F-Ribs And Sibs - Inflatable Boat Community

Hi this is only my second post on these forums so if I'm in the wrong place or this discussion already exists elsewhere feel free to let me know!

I've bought a 9.9hp 4 stroke Mercury Bigfoot outboard (2005 I think) which I run on a Honwave38. My outboard knowledge is zero, but from a bit of googling I think I understand that the 9.9hp engine is basically the same as the 15hp. One site I found said that a carburetor swap is all that is required.

Has anyone here ever done this upgrade or know how much work is involved??

Bit of an old post, but still deserves an answer...

It all depends on which exact engine you have. Generally speaking, manufacturers make one design of engine that'll cover several different models - the difference between them is mainly the fuel system, but other considerations would be the exhaust (might need to be bigger), and the propeller (no good having extra power if we can't transmit it to the water, so a coarser pitch might be needed).
So, a manufacturer's 4, 5 and 6hp engines do tend to have the same engine internals (crankcases, pistons, valves etc), but the difference will be the carburettor sizes, as that will control how much fuel and air get into the engine, and therefore how much power it will make.
The next group of engines tends to be the 8, 9.9 and 10hp models - they'll all have the same engine, shaft etc. Propellers might be different - the smaller engines will have finer pitched propellers than the big ones.
The next group up after that is usually 15, 20 and maybe 25hp - with modern engines these ones will have electronic fuel injection rather than a carburettor, but the principle is the same.

So, to answer your question as to what upgrade you can do, you need to know the exact engine you have and what others were in that group. If you have a 4hp, it might be possible to upgrade to a 6hp. If you have a 9.9, and the largest in the group is a 9.9, there's nowhere to go with this as you'd need a physically bigger engine block (more cc) to create more power.

Interestingly, a lot of people buy an engine without realising that they really should be going for the biggest one in that group of engines, as you're getting more power without any additional weight. Take the current Honda range, for example. The 8 and 10hp models both weigh 42kg (they're both 222cc, with different carbs) - to me, it doesn't make sense to buy the 'smaller' engine as you've still got to lug 42kg around. Might as well have the extra power. Same with the next ones up the range - 15hp and 20hp both weigh 46.5kg (short shaft, manual start), because they're the same 350cc engine, just with different fuel systems. Oh, and a different price tag too.... there's almost no justification for the 20hp being more expensive, except that more people want them.
And having said all that, a Suzuki 20hp weighs 42kg - same as a 10hp Honda.

Anyway, short version is that I can't be certain, but I think your engine is in the 8/9.9 group (have a look at the owners manual - it will cover all engines in your group!), so you've probably already got the biggest carb you can have on it. It is true to say that for some manufacturers the 9.9 is in the same group as the 15hp - those are the ones that can be upgraded, but it's still a bit of work and expense. The folks that usually do these upgrades are the ones who run on lakes that have a horsepower restriction (usually 10hp). Just to add to the confusion in this particular case, old Mercury 9.9 2 stroke engines were in the same group as the 15hp - so that's a conversion you can do on the older engines.

Plus, the bit I forgot to include above....

For most folks, when they're looking for more power they are actually looking for more torque. If we upgrade a 9.9 to 15hp, the extra 5.1hp all comes in at the top of the rev range - the torque curve is identical until you get to where the 9.9 tops out. So, the main effect of an 'upgrade' like this is to make the engine rev a bit more, and consume more fuel (about 50% more fuel...). In terms of top speed, even if you got the carburation, exhaust, and prop spot on, you'd probably only see a couple more knots out of it. If you want to go faster, get a 20hp... I think your current motor weighs in around 38kg, so for a 10% increase in weight you could have a Suzuki DF20A - which is fuel injected and therefore quite a bit more efficient (for a 20hp).