Changing the lower unit on a 125 force from 1.78:1 to 2:1

[Spivy30]

I was reading a post awhile back about parts being interchangeable for force outboards and somebody mentioned putting a a 85-90 hp 2:1 ratio lower unit on a 125-150 hp force. In the post they stated that the boat would run faster, my question is what would I have to do to make this conversion work besides getting a 2:1 lower unit, i.e. lower or high pitch prop etc.

or if its even worth doing, my boat is in really good shape and the my motor is in good shape and runs good with 150lbs of compression in all four cylinders but the lower unit is banged up, since it is in so Igood shape I dont mind putting a little money in it.

Any input would be great

 

[Frank Acampora]

Re: Changing the lower unit on a 125 force from 1.78:1 to 2:1

Yeah, that was me. My experience has been that when you substitute the 2 to 1 lower unit and go up in pitch by 2 on the prop, although the theoretical speed is about the same, the actual boat speed increases.
Depending upon how the boat is running and how it is propped, you might see a speed increase of between 3-5 MPH. Of course, it varies from hull to hull.

For example: With my older Chrysler 90 I had 1.78 gears. with a 17 pitch prop it went 36 MPH and with a 19 pitch it went about the same at a lower RPM. When I finally found 2 to 1 gears, the same engine now drove the boat to 42 MPH with a 19 pitch prop. With my 21 foot cuddy and an 88 Force 125, the Force drove the boat to 32-33 with a 17 pitch prop at less than recommended WOT running RPM. When I changed that to a 2 to 1, it now drove the boat at about 34-35 at a higher RPM with the same prop. Note that in this case I did not change the prop because the engine RPM was already too low.

So: the lower unit bolts right on, and the only other thing you need to do is experiment with props to find the best one. Now, this is for single exhaust lower units. I have not played with dual exhaust units and do not know if results would be the same.

Now, that's not really a big MPH gain, especially considering you would need to invest a couple of hundred for a lower unit. But, if you need to change the lower unit anyway, why not go the extra and look for a 2 to 1 ratio? --a couple of MPH is, after all, a couple of MPH. And if you ski a lot, the lower pitch prop on the 2 to 1 lower unit will give a good pop-out for the skier.

Me? I have a few units lying around from junk engines I have acquired so it is no big thing for me to swap them. Essentially, it costs me nothing. I have a lot of props too, so again, I can play with them without investing big bucks.