Making 150HP from 135HP Motor

[restridge]

I just bought a fish & ski boat with an '86 Mercury 135 Black Max outboard. The guy I bought it from told me the only difference between the 135 & 150 was the carb jetting. I looked thru the maintenance guide he gave me with it. I noticed the 135, 150 & 175 all had the same displacement & carb part #'s. The jetting for all three motors was based solely on elevation (altitude). So, what's the difference between these motors? Can my 135 be made a 150 or 175?

[boats.net]

Not sure there may be a difference in timming or gear ratio or even compression ratio.