Size of Mercury 20 HP 4 Stroke vs. 9.9

I heard the newer Mercury 15/20 HP motors are the same size as the older 9.9 in terms of exterior case. Anyone know if that is true. If so, how far back i.e. is a 2013 the same size as the 2015, etc? Reason I ask, looking to "re-sticker" the case with a 9.9 kit to push a heavier hull.

I can tell you that the 8 and 9.9 are 208cc and the 15 and 20 are 351cc. So I doubt the case could be the same size.

The 15/20 Mercury/Tohatsu/Nissan (all the same motor with the exception of the paint and decals) are considerably larger than the old 9.9's.

I must have misunderstood what I was hearing. The lake is restricted to 9.9 or less and I know for a fact most are running 15 hp outboards with decals. Maybe what I heard was the old 9.9 was the same size as the 15, but is no longer the case. The new 15hp is now bigger than the 9.9 hp, but same as the 20hp, so if you are going to cheat, then cheat with the 20hp. Does that sound more reasonable?

When you say "older" 9.9, do you mean a two stroke 9.9?

Not sure about size but newer ones are a lot heavier. A 15 hp is about 120# dry.


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Maybe this is what you have heard? The 9.9 and the 15 are the exact same motor, just a different carburetor- or possibly just the carburetor jet? So all you have to do to the 9.9 is change either the carb or the main jet and instant 15hp. I'm not sure if this applies to only certain model years or if it is current, but that might be what you are looking at.

Does anyone have a manual for a 20 hp Mercury?

bdrechsel said:

Does anyone have a manual for a 20 hp Mercury?

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They've been making 20 horse Merc's for a lot of years and they have made substantial changes over the last 50 or so years. You might want to be a little more specific and mention the year model. :lol:

jethro said:

Maybe this is what you have heard? The 9.9 and the 15 are the exact same motor, just a different carburetor- or possibly just the carburetor jet? So all you have to do to the 9.9 is change either the carb or the main jet and instant 15hp. I'm not sure if this applies to only certain model years or if it is current, but that might be what you are looking at.

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So if the 8 and the 9.9 Merc are the same, and this applies to the 9.9, I wonder if my '06 8hp can be converted...

jethro said:

Maybe this is what you have heard? The 9.9 and the 15 are the exact same motor, just a different carburetor- or possibly just the carburetor jet? So all you have to do to the 9.9 is change either the carb or the main jet and instant 15hp. I'm not sure if this applies to only certain model years or if it is current, but that might be what you are looking at.

Click to expand...

I had a 2003 15hp 4 stroke Merc

yes the carb is different but so is the exhaust housing (the inside of it not the outside)

so if youi wanted to make a 15 out of a 9.9 you would have to change the exhaust as well as carb work. the 9.9 is a detuned 15