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Old 08-05-2003, 02:22 PM
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how to figure out compression ratio?

just curios wut my compression ratio is for future reference.. would i be able to tell off my compression test i did? like 145 psi.. is there a way to convert that?? thanks guys

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Old 08-05-2003, 03:02 PM
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Basically,No,Trav.You will have to take the motor apart and
do some itty bitty measurements.
Go to,www.chevyhiperformance.com/techarticles/46778/
and it will all come clear.
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Old 08-05-2003, 03:04 PM
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you can get a rough estimate if you know what kind of pistons you have along with the size of your chambers. But then quench and gasket size plays a big role too, that's why I say a "rough" estimate.
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Old 08-05-2003, 03:38 PM
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You can get a REAL ROUGH estimate using the ideal gas law. Recognizing P1 x V1 / T1 = P2 x V2 / T2 (initial pressure x initial volume divided by initial temperature equals the same for final conditions.), then the final pressure divided by the initial pressure should be in the neighborhood of the compression ratio. There are a lot of assumptions there - that temperature doesn't change which it does a little, that there is no leakage of gas during compression which there is, etc.

Acknowledging all the errors and unknowns you can get a rough estimate by ratioing the final pressure to atmospheric pressure. Pressures must be absolute pressures so you need to add 15psi to both readings. For your example of 145psig (that is 'gauge' pressure, absolute pressure would be 145 + 15 = 160psia), and atmospheric pressure of 0psig => 15psia, the compression ratio would be 160/15 = 10.7:1. Real ratio will be less than calculated due to not accounting for temperature increase of the gas, and will be higher due to not accounting for air leaking from the cylinder.


Other than that, you will need to do what malc says.

I figured the compression ratio of my hemi with out CCing my head volume after boring it 1/8" over increasing it from a 331 to a 354. I knew the stock CR as reported by Chrysler. From that I could calculate the compression volume because I also knew the displacement. Assuming the compression volume stayed the same, using stock hemi pistons with the same deck height, I calculated the new compression ratio using the new displacement volume. As I recall, the original was 8.5:1 and the bore job increased it to 9.1:1.

Last edited by willys36@aol.com; 08-05-2003 at 03:44 PM.
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Old 08-05-2003, 04:59 PM
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A little more technical then i thought.. i was just curios if it was easy. but my mistake.. thanx for the replys and for the info.
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Old 08-05-2003, 09:09 PM
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compression ratio

first find the volumn of your cylinder, this is the bore radius squared x the stroke. Add to it the unswept area of the cylinder, and subtract or the ammount of area either filled by piston domes or increased by dish in the piston, compute the area inside your head gasket and add that and the combustion chamber area to it. This will give you the total volumn. divide this by the added figures of the combustion chamber, head gasket relief, and the total unswept area.
Example: You have a 350 cubic inch engine= 4inch bore and 3.48 inch stroke. convert this to CC's by multiplying by 16.39. = 716.39 CC's.
Combustion chamber size maybe around 68CC's.
gasket area 4.1 inch diameter and .040 inches thick.= .168 cu inches or 2.76 CC's.
Unswept area, distance from piston top to deck of block, say around .020 inches. = .25 cu inches or 4.18 CC's.
if you have flat top pistons, then there is no dome or dish to compensate for. To get the cc's of the dome or dish, refer to the manufacturer of the piston.
Your total unswept area is 68cc's + 2.76 cc's + 4.18 cc's = 74.94 cc's. Add this to the displacement of 716.39 = 791.33 cc's. now divide by the uswept area of 74.94 cc's = 10.56:1 compression ratio. This is the mathmatical figure, and compression ratio in no way is related to cylinder pressure.
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