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stock compression ratio?

gmcfan's Avatar
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04-14-04, 11:34 PM   #1  
stock compression ratio?

does anyone know what the stock compression ratio is for a 9 cubic inch, 3.5 HP vertical crankshaft engine is? I like to experiment with these engines, and would like to know if I increased the compression ratio, How would I find out how much I increased it? Thanks in advance!

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cheese's Avatar
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04-15-04, 12:25 AM   #2  
Hello gmcfan!

I really don't know the compression ratio. You might be able to find out by contacting Briggs. I imagine it would be about 9 to 1. You could measure the volume of the combustion chamber, and divide the total cylinder volume by that to get a ratio. It would involve filling the combustion chamber with water to find out how many cc's it takes to fill it with the piston at TDC.

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Join Date: Oct 2003
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04-15-04, 05:42 AM   #3  
You can measure the compression ratio yourself, as cheese said. Look in the Jegs catalog for a burett. Hot rodders measure compression ratios all the time and they sell stuff to measure it in catalogs pertaining to that sport. I bought the stuff several years back when I rebuilt my Chevy engine. You use a calibrated burett to dispense kerosine or brake fluid, don't use water. The burrett allows you to accurately measure volume. Mostly you need it to get the volume of the head assembly. The cylinder volumes can be gotten simply by knowing the bore, stoke, and the deck height. If you crank in the thickness of the gasket then the only unknown is the volume of the head. I would expect that a small single cylinder engine compression ratio would run between 8.5 and 9 to 1. Anything more would require high octain gas to keep the engine from pinging. Anything less would mean lower than normal power.

Last edited by jughead; 04-15-04 at 05:52 AM.
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