9.9hp outboard "upgraded" to 15hp?

[flightlead404]

In another thread a forum member mentioned having a 9.9hp outboard "upgraded" to a 15hp.

I know many of the 9.8/9.9 engines are downrated from higher hp outboards, but uprating them back to the higher hp easy or even possible typically?

I have a recent Yamaha 4-stroke caburated 9.9 and would love an extra 50% hp in the same weight.

[rslifkin]

It depends on the engine. Some 9.9s are a "small 9.9" where that was the highest rating (and it was likely offered de-rated to an 8hp). Others are a "big 9.9" where it's a de-rated 15 or 20. If it's a de-rated larger engine, they can often be up-rated. It's just a matter of figuring out what's actually different between the 2. Sometimes it's as simple as a different carb (the smaller one often having a restrictor plate and different jetting).

Don't forget, if you up-rate, you may also need to change props (depending on whether the up-rate comes with a different WOT RPM range, etc.)

[Rumrace]

There are intake kits to make 9.9 to 15hp. It’s a much bigger carb and intake. And your prop will need replacing.
Yamaha, Mercury, have parts lists for the conversion 400-$600.00

[HeywoodJ]

The question is how recent, but probably not possible. The current Yamaha 9.9 4-stroke is a 212cc/87lb engine that shares the same specs with the 8, so 8 to 9.9 is probably possible.

The current 15 is a 362cc/111lb engine that doesn't seem to share with anything. The Yamaha 20 and 25 are both 432cc, you could probably uprate the 20 to 25.

https://yamahaoutboards.com/en-us/ho...5-2-5-hp/25-15

The 15 and greater also use a computerized ignition system (don't know about the smaller ones), so uprating them becomes more complicated. Most of these kinds of stories were more true of older 2-strokes where carb, jet, and throttle limiter were the only things that you needed to touch to increase HP.

[Rumrace]

Yup that’s the case today.
Yamaha lists the 9.9 you can make a 15 from and they are all 2 stroke. Was a popular thing to do some time back.
The 4 stroke 15 Yamaha is a nice piece.
Too be honest I wouldn’t spend much on a 2 stroke.

[MicHughV]

Most 9.9's are simply a detuned 15. the real difference is about 500 rpm on the top end. There is no other difference between the engines, except for some different jets in the carb.
Some 9.9's are an up grade from an 8, same thing, an increase in rpm.

Off course, expect to pay 1,000's more for a few dollars of jets.

The 9.9's came about because some lakes and waterways have a limit of 10 hp.

Every other engine on the planet is a single number....5, 15, 25, 100, 150 etc...

One must remember that "hp" is a mathematical determined quantity..

Horsepower = Torque x RPM / 5,252, so you can see where adding rpm to the equation increases hp.

[winschwab]

FYI: I bought an inexpensive kit (under $100) to upgrade a recent 4-stroke Tohatsu from 3.5 to 5hp. Kit was prop and carb. Seems to work fine.

[Letterkenny]

Originally Posted by winschwab

FYI: I bought an inexpensive kit (under $100) to upgrade a recent 4-stroke Tohatsu from 3.5 to 5hp. Kit was prop and carb. Seems to work fine.

Any noticeable difference here? Thinking of doing this for my 3.5HP Merc (so Tohatsu).

[Cpt Mark]

You need to call a Yamaha dealer and go to the service department.

I just did this yesterday. I have a Honda 8hp I wanted to upgrade to a 9.9 or 10. All the specs looked the same, cc, weight, bore, stroke etc. What wasn't the same was the cam shaft. I looked up part numbers and photos. very different. had that been the same, then it would have been a new carb, $129, new prop, and probably a new CDI.

[Secondshift]

Did exactly that with our 2009 15FCESH.

As mentioned it depends entirely on the model.

Bought a cheaper after market carb online for under a hundred bucks.

If it lasts 3yrs it's value equal with the factory part and possibly made by the same supplier.

Had problems with the motor that seemed to indicate carb.

Turned out to be CDI but glad I bought that carb. Made a big difference on my little motor boat


Best Luck,

[Mark424]

I haven't seen anyting on the Yamaha, but the Suzuki 9.9, 15, & 20 are the same engine with different ECU, air intake, and maybe one other part. There is a youtube video of someone upgrading a 9.9 to a 20.

Originally Posted by flightlead404

In another thread a forum member mentioned having a 9.9hp outboard "upgraded" to a 15hp.

I know many of the 9.8/9.9 engines are downrated from higher hp outboards, but uprating them back to the higher hp easy or even possible typically?

I have a recent Yamaha 4-stroke caburated 9.9 and would love an extra 50% hp in the same weight.

[Rumrace]

Many manufactures don’t even show 2 strokes in their web page. Sailors tend to hang on to long shafts past their life time.
Hull speed may not improve with more motor and prop?
With electrics and hydrogen coming on strong the 4 stroke is loosing market share as it is.
I’d question if the fuel spent running a 2 stroke at max is not better replaced with the torque and fuel consumption of a 4 stroke. Then there is the pollution thing....

[winschwab]

Re 3.5 Tohatsu:
So far, so good. I use it as an auxiliary on a 20' racing sloop just to get out to race course and back, so not a lot of hours since the upgrade to "5hp." It does seem to be peppier and so far is not overly hard to start. The only reason I did the upgrade was that the proper carb did not seem to be available, and the motor had been a PIA ever since new. After many carb cleanings without great results, I went for the replacement.

[CarinaPDX]

It looks like @HeywoodJ got the right answer for the OP. Any "upgrade" of engine horsepower requires some research before proceeding: assuming that a 9.9hp and 15hp are essentially the same engine with different carbs is likely to lead to failure. The best way to proceed is to go through the parts catalog and check parts numbers for the two engines and note the differences. My old 15hp 2-stroke Merc appears to be identical to the 9.9hp but it is not. The ports in the cylinder are a different size, something that a performance automotive machine shop might be able to enlarge. Of course the carb is different, but so is the exhaust system (the 15hp has a tube that extends downwards inside the lower unit that is not on the 9.9hp). There might be other changes as well - it has been a long time since I looked at it. For a 4-stroke a change in the cam shaft would accomplish the same thing as the different port dimensions in a 2-stroke. If you are lucky and the upgrade really is just a change in carb and labels then great - but that should not be assumed.

Greg

[Caravela1986]

I had my Tohatsu 9.9 HP 2 stroke upgraded in Panama City Panama local Tohatsu dealer
and have been very satisfied.

It is For Sale in NJ for US $$ 500 Send me personal message if interested