what i have found. Doing a compression check on it before taking it apart it was 125-124 on all cylinders. I have purchased the mopar head gasket set that is .20 instead of the stock .40. I need to know how much to get mil'ed off the heads to get it in the 9.5 area safely with those gaskets. I do understand that doing the deck would be a lot better but i plan on changing the engine down the road anyways. Also something that might help the heads are 4006 452 casting, the car weighs roughly 3200lbs it has a 8.3/4 rear end. Any exact numbers that i can tell my machine shop would be extremely helpful. I'm by no means a top notch mechanic but this is a project me and my buddy's work on for fun and don't wanna mess it up. If any more info is needed i can supply.

Rising the compression two points takes a lot of cutting, block and head's. I have done this on a 440 you need to get the heads down to 80cc from 88 - 90 , then cut the intake side so the it fits, I will say that the valve cover sealing area remaining is very thin, -1/8".

You need to deck the block so the piston is down 90 in the hole.

If you want/need compression buy new forged pistons in the range you want. You get the benefit of a better piston and a few more cubes.:)

The heads you have are great up to 560 lift, but like a little work under the valves. If you can unshroud and blend the bowels it would help. A set of 2.14 / 1.81 valves is re a requirement, I have a mild 9.5 440 with these heads 520hp , 543tq , so they can work.:)

I had to look this up its been years since I did the heads and this is out of the Direct Connection Performance Book, 1980 something.

Remove.0042 from the head surface per 1cc of chamber volume.
For each .010 removed from the head,.0123" must be removed from the intake face of the head.

Like I've said before you want the heads at 80cc. So you're going to cut 0.0336 of the face and 0.04059 of the intake side for a 8cc reduction in volume.

You might want to get the cost for doing this and apply to the pistons instead because I think without decking the block its only going to rise the compression about one half point.
Good Luck

Forgot to say welcome to the forum, I'd like to here more about your a body.:)

Thanks. it good to be here. I understand decking the block is the better way to go but how much can i get off the heads safly would you say

It's all in my first post, that's about it!:) MAXED OUT

thanks man, one last question the head gaskets i purchased the numbers you gave me is that including them or not? because they should help with the compression right?

The best thing you can do is find a set of 915 heads and cut them to 70cc.:)

The gasket help but your looking for two full points and that's a tough one to come up with.

And its not that decking the block is better ,its that you have to do both to get the compression ratio up.

Go to 440source.com and see what that have for pistons because the stockers are going to be limited by RPM also. There parts are priced responsibly.

okay i will take a look thanks man

With the cost of machine work ? If the cylinders are still good ? Hone & Replace the pistons with what you need for compression

I'm looking at .060" mill to get 9.2 guessing .090" down the hole. I have "heard" cutting up to .100" with no problem. All guessing aside my concern would be valve to piston clearance with no piston valve notches, bigger cam may get close Intake valve opening event "valve chasing the piston".

We did something redneck like this on a small block and with .488" lift it got tight. My son raced this thing and never blew up but when we take it apart I'm expecting to see smiles on the piston.

But why spend the time and cash to use a stock cast piston?:)