Re: Difference between a 9.9hp and 15hp Johnson?
The search function on this site isn't very good. It seems you need more than three characters per word, so just entering "9.9 15" will return zilch. Entering "Johnson 15" will return everything with "Johnson" but the "15" will be ignored. However, entering "15hp" will return results, but any posts that might have contained "15 hp" (space between) will not show up.
However, I'll give you some info on the differences:
Standard propellers are the same for both 9.9 and 15 thrughout the 1974-92 lifecycle of these engines, 9.5"x10"
1974-78: No difference except for the carburetor. Both motors share a square exhaust tube that seals with the gearcase, creating a "wall of water" from spilloff accumulation in the midsection that helps silence the motor. The water overflow exits through the top exhaust relief ports. Exhaust gas exits through the prop.
Note: One of the 1977 OMC brochures (Johnson or Evinrude) claimed an "increase in top speed" for the 15 starting that year, but I don't know of any additional difference from the 9.9 that accomplished this. The claim was definitely made before the spacers were added in 1979 (see below). My best guess is the carburetor bore was made even larger on the 15 starting in 1977, but perhaps someone else could confirm this.
1979+: spacers added at intake reeds on the 15 to improve throttle response.
1981-84: 15 now has tuned exhaust (shorter, rounder tube) to increase torque. This results in more mixing of the exhaust and exiting cooling water. Little (if any) water exits through the top of the midsection's exhaust relief port. Some believe this made the 15 louder. There were also changes to compression ratio & timing (according to 1981 OMC brochures). 15's full-throttle operating range increases by 500 rpm over pre-1981 model. All these changes apparently gave a true 15hp measurement at the prop instead of the flywheel (OMC may have been prompted to do this due to increased market competition, and possibly because some felt the 15 wasn't a huge enough power leap from the 9.9).
1985: 15 has exhaust tube reshaped again, squarish but still short.
1990: 9.9 now shares same short, square exhaust tube with 15.
In various years there were changes that affected both motors at the same time, such as a powerhead redesign that placed the thermostat at the top, and a push-pull throttle cable instead of the metal twist linkage.
