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PostPosted: 07 Jul 2011, 07:50 
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Joined: 26 May 2009, 05:47
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Location: West Olive Mich
You hear people talk about compression numbers.Do any of you people know what a newer motor compression numbers are #-o

A newer 25hp Merc will have around 145 to 150 #'s.I have purchased older Mercs & have had compression readings around those numbers also.Some times the rings may be stuck, but a little Marvel Mystery oil will take care of that if not too bad.I have a 1974 85hp Merc that the numbers are around 160# with no motor work done.

You have to make sure you have a good compression gauge.I use a Snap on.

So when somebody says 125# is not that good, I feel the same way.



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PostPosted: 07 Jul 2011, 17:21 

Joined: 24 Mar 2010, 00:02
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Well, comparing Mercury motor compression values to Evinrude/Johnson motor compression values is not comparable. That is like comparing apples to oranges, yes they are both fruit but they are different!!

I am a Johnnyrude guy, so I don’t have much experience with Mercury motors. It is my understanding that Mercury motors have always been high compression motors.

Also, you can not use the same values of compression when talking about higher hp motors compared to lower hp motors. Higher hp motors will have and require higher compression psi numbers compared to lower hp outboards. Also using compression numbers from new to late model outboards and comparing them to older outboards is not justifiable.

I am going to make some comments about compression related to Johnson and Evinrude motors, because am more familiar with motors from the 1950’s to mid 1980’s motors.

*** I own several Johnson/Evinrude outboard service manuals and none of them given compression reference ranges.** Over the years outboard mechanics would document in their notes service done on new or relatively low hour motors of compression and work they had done and how the motor ran. As time went on these same mechanics serviced these same make, model and era motors and would document the compression and work done on these now older and higher hour usage motors and how they ran. So, over a period of time the service industry developed an acceptable range of compression based on the make, model and year motors. Some motors were known to be lower compression motor off the bat.

Excellent example of what am referring to is the 9.5hp Evinrude/Johnson motor that where built from 1964 to 1973. It is not uncommon to find compression readings ranging from 55 psi to 80psi on these motors. Most of them are in the 60’s and 70psi range and they run fine. Some people have those same motors with the psi range in the 50’s but they do not have as much power and have dropped a few mph, but idle ok and are dependable.

You can not compare compression ratios from 25hp motor to that of 6hp motor.

25hp Evinrude = great compression would be > 125lb, (but run and idle ok at 100psi)

but

6hp Evinrude = great compression would be >85 to 90psi

Most of the Evinrude/Johnson motors 5.5hp to 6hp from 1950’s to late 1970’s had decent to good compression range from 60psi to 80psi. Then again, most of the motors from that era are found to be running fine with psi ranges from 60psi to 70psi.

So, they are not speed demons, they don’t have quite as much power and has lost a mph or two over 40 to 60yrs. That is pretty dang good if you ask me. If the motor is running fine, idles ok, gets you to the fishing hole and back and is dependable…..then that is what most of us are wanting!! Most of us can find those motors for less than $300 or $400 dollars. Beats paying couple of thousand dollars just for the motor. Any hoo off subject…sorry!


Again you can not compare different makes of outboards and different hp of outboard compression values and extrapolate if the compression is adequate or not.

Please be very careful when telling someone that has little or no knowledge about outboards that there compression values are too low and that should get rid of the motor or junk it. Unless you are very familiar with the year, make or model of the that outboard, you may be giving poor advice and that treasured outboard that belonged to their dad, grandfather or their only affordable means may be junked!!!


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PostPosted: 07 Jul 2011, 18:06 
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Joined: 15 May 2010, 00:38
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Location: Central Florida
Right on the money, Cajun.
Another good example of compression changing throughout the years would be in the OMC crossflow V-4 line. The older 115s and 135s had compression up in the 120s, 130s or higher but the later model engines designed to run on lead free low octane fuels went all the way down into the 90-100psi range.
Some of the OMC Looper V6 engines have staggered compression as well meaning that cyls #1-4 will have higher compression than cyls 5-6 will. Also the numbers will be different bank to bank. The standard for compression these days is and has been that overall, the cylinders should read within 10% of eachother whatever the reading is. That takes the differences in model years, engine models, brands, and compression gauges out of the game.


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PostPosted: 07 Jul 2011, 18:09 

Joined: 07 Jun 2010, 08:18
Posts: 510
Location: NY
I would go a step further and say there might not be any general rule to follow. Below are the SM specs for 2 stroke Nissan/Tohatsus:

Model kPa psi
2.5-5 hp 539 78
8-9.8 hp 392 57
9.9-18 hp 760 110
25-30 hp 755 110
40 hp 735 107
40-50 hp 670 102
60-70 hp 833 121
80- 90 hp 804 117
115-140 hp 882 128


Specs from Merc SM for 20 & 25HP:

Each cylinder within 15%, and less than 120 psi may indicate worn engine.


Out side of the basic displacement to head volume compression (static), you have to take in consideration the height of ports for dynamic compression.

I 100% agree on keeping a yearly record. While I have been good on doing that on my I/O, not so much with my 2 strokes.



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PostPosted: 07 Jul 2011, 18:18 
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Joined: 26 May 2009, 05:47
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Location: West Olive Mich
I will admit that most of my motors are Mercs.

My post did what I wanted,brought different views to the table & to open readers eyes.



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