90hp to 115hp conversion for Johnson

[kai1414]

I'm sure this has been asked before but I couldn't find it anywhere so here goes.<br />My father owns a late 80's 90hp Johnson outboard without VRO. We've heard that there is a simple conversion from the 90hp to the 115 hp becuase they are the same block. Has anyone done this or could anyone outline whats involved or put me in contact with someone who is involved. Thanks.

 

[walleyehed]

Re: 90hp to 115hp conversion for Johnson

Simple, yes, if you have the extra money for carbs, exhaust filler plates and the bubble back.<br />25HP gain???...No, actually about 5-7HP gain.<br />It's not worth the time, effort or money unless you have nothing else to do.

 

[kai1414]

Re: 90hp to 115hp conversion for Johnson

Originally posted by walleyehed:<br /> Simple, yes, if you have the extra money for carbs, exhaust filler plates and the bubble back.<br />25HP gain???...No, actually about 5-7HP gain.<br />It's not worth the time, effort or money unless you have nothing else to do.

 

[kai1414]

Re: 90hp to 115hp conversion for Johnson

Alright, well I'm curious then, why are they called a 90hp and a 115hp respectively if giving the 90hp the 115 components anly makes another 5 to 7 hp gain? This is assuming that all other aspects are the same. Do you know of anyone that has Dyno information and anything like that? I'm not really well versed on the components, but could you explain the "exhaust filler plates" and the "bubble back". I was alson under the impression that you had to change the intake manifold, because the port sizes were different. Either that or they had to be machined. Again I haven't had it apart, so I apologize if I'm sounding like a complete idiot. This is basically preliminary research. Thanks.<br /><br />Jeremy

 

[jimmbo]

Re: 90hp to 115hp conversion for Johnson

the 90 has different porting than the 115, the ports are the tranfer holes in the cylinder walls, not the intake manifold. The 90s carbs are smaller. Cylinder heads will be different too. Not all 115s will have the bubble back exhaust housings, only the ones that were rated at the prop, 1985 and later. The bubble back and the exhaust filler plates create a megaphone for the tuned exhaust. It was really the only difference between the old crank rated 115s and 140s. By the way the old 140s could only put out about 125 hp on a really good day. Hence when the engines became prop rated it became the 115.