Help with compression test

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    Posted: 17 Jan 2019 at 2:47pm
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Hi all

I'm looking at a 40hp mercury 4 stroke second hand and compression results,are 145 . I'm assuming this is even on each of the 3 cylinders but will confirm later.

I know it's important to have them near or the same - just don't know if 145 is low for a 4 stroke ?

Any one care to shed some light on this?

Thanks in advance
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Post Options Post Options   Likes (0) Likes(0)   Quote Mr Moritz Quote  Post ReplyReply Direct Link To This Post Posted: 17 Jan 2019 at 3:26pm
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A good 4 stroke Suzuki 115 gave 210 psi. All cylinders within 5 psi

Have a vague memory that you take actual atmospheric pressure (say 1009 hPa) multiply by compression ratio of outboard (say 12 : 1). 1009 X 12 = 12108 hPa convert that to pounds per square inch using computer = 175.6 psi. but many years since I did it
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Post Options Post Options   Likes (0) Likes(0)   Quote Tagit Quote  Post ReplyReply Direct Link To This Post Posted: 18 Jan 2019 at 4:19am
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Was the comp test done on warm engine with throttle wide öpen? Would have expected 200psi or so
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Post Options Post Options   Likes (0) Likes(0)   Quote Staystoked Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jan 2019 at 2:05am
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Thanks all who replied
Much appreciated
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