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Bill W

Acceptable compression on engines?

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Bill W

Looking again at buying our first Bu (fingers crossed) Surprised.gif so I have been talking with folks about compression checks, etc. Did a forum search but didn't find much about the specifics and since I don't know a lot about engines in general - I it relevant to ask The Crew.

So what's an acceptable range of compression (drop) for these engines? I know the "born with" levels that they have when they are new differ between the various models but is there a guideline to use, or a % / yr ratio, etc?

I think I can safely assume 0 PSI is bad :blush: but between that and whatever the original full compression was - how is one to know what the compression numbers tell you?

Edited by Bill W

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electricjohn

You look for consistency amongst the cylinders, but I would GUESS that anything below 100 would put a flag up for me. Last time I checked,I was at 125 in all eight. An engine will run(weakly) as low as 60.

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D-GOOSE

This thread should start a great fight. Crazy.gif

So do you check compression with all the plugs out or one out at a time. :lol:

JM2C but I wouldn't want any thing less than 150psi and no more than 10% difference from the highest cyl to the lowest.

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electricjohn

I do all plugs out, throttle wide open, and two compression strokes of each piston.

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RTS

The Indmar service manual says:

Check compression by removing all spark plugs, throttle wide open, 5 revolutions of engine. Electricjohn is right on about this, as five revolutions will give you 2 1/2 compression strokes.

It goes on to say that low compression is evident if you've got less than 100 psi on any cylinder or a differential of 10% between any cylinder.

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D-GOOSE

My weedeater run's on 100psi Crazy.gif

5-7 Rev would be right Yes.gif

Yes an eng will run at 100psi or a little lower but it will be gutless.

I'm also going off of

So what's an acceptable range of compression (drop) for these engines
and that he is looking at buying a used boat.

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Pistol Pete

All this info. is correct I might add that technically, you're supposed to run the test with engine cold and engine hot. I'd also go with 8% variation between all cylinders.

edit: I'd also like to see ~150 psi on all cylinders

Edited by Pistol Pete

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Bill W

Thanks Guys, this looks to be just what I was looking for.

So to clarify in an example: if the original compression of an engine is 155psi and the current compression is 148psi in all cylinders (over 450hrs), it's a bigger deal that the compression across all cylinders is the same first - as long as it's not too far below the original levels. Is that correct?

Also - Is there a "normal" rate or amount of compression reduction over time related to wear/use?

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D-GOOSE
Thanks Guys, this looks to be just what I was looking for.

So to clarify in an example: if the original compression of an engine is 155psi and the current compression is 148psi in all cylinders (over 450hrs), it's a bigger deal that the compression across all cylinders is the same first - as long as it's not too far below the original levels. Is that correct?

Yes.gif Your right on with that one. If that are the original #'s and the current #'s then the engine is in great shape.

Also - Is there a "normal" rate or amount of compression reduction over time related to wear/use?

No.gif There are to many factors that can change your compression. I.E. the person didn't change to oil at reg intervals, not letting the eng warm up before WOT, running the eng with low oil, brain fart, help me out with a few more guys.

These are all great idea's JM2C

8%-10% variation between all cylinders.

you're supposed to run the test with engine cold and engine hot.

Check compression by removing all spark plugs, throttle wide open, 5-7 revolutions of engine.

150 psi on all cylinders min.

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99response

170 to 185 is where you'd want to be, like people have said a cylinder that is 10% under is a problem

Edited by 99response

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