lower unit swap from a 85hp to a 125hp

[PatrickGuevara71]

so I have a force 125hp from 1984 but the lower unit is bad and want to replace it, but i could only find the lower unit for the 85hp. will the 85hp lower unit work for the 125hp

I found a post on this forum about swaping the lower unit from a 125hp to a 85hp and it said that it would decrease the performance. so if I swap the lower unit from a 85hp to 125hp will it increase the torque?

 

[pnwboat]

The factory gear ratio on the 1984 125HP is 1.73. That is for every 1.73 revolutions of the motor, the propeller will turn 1 revolution. The factory gear ratio on the 85HP is 2.0. That is for every 2 revolutions of the motor, the propeller will turn 1 revolution. Generally, this will increase the torque, however, the motor wil be turning at a higher RPM for a given speed. You can compensate for this somewhat by going to a higher pitch prop. All of the later model 120 HP motors have a gear ratio that is 1.93 or pretty close to 2.0. My personal opinion is that you will see an increase in performance by putting the 85 HP lower unit on your 125HP motor. You can verify the gear ratio by marking the propeller and turning the drive shaft. See how many turns of the drive shaft it takes to turn the prop 1 turn. If it takes about 1 3/4 turns of the driveshaft to make the prop turn 1 revolution, then you have a 1.73 ratio lower unit. If it takes 2 turns of the drive shaft to make the prop turn 1 revolution, you have a 2.0 ratio lower unit.

 

[jerryjerry05]

I believe after 91 the gear ratio was the same for ALL of the bigger Forces.
That includes the 150/120/90

 

[The Force power]

I believe after 91 the gear ratio was the same for ALL of the bigger Forces.
That includes the 150/120/90

Is that because they became Mercs. gear-assemblies?

 

[jerryjerry05]

No they went to Mercury in 94
and yes some early 90's used Mercury gears but
they were used in the Force setup/style.