Upgrade to a New Johnson 15 hp

I'm considering upgrading to a 2005 15 hp johnson outboard but the rpm operating range and prop diameter concerns me. I have an 8' 1985 AquaLark and it runs well (24mph) with the 1974 15 hp Johnson (which I converted to CD ignition) I currently have. My engine runs 6000-6500 rpm. A friend has a 1996 Johnson on his AquaLark and it runs very well but I wasn't able to locate any information on the rpm operating range for that year to compare it to a new 2005 model. Also, the prop diameter seems to be smaller on the 2000 and newer models. I don't really want to spend $2500 on the new engine to have it not be able to plane out or get a very high top speed. Can anyone offer any advice?

Re: Upgrade to a New Johnson 15 hp

Is this a two-stroke or four-stroke engine? If its a two-stroke, the lower unit gear ratio is 2.42:1 which would very likely have a larger prop (more pitch) than the four-stroke which has a 1.92:1 ratio. The four stroke RPM range is 5400-6000 RPM while the two stroke is 5500 - 6500 rpm. Prop diameter has far less effect on performance than does pitch. Slightly smaller diameter props with less pitch are typically used on engines with higher ratios than engines with lower ratios. Without knowing what prop pitch you have now and what the replacement engine has, its hard to make a comparison.

Re: Upgrade to a New Johnson 15 hp

The new Johnson 15hp, assuming 2-stroke, is the evolution of your 1974 15hp which was it's first year. It used points and condenser ignition system and had recalls on the carbs and cylinderheads back then. They were also known for fouling plugs.<br /><br />The 2005 15hp has more cubic inches for more torque, a reliable electronic ignition with no plug fouling, and best of all it puts out more HP and performance than the older motors. <br /><br />Your 15hp was rated at the crankshaft and the new 15 delivers that hp at the prop. The operating range is 5000-6000 rpm.<br /><br />You will definitely notice an increase in performance and have a quieter and smoother running engine also.

Re: Upgrade to a New Johnson 15 hp

Unfortunately, I had much experience with that plug fouling this summer before I converted it to a 1984 CD ignition system about 2 weeks ago. I am referencing the 2-stroke models. I am currently running a 9-1/4 X 9 stainless prop. The gear ratio is the same on my 1974 as the 2005 so, assuming 500 less rpms, that translates into 206 less prop rpm. So, I would just need to go to a steeper pitch, right? These boats are very difficult to get up on plane with such a small engine - should I worry about a steeper pitch prop making it worse? Does anyone know what the operating rpm range was for the 1996 model? There are approximately 20-30 of these Aqualarks on my lake and the newest engine that I have seen so far on them is the 1996. Thank you very much for your help!