99 to 15 HP conversion

My mechanic was telling me the

My mechanic was telling me the Honda 9.9 and 15 4-stroke are the same displacement and its possible to convert to the 15. Anyonw have more details about this?

"Jason, most 9.9's/15'

"Jason, most 9.9's/15's are the same powerhead regardless of brand. And yes they can be converted - have done a number of 2 stroke conversions.

Can it be done, yes, is it worth it - not at all. The cost increases significantly with a 4 stroke. Primary difference is the carbs and sometimes the exhaust as well - has to be able to "inhale and exhale" better to create the 50% horsepower increase.

On the 4 strokes there is usually the added pain of swapping out the cam, since the valves now have to move differently.

And if you decide to go ahead with all the cost and work (usually on an engine that is running fine in the first place) this is what you get -

Personal test, not scientific, but close enough I guess...

14 foot boat, total displacement with 2 people 850lbs.

Top speed 9.9, cool, calm day, no waves/wind, 18mph - same conditions etc, 15 horse, top speed 21mph.

Horsepower increase of 50% resulted in less than a 20% increase in speed at significant cost in parts - oh ya, did notice a 40% increase in fuel/oil burned at about 3 bucks a gallon.

A good tune-up, a high quality prop and keeping the boat balanced will do more than a 9.9 to 15 conversion will do for you - plus you will save hundreds in parts and gas over the long run.

If you want more power, sell your 9.9 and buy a 20 or a 25...

My 2 cents..."