Re: Weird JW compression test.

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Pete

US Member - 2 Years
Replies: 154
Topics: 6
#79502

Certainly weird results but expected with these tests. If you do hand pull you will probably do 5 pulls to get a max gage reading and there is time between pulls for built compression to leak that the next pull needs to make some up to further advance the gage needle, maybe there is a slight drop in the gage reading to hold pressure? maybe you have a faulty gage and hose ❓ How long is the gage hose ❓ With a drill it will be 12 or more revs and a better chance in less time to get the max pressure in the gage. I think pull or drill shouldn’t be that different if good gage, short hose and no leaks .

I see you get 6 psi different. Neither is absolute accurate just a relative comparison is being done. We do not know the complete detail conditions of your test ❓

Note for me hand pull gives more info. For ex. I pay attention to the 1st reading that is very indicative of the lower compression ratio (CR) based on a CR using the cylinder volume above the top of the exhaust port to TDC which is less than the full volume CR from TDC to BDC. Most 2 cycle outboards I have checked have about 9:1 based on full volume (TDC to BDC) and about 5.5:1 based on TDC to the top of the exhaust port. So 1st pull tells me how that builds with the lower CR. Several pulls thereafter give info on the compression build to max. I stop at 5 pulls more than enough. For future testing I keep track of the compression readings for each pull and compare to previous readings to see the pressure build. That gives me a better indication of the cylinder and rings wear condition rather than just one max psi reading that you would get just recording the max reading. In my experience as engines wear they take more pulls to reach max pressure in the gage.

I calculate expected compression psi from a known CR using the thermo gas laws to compare to gage test pressure.

I don’t like rotating the engine a lot (like a drill) with no fuel+ oil ingesting. I always test dead cold so temp is easier to control and one less variable out of the picture + throttle wide open. I do another test with a several oil squirts of oil in cylinder pull over to distribute then test to see what about 100% ring seal will give, difference without oil indicates how much leakage there is . Leak down test can also be done for better idea of cylinder and rings condition.

I always use the same gage this most important for consistent results. Carefully store the gage.