upgrading merc 75hp to 90hp

[Scott Wright]

I have a 2000 640lx powered by a classic Merc 75hp 2-stroke. It's been a good motor and hasn't given me any problems, however the boat seems a bit underpowered. Hole shots are not quick by any means and my top end is OK. What I've been pondering is modifying my motor to boost the horsepower to the maximum rating (90)without having to buy a new motor or trade in my existing motor.



I took a look at the Merc Marine website to compare the technical stats on the 2-stroke 75 to the 2-stroke 90 and found that the motors have the exact same displacement, bore, stroke, dry weight, ignition, etc... So then, it seems that my existing motor could be easily upgraded by replacing perhaps just the crankshaft. Does this sound right or is it just too easy? What am I missing here?

 

[Michael Snow]

If the stroke is the same, how would a different crank help? Usually it's the aspiration that changes. More air/gas in, more exhaust out. My boat's rated for a 75 (or maybe a 70, I forget), wanna trade it for a 40?

 

[TOMMY RICH]

Yep....What makes the diff between a 200 HP 350 and a 400 HP 350?....Fuel and air mixture. In other words, Intake

 

[Michael Snow]

Port timing.... in a 4-stroke, that's a camshaft. In a 2-stroke, would that be the crankshaft (hence making my original post WAY wrong)?

 

[Scott Wright]

Thanks to all for your input. What I realize now is that it's not as easy as I'd have hoped to upgrade my motor, nor is it likely worth the cost or time committment. I'll be keeping my 75.



Scott