Proper Way Of Testing Compression

[Margarita Man]

I heard different ways of testing engine compression and now I am thoroughly confused as to the proper method. Can someone walk me through the proper way of testing engine compression on a 1985 Johnson 4 cylinder?

 

[angus63]

Re: Proper Way Of Testing Compression

Pull all plugs, wet the cyls with a little oil, insert compression gage into a cyl, crank a bit, and record the peak pressure. I'm sure you'll get many opinions, but if you do it the same way with the same gage each time, thats what is important to see if any of the cyl's change over time.

 

[reload]

Re: Proper Way Of Testing Compression

All compression gauges are somewhat different. Most folks like to use the type that screw in to the hole. I've never had that type so I use one which I have to hold it tight to the head while someone cranks it for you. Also the length of the hose, pipe etc from the gauge to the plug hole also will effect the reading because it in effect changes the comp. ratio.

Just some more nice to know info.

 

[HighTrim]

Re: Proper Way Of Testing Compression

I personally like the screw in type testers, especially if im alone doing the job. I dont want to start the debate about testing on a hot or cold motor, but most of the advice ive gotten is to bring your motor to operating temps. But like angus says, you just want to make a benchmark to compare to in future tests, as long as your original test turns out ok of course. Aim for usually 100 psi per cylinder with no more than a 10 percent difference between cylinder.

 

[77Lark]

Re: Proper Way Of Testing Compression

I had 125 in two cyl. and 130 in two cyl. I thought it was good. Is that too high? I keep hearing shoot for 100. I'm just thinking maybe this means I have some gunk buildup on the rings or something.

 

[HighTrim]

Re: Proper Way Of Testing Compression

100 is a minimum, ur numbers are great, i wouldnt worry about them. If you had build up you r numbers would be lower.

 

[Margarita Man]

Re: Proper Way Of Testing Compression

Thanks for the advise guys! I guess the hot and cold testing is not much of an issue if each cylinder readings are within 10 psi hot or cold. But I will test it both ways to see if there is really any difference.

 

[luckyinkentucky]

Re: Proper Way Of Testing Compression

I was always told to test at wide open throttle! Also, I was told NOT to add oil to the cylinders until one or the other falls low. That way you can troubleshoot a worn piston ring! If the compression goes up with the oil addition then you have a worn or sheered piston ring. I got my info from an Evinrude Tech who works for Evinrude.

 

[bear58]

Re: Proper Way Of Testing Compression

Glad I stumbled on this thread, was about to post a simular one.
I have a 1993 40hp johnson, oil injected, was wondering about the compression should I be looking for a min. of 100 psi as well.

Thanx .... great site, frequent lurker, first post

Bear58

 

[HighTrim]

Re: Proper Way Of Testing Compression

the number vary depending on the gauge used, temp, etc. the main thing is the difference between cylinders, up to 10 percent is acceptable. But yes, if you're in the 100 psi range with equal numbers between cylinders you're looking good.If you're a little low, you can always de carb, sometimes helps to bring ur numbers up.

 

[bear58]

Re: Proper Way Of Testing Compression

the number vary depending on the gauge used, temp, etc. the main thing is the difference between cylinders, up to 10 percent is acceptable. But yes, if you're in the 100 psi range with equal numbers between cylinders you're looking good.If you're a little low, you can always de carb, sometimes helps to bring ur numbers up.

engine will be cold, and non screw in type gauge will be used.

Thats great and thanx
Bear58