The compression debate

[NJCoastFlyFish]

I am going to check out an engine saturday, and going to do a compression test on a 70hp johnson 1989, the owner said he did the test and all the numbers were the same but he said he didn't remember the exact number, whether 110psi all around or 90psi all around.<br /><br />I remember mixed feelings on compression, some say as long as it is ALL within 10% your fine, others say it has to be above 100psi and 10%, others say if it aint within 5% walk.<br /><br />Not sure what number I should say no thanks at? I think I would be ok with 110, very happy at 120, but questionable at 90. Any help would be great!

 

[Texasmark]

Re: The compression debate

We discussed that the other day and I think it was on the Merc forum. I got numbers out of my owners manual (like 150) and some of the more senior members said 125 was great. 100 may be a little low. The 10% was a good number we all agreed on (my manual said 15 was ok).<br /><br />Compression could be misleading. If the rings are caked up with carbon, it could show low. If the piston crown is packed with carbon it could show high.<br /><br />Johnson back then (as I recall) did use a head gasket so you could have a blown headgasket if the numbers varied alot. Merc didn't in all the engines I am aware of.<br /><br />Mark

 

[NJCoastFlyFish]

Re: The compression debate

HEhe- sorry I don't visit the merc forum much <br /><br />Did mercs and omc engines back then come out of the box with different compression numbers? I know someone who just had a new powerhead put onto a 1979 johnson 70 and it tests 120psi all around-

 

[Paul Moir]

Re: The compression debate

Yes they do. You see big changes sometimes from year to year as they messed with port timing and compression ratio. Some of the big V-6s measure 85 or 90 new. Depends on the particular outboard.<br /><br />Compression testing on a 2-stroke is only useful in a relative manner for since the specific compression reading varies greatly depending on a multitude of factors. Altitude, pressure, how much residual lubrication there is in the cylinder, whether the engine is hot or cold, how fast and how many times the engine is cranked, carbon buildup, even in extreme cases exhaust back-pressure, all have great influences on the actual number. Not to mention hardly any two compression gauges ever seem to agree. OMC avoided the whole mess by never publishing the numbers, and saying 10% variance was allowable. Of course, you've got to read it with reason - obviously a 30/35psi engine isn't healthy.

 

[Mark42]

Re: The compression debate

I suggest bringing a small flashlight and few spark plug sockets with you so you can inspect the piston top and depending on design, sometimes the cylinder wall and ports. If the piston is heavy with black carbon you know its runing rich. Sometimes you can see the port when the piston is at BDC and look for buildup in the ports. Visual inpection will also tell you if all cylinders are running the same or if there is a wide variation in appearance between cylinders.

 

[koolerb]

Re: The compression debate

I've got a differential compression tester I used to use for airplane engines. The drill for one of those is you pump 80 psi into the cylinder at TDC and measure the leakage. Have any of you used one of these on an outboard?

 

[ob15]

Re: The compression debate

The leak down test is supposed to be the best way to test.

 

[gatorred]

Re: The compression debate

all should be even in compression 125 or close if not it worn some i could be off some 100 is good but one shouldn't be 25 or that low <br />i edit it i working on a car for 125 reading opp's

 

[pj_elia]

Re: The compression debate

Did anyone understand that??

 

[ondarvr]

Re: The compression debate

pj_e<br /><br />That's Gregg, he's a good guy and has many interesting things going on in his life.

 

[koolerb]

Re: The compression debate

Not a word.

 

[cobra 3.0]

Re: The compression debate

uh?

 

[voodoo]

Re: The compression debate

My 89 65 Johnson reads 100 lbs, fresh rebuild.<br />I pulled the head to verify this

 

[dajohnson53]

Re: The compression debate

My 82 (or so) Johnson 235 powerhead measures 85-90 pretty much even in all 6 cylinders. It was remanufactured in mid-90s (they put a 175 cowling on after reman - don't know why, but the powerhead is 235). It runs very strong, idles and accelerates well, starts as well as any big carb'd 2 stroke I know, after learning the precise "secret sequence". The mileage seems to be about what you'd expect for a big 3 carb'd engine - hear that sucking sound? Seriously, gas mileage isn't significantly different than a friend's new carb'd Yammie of about the same size, similar boat.<br /><br />I've been told that, to a certain point, the above performance features, coupled with EVEN compression are really what you need to know. Compression is more of a "is it changing since last year" sort of thing and also to give you red flags for certain cylinders.