Outboard question

[brackish]

I searched this question before without getting any reasonable answer.  Many outboards are built on the same displacement package but have different HP ratings.  This is particularly true for 9.9's and 15's and that is the case with my 2005 Suzuki 9.9 hp four stroke.  While this motor was on the 23 I didn't have any desire to determine what it would take to change it from a 9.9 to a 15, as 9.9 was more than it needed.  But now that I'm putting it on a skiff that can handle up to a 25 but a 15 is considered optimal, I do.  A search gets all kinds of answers but not specific to this motor, and not specific to 4 strokes.  They talk of jet changes, whole carb changes, reed (what is that and does a 4 stroke even have such a thing) changes, possibly valve changes. 

I have a repair manual (not factory) for the motor and going through the manual most of the systems have parts lists where both the 9.9 and 15 are listed side by side.  The only difference that I've found in all these listings is the number for the carb jets.  Could it be that simple?

[Craig]

Possibly. There may be some changes in timing specs, and low speed/idle adjustments but should not be rocket science to up your HP.

[Salty19]

Reeds are two stroke specific.

[capt_nemo]

Understand the same thing is true with my YAMAHA 4 stroke 4HP and the 6HP offered. The difference in max HP output appears to be a result of increased operating RPM of the engine brought about by different carburation and whatever else. Don't know if conversion kits are available or even required.

There sure is a difference in original cost though. My friendly YAMAHA Dealer advised me not to pay more for such a small increase in HP especially if not really required.

capt_nemo

[Shawn]

I doubt jets alone would do it as you would also need to increase the engines ability to breathe to be able to properly burn the additional fuel larger jets would give you. That would potentially mean a larger carb. and/or removing some type of restrictor on the smaller HP engine. Jets alone would likely overly richen the mixture and cause a loss of power.

Shawn

[skip1930]

On my  hp 2 stroke Mercury O/B engine the owner's manual cover's both 4 and 5 hp.
HP= 4 ~  5
kilowatts = 2.98 ~ 3.73
Full rpm  =  4500 to 5500
Idle rpm  =  850 in gear and 1000 in N
cylinders = 1
Piston displacement = 6.2 cu inch [102cc]
Cylinder bore = 2.165 in [55 mm]
Piston stroke = 1.693 in [43cc]
Plug = NGK BPR7HS-10 or Champion RL82YC
Gap = 0.40 [1mm]
Gear case lube = 6.8 fl oz.
Gear ratio 2.15 : 1
Mix = 25 : 1 break in, 50 : 1 afterwards.

The 5 hp offers an optional battery charging set-up.

How Mercury gets two different hp's with this short stroke, over square [bore larger then stroke] engine, high rpm engine. I have no clue since the carb numbers are the same part number.
It's just not as simple as drilling out jets. It could be the development of two different jets.

Jets are NOT a straight line bore, but rather tapered like a venture, I'm guessing in the micro operation of a jet ...  to very the speed of the fuel. Slowly at the entrance to the jet. Through the venture to speed it up. And slowly again at the exit of the jet [all this in a 1/4 of an inch or less] in order to mix with the induction of air and further break up the mixture into smaller 'droplets' on there way to be vaporized prior to induction into the combustion chamber. [Liquids don't burn, vapor burns. That ratio with gasoline is 14.7 :1 that never changes during normal operation] Both the 4hp and 5hp are the same castings, machined and tuned by Yamaha who did the basic research for Mercury in Japan.  

Mercury is always under rated, making more hp then stated.
I think it's just marketing.

skip.