I did some limited research into "upgrading" a 1988 9.9 Evinrude to 15 hp specs. From what I was able to gather was for this model the carburetor was the only different part number between the two engines.
What I was able to determine from reading different material online was it seems to be a difference in the internal bore size between the the units which would allow the "15 hp." carb to draw in a slightly larger amount of air. Whether the jetting was different I had not determined that for certain as I did not have the larger carb to pull down and compare.
Also whether there was a difference in the amount of factory "full throttle adjustment as in relation to the carbs butterfly full open position between the two carbs as factory installed and adjusted again I never did determine that as well.
It did seem that both carbs used the same part number rebuild kit though.
I never found any differences in exhaust, reeds or reed cages and so forth between the motors on the 1988 years model as far as I could determine from comparing part numbers from the two engines.
The original operator manual that came with the motor new is the same manual for the 9.9/15 hp models. All specs in the manual for the motors are the same including spark plugs, capacities and weights between the two motors. Even the manual list the factory supplied prop to be an Aluminum 3 blade 9 1/2" diameter x 10" pitch to be the same as supplied for both engines.
The only difference in specs in the manual are as follows,
Power at propeller shaft 9.9 model 7.35kW at 5500 RPM
Power at propeller shaft 15 model 11.2 kW at 6250 RPM
Full throttle operating range 9.9 model 5000-6000 RPM
Full throttle operating range 15 model 5500-7000 RPM
So it seems the power increase from specs released by the manufacturer is the result of the engine being operated at a higher RPM to gain the additional hp between the two models. So from that information as supplied in the factory owners manual I would be of the opinion if the motor was jetted properly and propped to run at maximum full throttle levels then the main hp gain would be above 5500 rpm for the 15 hp model.
So actually on a case by case basis the amount of increase by the carb upgrade would be determined by how optimum your set up was for running at the listed specs to begin with. A tachometer would be a requirement to evaluate where you were at and what you may be able to gain with any changes and without that you would really be clueless.
Granted that if as indicated by the factory specs information then longevity of the motor at the 9.9 levels should be somewhat better at full throttle levels as the engine is not being stressed as hard at the lower RPM loads generated by the 9.9 maximum limits as long as the motor is not run in too lean of a condition as compared to the 15 hp although the factory would be keeping stress levels at a safe point across the board.
Again when you are dealing with a carbed engine altitude above sea level the motor is operated at will also affect power and jetting and factory settings are usually a compromise to work at an acceptable level under a fairly wide range of conditions and for different circumstances could be improved upon.
So I think the way a person prefers to run their motors is also something to consider before starting to modify an engine for higher hp levels. For myself I do not like to run an engine at its very maximum for extended time periods for engine longevity reasons.
After seeing where and how the power is gained between the two factory levels of performance I have less of an aversion at the factory 9.9 power level to run the engine at the wide open level for an extended time as long as I have the jetting and oil mixture correct as in reality the 9.9 at wide open would be the same stress wise as running the engine in the 15 hp mode at about 75- 80% of maximum throttle level which would probably be about the level I would maintain on a normal basis anyway.
Everyone looks at things differently and has their own opinion and should do what they feel comfortable with.
What I was able to determine from reading different material online was it seems to be a difference in the internal bore size between the the units which would allow the "15 hp." carb to draw in a slightly larger amount of air. Whether the jetting was different I had not determined that for certain as I did not have the larger carb to pull down and compare.
Also whether there was a difference in the amount of factory "full throttle adjustment as in relation to the carbs butterfly full open position between the two carbs as factory installed and adjusted again I never did determine that as well.
It did seem that both carbs used the same part number rebuild kit though.
I never found any differences in exhaust, reeds or reed cages and so forth between the motors on the 1988 years model as far as I could determine from comparing part numbers from the two engines.
The original operator manual that came with the motor new is the same manual for the 9.9/15 hp models. All specs in the manual for the motors are the same including spark plugs, capacities and weights between the two motors. Even the manual list the factory supplied prop to be an Aluminum 3 blade 9 1/2" diameter x 10" pitch to be the same as supplied for both engines.
The only difference in specs in the manual are as follows,
Power at propeller shaft 9.9 model 7.35kW at 5500 RPM
Power at propeller shaft 15 model 11.2 kW at 6250 RPM
Full throttle operating range 9.9 model 5000-6000 RPM
Full throttle operating range 15 model 5500-7000 RPM
So it seems the power increase from specs released by the manufacturer is the result of the engine being operated at a higher RPM to gain the additional hp between the two models. So from that information as supplied in the factory owners manual I would be of the opinion if the motor was jetted properly and propped to run at maximum full throttle levels then the main hp gain would be above 5500 rpm for the 15 hp model.
So actually on a case by case basis the amount of increase by the carb upgrade would be determined by how optimum your set up was for running at the listed specs to begin with. A tachometer would be a requirement to evaluate where you were at and what you may be able to gain with any changes and without that you would really be clueless.
Granted that if as indicated by the factory specs information then longevity of the motor at the 9.9 levels should be somewhat better at full throttle levels as the engine is not being stressed as hard at the lower RPM loads generated by the 9.9 maximum limits as long as the motor is not run in too lean of a condition as compared to the 15 hp although the factory would be keeping stress levels at a safe point across the board.
Again when you are dealing with a carbed engine altitude above sea level the motor is operated at will also affect power and jetting and factory settings are usually a compromise to work at an acceptable level under a fairly wide range of conditions and for different circumstances could be improved upon.
So I think the way a person prefers to run their motors is also something to consider before starting to modify an engine for higher hp levels. For myself I do not like to run an engine at its very maximum for extended time periods for engine longevity reasons.
After seeing where and how the power is gained between the two factory levels of performance I have less of an aversion at the factory 9.9 power level to run the engine at the wide open level for an extended time as long as I have the jetting and oil mixture correct as in reality the 9.9 at wide open would be the same stress wise as running the engine in the 15 hp mode at about 75- 80% of maximum throttle level which would probably be about the level I would maintain on a normal basis anyway.
Everyone looks at things differently and has their own opinion and should do what they feel comfortable with.
