Converting Johnson 90 to 115HP

[chris1573]

I have a 1996 Johnson 90 V4 60 degree model number BJ90SLEDR

I have been offered the same age 115HP version of this engine for spares model number J115SLEDR

I have had a look at the BRP parts website and initially it looked as simple as swapping the Carbs and Cylinder Heads (the 115HP is higher compression). Even the exausts appeared to be the same.

However upon looking more closely at the BRP drawings and part numbers it appears that there are 2 part numbers for what they call "CYLINDER & C'CASE ASSY.," which appears from the drawings to be the two halves of the Crankcase, there is one part number for the 90 and one for the 115.

I was under the impression that the Blocks for these two engines were the same, is it just that there are differences to the ports in which case am I wasting my time swapping over the other bits?

Can anyone help me?

Thanks

Chris Stevens

 

[WillyBWright]

Re: Converting Johnson 90 to 115HP

That would be likely.

 

[Dhadley]

Re: Converting Johnson 90 to 115HP

Correct, the port timing is different on the 60* motors.

Max out the set up on your 90 and you'll likely not see any difference between the 90 and the 115. Same bore, stroke, intake, ignition & lower unit including gear ratio.

 

[chris1573]

Re: Converting Johnson 90 to 115HP

Thanks for the info guys Dhadley, when you say max out the set up of my current engine what else could I do?

So far I am running a BRP 19" Aluminium Prop
Have the correct Plugs (Champion)
Turning 5,250 RPM at WOT
Getting about 36 - 37 knots in a 5.25 metre Osprey RIB

Is there much I could do to improve upon this?.

I also have an 18" Quicksilver Silverado SS prop that I have not tried yet would this provide better top end speed?

as I said my engine is a good one and runs well, I have just used a can of engine tuner (although I don't think it really needs it) to make sure there is no carbon build up.

I run standard unleaded fuel (95RON) in the UK although we do have the option for super unleaded (98RON)? would this make much difference?

Chris

 

[Dhadley]

Re: Converting Johnson 90 to 115HP

With the 19 you're getting about 10% slip which isn't bad. 5250 is a bit low for that motor and is lugging it. The ss prop will probably allow you to raise the motor without losing bite. That will improve performance but you may not see the improvement you'd like because it's a smaller pitch. If it were mine I'd try to gain rpm with the set up rather than losing pitch. Sometimes the RIBs can be tricky to get set up right.