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!!!