Engine Compression Ratio - Team Camaro Tech
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post #1 of 7 (permalink) Old Jun 8th, 19, 04:44 PM Thread Starter
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Engine Compression Ratio

Is there any way to calculate the compression ratio with the engine installed? Is so, what is the information I need?

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post #2 of 7 (permalink) Old Jun 8th, 19, 05:32 PM
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Re: Engine Compression Ratio

You would need to know piston dome volume, position in bore at top dead center (depth below deck, deck zeroed), combustion chamber volume and thickness of the had gasket once compressed to calculate static compression. Much easier to borrow a compression checking tool and spin the motor over without and spark plugs in it.

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post #3 of 7 (permalink) Old Jun 9th, 19, 11:06 AM Thread Starter
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Re: Engine Compression Ratio

Checking the compression of each cylinder without spark plugs only tells you if you have good compression or if something is wrong with one. Is there math that can be used to convert some number over to get the compression ratio if all you can do is a compression test?

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post #4 of 7 (permalink) Old Jun 9th, 19, 12:31 PM
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Re: Engine Compression Ratio

To determine your static compression ratio you use a compression gauge. To determine if you have a bad cylinder you use a leak down tester.

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Is there math that can be used to convert some number over to get the compression ratio if all you can do is a compression test?
You take the average number of your compression test and divide by 14.7 to determine your static compression expressed in atmospheres (the way they sell pistons). Ball parking it with simple numbers if your static compression test says you have 150 psi in your cylinder divide by 15 to get 10:1 compression.

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post #5 of 7 (permalink) Old Jun 9th, 19, 12:52 PM
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Engine Compression Ratio

Donít confuse static with dynamic compression ratios. Measuring cylinder pressure can give you the dynamic ratio as the cam greatly effects the result. Static ratio is mathematically determined by measurement of the components that make up the combustion chamber.
There are calculators online to add in the cam events to determine the dynamic compression ratio which is always less than the static ratio.


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post #6 of 7 (permalink) Old Jun 10th, 19, 04:26 AM
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Re: Engine Compression Ratio

Yes, one must know the head chamber volume, then one calculates gasket volume, TDC volume for the divisor, and swept volume (stroke).
Combine swept volume (BDC) with above (TDC) as this amount becomes the dividend, find the quotient.
Thank you, Dave for the tip of using compression figure.

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post #7 of 7 (permalink) Old Jun 10th, 19, 08:35 AM
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Re: Engine Compression Ratio

Below are a couple links to some compression ratio calculators. The first link will calculate your Static Compression Ratio... and the option to determine the Dynamic Compression Ratio if you input some additional information. The links are pretty easy to use by plugging in the requested information.

Note: When typing in the "Piston Head Volume", use a positive number for a dished piston (do not add a + symbol in front of the number)... and use a negative number for a domed piston (add a - symbol in front of the number).
https://uempistons.com/p-27-compress...alculator.html

If your cam information only lists the "Intake Valve Closes ABDC @ .006" , then you can use the calculator in the link below to determine you Dynamic Compression Ration after you determine your Static Compression Ratio using the calculator in the above link.
Wallace Racing: Dynamic Compression Ratio Calculator
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