9.9hp to 15hp Conversions

[Mr. Bwok]

I just thought I would take a second to post my findings after researching many resources before using my 1980 15hp Evinrude parts motor (bought for $30 with a broken connecting rod) to upgrade my 1976 9.9hp Johnson (low hour two owner beauty that is seeing hard duty in the hands of two boys, 10 and 13, on a 10' Zodiac). The upgrade results are fantastic 21mph GPS top speed with 9.9hp is now 27mph with the 15hp conversion. See below what I found out from retired OMC professionals in Peterborough Ontario Canada and LeeRoy Wisner's fantastic and life saving knowledge he shared on these outboards. http://www.sschapterpsa.com/ramblings/OMC/OMC info.htm

The 9.9 hp carburetor?s INTERNAL THROAT is .625 dia., 15 hp internal throat is .875 dia. up until late 1987 when they went to a plastic topped carburetor. Please notice the word INTERNAL THROAT, not front and back openings. There are 3 different stages converting to a 15 hp and it depends on year and how much work you are willing to do. The common factor and the single largest piece of the puzzle that will increase power regardless of year is the CARB. Here are the requirments for conversion with corresponding years. (1) The 15 hp from 1974 through 1978 change carburetor only with impressive results, it is amazing what a 50% power increase will do.....lol, however trolling is a little rougher (may just be my engine). (2) 1979 through to early 1981 change Carburator and if you are ambitious add a 0.0125" shim to the reed assembly. This not necessary, but does seem to allow a bit more air flow
(3) 1981 through 1987 Carburetor, shim and now they have a tuned exhaust tube. (more trouble to change that what it is worth in my opinion)

Realistically if you are looking for an increase in power, but not concerned with dyno numbers matching exactly 15hp...........Change the CARBURETOR only. Do not get confused with people saying the intakes are different, or the head is different or the ignition is different.........THEY ARE NOT. 90% is all CARB. The shim and the exhaust will add a bit, but are not necessary to make that old 9.9 an old 15.

I hope this helps out.

 

[jmendoza]

Re: 9.9hp to 15hp Conversions

Years before the internet I did this to my 1975 9.9 under the direction of a Master OMC mechanic who had just been to school on the then new 9.9/15 series outboards. The marina who employed him had a large rental fleet of fishing boats with these engines, so they were his specialty.

There has been quite a bit of disscussion on this, sometimes heated, as there were so many changes made to theses engines over their 20+ year production run, and several style carbs and exhausts. This does lead to some confusion for sure. Anyway, back when I did the conversion in 1978, there were only a few updates so it was easier to do, and not so confusing as today, so looking back, I was fortunate to do it when you could still buy the stuff from OMC, and the information was current. Not so easy today...anyway, I digress.


The mechanic had me order the 15 carb for a 1975, same year as my engine, a new carb gasket, and a fuel line and clamps, it came to $78.00 dollars back then which seemed pricey at the time, but in retrospect it was a deal. To update the engine, he then gave me a used 8 bolt Evinrude head, and the brown Delrin magneto plate ring as they both improved the idle for trolling. The head had more even cooling, the Delrin ring took some slop out of the timing plate. This was to reduce plug fouling and smooth out low speed running I reccomend this mod for anyone who has a 1974-76, but unfortunateley today the carbs are hard to find.

I cannot comment on measured speed, MHP increase, or measured HP, I have to trust the OMC published specifications, but the engine was stronger and would now plane with 3 people in my boat easily, as fast as it did with two people when it was a 9.9 HP. Besides, this was before GPS!


Recently, I bought a used 1975 Evinrude 15HP power head to replace my tired one. This gave me the chance to scutinize the internals: the crank, the rods, the pistons, the ports, reeds, manifold, ignition, and combustion chambers are all identical, I could find no differances between the two blocks other than paint color. I was a little dissapointed as I though just maybe the 15 would be different, but it was not, as my mechanic had told me long ago. Oh well!

Picture of converted 1975 9.9HP and boat below: