compression ratio ?
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Senior User
Joined: Nov 2003
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compression ratio ?
I have been reading a lot of old topics this past week and have come to the conclusion that if I use some 360 pistons I have, my compression ratio will be bumped up a little bit and also give me better quench. My ? is what exactly will my compression ratio and quench be if I threw thoses pistons in the motor. They are .030 over and I would use a Mr.Gasket steel shim(unless I would have problems using it).
Post Fiend
Joined: Jan 2003
Posts: 8,600
Likes: 4
From: Iowa
Hard telling. There's too many variables. I could calculate what it theoretically should be, knowing the specs on everything. However, this rarely comes out in practice. For instance, one manufacturer's "360 pistons" could be 1.755" compression height, while another's could be 1.78".
I'd advise you to do a mock-up build, and measure everything. That way you'll know exactly what you've got.
I'd advise you to do a mock-up build, and measure everything. That way you'll know exactly what you've got.
Post Fiend
Joined: Jan 2003
Posts: 8,600
Likes: 4
From: Iowa
Theoretically then:
Valve relief volume = 10cc
Compression height = 1.776" (this is the big one)
Deck clearance = 10.17-1.892-6.488-1.776 = 0.014" (you can directly measure this)
Head gasket bore = 4.22"
Head gasket thickness = .020"
Cylinder bore diameter = 4.080"
Crankshaft stroke = 3.784"
Head CC (measured) = 69.3
So we get a 10.33:1 static compression ratio. However, change the pistons to 1.759" compression height, as some aftermarket "replacement 360" offerings are, and look what happens to the compression ratio. It goes down to 9.96:1. The Mr. Gasket steel shim head gasket really raises compression. If you used a Fel-Pro with the following dimensions:
Head gasket bore = 4.4"
Head gasket thickness = .041"
Then compression goes down to somewhere between 9.77:1 and 9.44:1, depending on pistons.
This is why it's so important to actually measure everything.
Valve relief volume = 10cc
Compression height = 1.776" (this is the big one)
Deck clearance = 10.17-1.892-6.488-1.776 = 0.014" (you can directly measure this)
Head gasket bore = 4.22"
Head gasket thickness = .020"
Cylinder bore diameter = 4.080"
Crankshaft stroke = 3.784"
Head CC (measured) = 69.3
So we get a 10.33:1 static compression ratio. However, change the pistons to 1.759" compression height, as some aftermarket "replacement 360" offerings are, and look what happens to the compression ratio. It goes down to 9.96:1. The Mr. Gasket steel shim head gasket really raises compression. If you used a Fel-Pro with the following dimensions:
Head gasket bore = 4.4"
Head gasket thickness = .041"
Then compression goes down to somewhere between 9.77:1 and 9.44:1, depending on pistons.
This is why it's so important to actually measure everything.
Elder User
Joined: Feb 2005
Posts: 669
Likes: 1
If you're comparing the 360 pistons to stock truck 390 pistons then you will get a serious increase in compression ratio. If you were to run rusty's calculations and change to the stock 390 piston I would guess that the compression ratio will drop to under 9 to 1. BTW...what head casting do you have?? Those 69.3 cc heads are really helping your compression....most of the later FE heads check out at quite a bit higher cc.
Good Luck
Tracy
Good Luck
Tracy
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