Originally Posted by gaviotapete
ah nos,glad to see that you are around b/c you are probably the best one to ask about doing a horsepower upgrade to a 40hp Mercury. have you tried it? is it possible?? how about yammie 75 going to 90??
Sorry, only just saw this Pete.
Don't know about the yammies-never owned one.
That 40 merc of yours has a powerhead that looks strikingly similar to the old blueband 50-which never actually made more than 45hp at the prop anyway as they were crank rated at 50hp. If it's the same powerhead I'd leave it as a 40. I've never looked at the 2 side by side though so I'm not really qualified to comment.
The trouble with wanting to up horsepower in general on larger outboards is that unless you've got the blueprints or both engines in front of you then there's no way to tell what to do.
Just because an engine uses the same block doesn't mean it uses the same carbs,main/idle jets,head profile,or pistons for a start-or the same size reeds,transfer or exhaust ports. You'd need to know the full specs for the ignition system too. It's very easy from a manufacturing POV to change a programme on a CNC machine to change port heights/sizes on the assembly line and get a few assembly monkeys to fit parts from a different box but a totally different proposal to do it all by hand.
The other thing to beware of is something it appears Yam used to do on motorcycle engines back in the 70's (It probably doesn't apply now and it's an observation based on what I've seen so don't sue me!).
The Yam xs250 (25hp) and xs400 (40hp)SOHC twins had the same bottom end bar the output sprocket-the only differences were the barrels,pistons,head and cam. Yet if you fitted the 400 top end to the 250 they didn't last very long-usually throwing main or big end bearings. The only thing anyone could suggest as the reason for this was that the larger hp/capacity engines got the cranks that were better balanced and closest to machining tolerances and vice versa.
It's realistically cheaper and less hassle to sell the engine you've got and buy a bigger one if you want more HP.