compression question.
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Freshman User
Joined: Jan 2005
Posts: 33
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From: Stephens City, VA
compression question.
Is there a formula for figuring out how much millin the heads will raise compression. I have a 400, 40 over that I want around 9.2 comp.I currently have new (free) 8.4 pistons, and want to mill the heads to get the compression I want.
Thanks in advance!
Thanks in advance!
Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
You can do ithe hard way or the easy way. The easy way is to download a free engine calculator like this one.
http://www.virtualengine2000.com/ProgramDownload.htm
The hard way is to find the ratio of the swept volume of the cylinder to the volume of the head, gasket, deck and dish.
A 400 head has about 78.4cc combustion chamber. The gasket is 0.040" thick. The gasket bore is 4.1". The piston at top dead center is 0.0565" below the deck surface. Pistons which will give you 8.4:1 CR have about 13cc in the dish and valve relief.
To get 9.2:1 CR you need to reduce the unswept volume by 11.4cc. You can do this by milling the heads or by decking the block or a combination of the two.
If you zero deck the block, it comes out to 9.23:1. That requires removing 0.0565" material from the block. You may want to take some or all from the heads.
You should cc your heads to make sure that they are 78.4 ccs before doing any cutting. You should have your machine shop verify that it is safe to take 0.0565" from the block or heads. If you mill the heads, you should cc the heads when you are finished to make sure that you have taken the right amount off. Your machine shop will also have to remove material from the intake manifold mating surface to make the intake manifold fit.
http://www.virtualengine2000.com/ProgramDownload.htm
The hard way is to find the ratio of the swept volume of the cylinder to the volume of the head, gasket, deck and dish.
A 400 head has about 78.4cc combustion chamber. The gasket is 0.040" thick. The gasket bore is 4.1". The piston at top dead center is 0.0565" below the deck surface. Pistons which will give you 8.4:1 CR have about 13cc in the dish and valve relief.
To get 9.2:1 CR you need to reduce the unswept volume by 11.4cc. You can do this by milling the heads or by decking the block or a combination of the two.
If you zero deck the block, it comes out to 9.23:1. That requires removing 0.0565" material from the block. You may want to take some or all from the heads.
You should cc your heads to make sure that they are 78.4 ccs before doing any cutting. You should have your machine shop verify that it is safe to take 0.0565" from the block or heads. If you mill the heads, you should cc the heads when you are finished to make sure that you have taken the right amount off. Your machine shop will also have to remove material from the intake manifold mating surface to make the intake manifold fit.
Thanks
