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Old 03-30-2005, 05:49 PM
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piston cr

having trouble finding a post or a method for determining an engines compression ratio. the engine rebuild was done several years ago and i can't recall what pistons cr i bought. however i seem to remember having the option of 9/-9.5/-10 : 1 ratios and knowing myself i'm sure i would have went for the 10's. these are 350 sbc .030 flat top pistons with 4 valve reliefs ontop. the heads are 64 cc and i'm wondering if (short of removing a piston) if there is a way to determine cr . for example by doing a cylinder compression test are there compression numbers that would typically equate to certain "ballpark" cr. anything at all. thanks for your time.

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Old 03-30-2005, 06:11 PM
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10.3 to 1 compression with a 5.7 rod, .030 bore, 64 cc head, and 4 valve relief piston.
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Old 03-30-2005, 06:27 PM
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piston cr

sorry, i was unclear before. how do i check to confirm that the pistons are 10:1 ratio. i was hoping to confirm the pistons and cr that i have. i am not 100% certain that they are 10:1 but that is what i thought i had. thanks again.
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Old 03-30-2005, 08:55 PM
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figuring compression ratio

First you need to know the compression height of your pistons, that is the distance from the center line of the wrist pin to the top of the piston.
Second is rod length.
Third the deck height of your block.
Subtract the total of 1/2 your stroke, the rod length, and compression height of your piston from the deck height of your block, which is the distance from the centerline of the crank to the deck of the block.
This will tell you how much unswept area you have in your cylinder.
Figure the displacement of the bore and thickness of your head gasket.
Find out the CC's of your valve reliefs.
Convert the volumn of your cylinder to CC's, as well as all the other dimensions.
Add all these figures together. If you are running a domed piston, subtract the CC's of the dome from the total of the unswept area.
Unswept area is that area left for combustion when the piston is at top dead center.
Generally, this applies to the valve reliefs, the difference between piston top and the deck height of the block, cylinder head gasket, and combustion chamber.
Divide the total figure by that of the unswept area, and you have the compression ratio.

IE: 350 CID engine (SBC for the Bow Tie brigade). (4 inch bore X 3.48 inch stroke)
1 cubic inch = 16.39 CC's.
One cylinder = 716.39 CC's
Head gasket (rough numbers) thickness = .050", diameter of cylinder hole, 4.1". = 10.8 CC's.
Combustion chamber size= 64CC's
Valve reliefs,= 6 CC's.
Cylinder relief ( rough guess0 .020" = 4.12 CC's.
716.39 + 10.8 + 64 + 4.12 = 795.31 CC's.
Divided by the unswept area, 10.8 + 64 + 4.12 = 78.92. = 9.08:1 compression ratio.
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Old 03-31-2005, 06:13 AM
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If you did a cylinder psi test, and you could tell me where the cam was installed, per intake degrees i could get you close. But with out that info the only way to know is pull a head and CC everything.... Or find the part numder of the pistons you installed..

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