compression test

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J

jivemother33

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i want to do a compression test on my 340 i have the gage what do u guys recomend the proper way to do it , engine cold or hot? how many times role the engine over ? also is there a way to convert the psi number to compression ratio or atleast a ball park idea thanks .... :lol:
 
you can do it ether way. Hot or cold. Pull all the plugs.
Put the tester on each hole. Crank the engine over 4 times.
See what the compression is and go to the next hole. Do this
till you have all the cylinders covered. Then if compression is
low. Do it again on each cylinder with a couple squirts of oil.
If it come up then you got bad rings. Good luck, Green Duster.

duster work.gif
 
You should have your carb wide open as well. As you're turning the engine over pay close attention to how fast the pressure comes up on each cylinder. Example, do most of the cylinders come to max pressure after only 2 of the 4 revolutions and a few of the other cylinders take the full 4 revs to come to full pressure.

Terry
 
i am not having engine problems just want to know about what compression i am runnin for future info thanks
 
agree with all above, I also make sure all plugs are out at the same time, do like the engine a little warm and I use a battery charged to keep the cranking speed even too..
:coffee2:
 
The only problem is your cam can change the cranking compression pretty radically so its not a really good foundation to go off of-it can give you a rough idea of the ball park number.
 
to the answer to your question about calculating cylinder pressure to compression ratio i had the same question, and after doing some searching i finally found a chart that figured it. don't know how accurate it is but looks pretty close to me. the address is http://www.geocities.com/terminalvelocitycoaster/adr/ maybe this will help with your question. :thumbup:
 
345man that table is pretty close. For example my 340 in my Valiant is 14:1 and when I did a compression test the cylinders were between 270-280psi which is right in line with the table. I also compared it to some of my old info from when I raced Buicks and those motors were 10.5:1 with 180psi which also matches the table. I know that cam shaft opening and closing rates effect cylinder pressure but this table is close enough.


Chuck
 
it was the only chart or calculation i could find anywhere that didn't take a rocket scientist to figure out. cam choice does make a difference and to get a perfect ratio you would need all info. i thought that chart would be helpful to everyone trying to get a pretty close ballpark figure on compression for future motor mods or just need to know info. im glad to here your compression and pressure came out close like mine did when i checked. :thumbup:
 
Also on the first comp stroke you should have about 80 % of the total. This is a good indicator of the condition of your ring/cylinder seal. You should be at max comp on three comp stokes,four at the most.
 
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