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I have read lots of posts- with no one really coming to the truth. I have a2005 F75 4 stroke EFI. I wanted some more grunt so I did the research. The only 3 parts I could find different are the intake hose , the ECU and the lower unit. The intake inlet has an aluminum restrictor on the F75 - on the F90 it simply is not there. The part number on the ECU for the 75 & 90 are different which is the real trick. The lower unit on the F90 is slightly ( very slightly) higher geared. So I bought a used F90 ECU on ebay for $250 - then I sold my F75 ECU for $100 . I removed the restrictor and left the lower unit alone.
So - when my motor was only 75hp I needed a 15 pitch prop to push my 17ft crestliner at about 32 MPH ( flat out ). So - I started with a 17 pitch prop after making the 90hp conversion and it flew out of the hole and hit the rev limiter at about 34 mph. So - I then installed a 19 pitch prop and it still comes out of the hole fast and runs 40 mph without hitting the rev limiter. I think I could go to a 20 or 21 pitch without a problem. Since I like to run at about 28 mph cruising speed I now use a LOT less fuel and the motor runs perfect. Real case real truth. So - for $150 I turned a 75 into a 90
So - when my motor was only 75hp I needed a 15 pitch prop to push my 17ft crestliner at about 32 MPH ( flat out ). So - I started with a 17 pitch prop after making the 90hp conversion and it flew out of the hole and hit the rev limiter at about 34 mph. So - I then installed a 19 pitch prop and it still comes out of the hole fast and runs 40 mph without hitting the rev limiter. I think I could go to a 20 or 21 pitch without a problem. Since I like to run at about 28 mph cruising speed I now use a LOT less fuel and the motor runs perfect. Real case real truth. So - for $150 I turned a 75 into a 90