Mercury I6 90 & 115 HP powerhead differences

[gregpro50]

What is the difference between an I6 powerhead that is a 90 and one that is a 115?

I had a powerhead go bad on an '86 115 and I swapped over a good powerhead from an '81 motor but it feels like it is a 90 HP powerhead since it doenst have the power of the old one although it is running great.

 

[jimg984]

Re: Mercury I6 90 & 115 HP powerhead differences

82 and newer the horsepower was rated at the prop shaft, so 115 hp =115hp
before 82 hp was rated at power shaft so this would pre 82 115hp at powerhead shaft = 90 hp on prop shaft, so very well could feel 90hp

 

[Faztbullet]

Re: Mercury I6 90 & 115 HP powerhead differences

The internal differences are:
1- 115 power ported piston...................90 no ports
2- 115 power ported block....................90 no port
3- 115 wider exhaust ports ............... ..90 small square ports
4- 115 bigger main jet in carb
5- 115 usually had different gear ratio than 90 (1.87 vs 2.1)

 

[gregpro50]

Re: Mercury I6 90 & 115 HP powerhead differences

OK so it is some difference in the block and rotating assembly. The book I have shows just about everything the same (bore, stroke, etc) except for a minor variation in carb jetting. I used all the electricals and carbs from the '86 powerhead and put them on the '81. I think you guys are right and the motor really is a 115 just rated different. I have it tuned out the best I can and timed right and it runs really good but just doesnt have the power of the old powerhead. Unfortunately the old powerhead isnt rebuildable either.

So if I were to find an '82 or newer powerhead it would be the same as the '86 that self-destructed?

 

[gregpro50]

Re: Mercury I6 90 & 115 HP powerhead differences

bump