Same size motor but different size HP

[techwriter]

Hi everyone,

I have searched the forum and have not found the answer to this. Something that I would like to know is exactly what are they doing or setting for different levels of horsepower on the same engine. The OMC built 9.9 and 15 hp are the same engine. On various brands the 50, 60 and 70 hp may be listed in the specs as having same displacement and that would mean the same engine most of not all of the time.

I have a 1993 25 hp Johnson 2 cyl thru hub exhaust on an old boat that I just fixed up. The owners manual with this engine lists this same engine as being available as a 20, 25, 30 or 35 hp engine. It lists the 20's as using a 10 X 13 prop and the 30's using a 10 X 15. It also lists the full throttle operating range as being 4500 - 5500 rpm on the 25's and 5200 - 5800 rpm on the 30 & 35 hp.

My guess would be different jets or carburetor to dump in more fuel and air to obtain the higher rpm with the greater load of a steeper prop. Can someone tell me exactly what the difference is.

I will mention that I am not trying to over power my boat. The boat that this 25 is on is rated at 35 hp anyway. It is just that I would like to know.

Thanks for any replies.

 

[glengold]

Re: Same size motor but different size HP

boats.net will list interchangeable parts. Spend some time studying the differences between motors and your questions will be answered.

 

[boobie]

Re: Same size motor but different size HP

The different ratings are affected by carbs, intake manifolds, exhaust tuning, rpm specs and sometimes block porting.

 

[emoney]

Re: Same size motor but different size HP

What Boobie said. Plus, it's usually just as easy to sell your 25 and then buy a 35 as it is to spend all the money moving yours "up" the ladder. At least in these parts, with the availability of outboards it is.

 

[techwriter]

Re: Same size motor but different size HP

Thanks everyone for all of the good information. I researched this some by comparing various part numbers of the two sizes. It looks like it would take a different carburetor and probably a different intake manifold. The cost would be a little less than $200 with used parts plus a prop or just under $400 for new parts plus the prop. boats.net lists the new prop at $88.40 in aluminum.

It is nice to know these things if I someday had to buy a new carburetor anyway. Otherwise I do not need the additional 5 hp and about 5 mph anyway. If I really want to go faster I can just use the bigger boat with 110 hp and buy more gas.

 

[watergecko]

Re: Same size motor but different size HP

On the newer motors, like the etecs, alot of it is how the computer is programmed. For example the only difference between a 55 and 60 hp model is the program, no diff parts, just software...and $500.