Just an FYI.....I've experimented over the years with different lower units on my 1988 Force 125HP motor. Switching from the earlier 1.73 :1 ratio 4 cylinder lower to a 2:1 ratio 3 cylinder 85HP lower really woke the motor up. Had to switch props though. Had to go to a higher pitch prop.
This weekend I switched to a later 1.93:1 ratio later model 2 piece drive shaft lower and it seems that there is added performance over the older 2:1 ratio unit. The earlier units are single exhaust. That is....all the exhaust gases exit through the snout on the trim tab. The later model referred to as a "dual exhaust" has an additional exhaust passage that directs most of the exhaust through the center hub of the propeller as well as the passage that goes out the snout on the trim tab. I suspect that this may result in lower exhaust restriction? I don't know...not an expert here. The 1.93:1 ratio unit has the water pickup vents on the side of the lower unit like the Mercury units are set up. The older single exhaust lower unit has the water pickup vents on the torpedo shaped section of the lower unit. Any ways, my RPM's dropped by a hundred and fifty or so and picked up about 3 or 4 MPH at WOT. That was with a full tank of gas and two people. Before I would have to have no more than 1/2 a tank of gas and 1 person to get anywhere close to what I was seeing this weekend. I also noticed that my water pressure at lower RPM's was higher. With all of the various single exhaust lower units, I never saw more than about 5 PSI at idle. With the dual exhaust I'm seeing about 10 to 12 PSI at idle. Slightly higher water pressure at WOT too. Single exhaust unit I saw about 18 PSI. With the dual exhaust unit, I'm seeing about 23 PSI.
I did run into some issues when installing the lower unit though. The water pump on the dual exhaust unit has an additional bolt right behind the water pick up. There is a casting inside the drive shaft housing that goes from one side to the other. This thin casting hits that bolt. Just had to take a hacksaw and trim away enough of the casting to clear that bolt.
I don't know if my old 2:1 ratio lower unit has a problem with it, but the later model 2 piece dual exhaust unit seems to perform just a tad better so far.
This weekend I switched to a later 1.93:1 ratio later model 2 piece drive shaft lower and it seems that there is added performance over the older 2:1 ratio unit. The earlier units are single exhaust. That is....all the exhaust gases exit through the snout on the trim tab. The later model referred to as a "dual exhaust" has an additional exhaust passage that directs most of the exhaust through the center hub of the propeller as well as the passage that goes out the snout on the trim tab. I suspect that this may result in lower exhaust restriction? I don't know...not an expert here. The 1.93:1 ratio unit has the water pickup vents on the side of the lower unit like the Mercury units are set up. The older single exhaust lower unit has the water pickup vents on the torpedo shaped section of the lower unit. Any ways, my RPM's dropped by a hundred and fifty or so and picked up about 3 or 4 MPH at WOT. That was with a full tank of gas and two people. Before I would have to have no more than 1/2 a tank of gas and 1 person to get anywhere close to what I was seeing this weekend. I also noticed that my water pressure at lower RPM's was higher. With all of the various single exhaust lower units, I never saw more than about 5 PSI at idle. With the dual exhaust I'm seeing about 10 to 12 PSI at idle. Slightly higher water pressure at WOT too. Single exhaust unit I saw about 18 PSI. With the dual exhaust unit, I'm seeing about 23 PSI.
I did run into some issues when installing the lower unit though. The water pump on the dual exhaust unit has an additional bolt right behind the water pick up. There is a casting inside the drive shaft housing that goes from one side to the other. This thin casting hits that bolt. Just had to take a hacksaw and trim away enough of the casting to clear that bolt.
I don't know if my old 2:1 ratio lower unit has a problem with it, but the later model 2 piece dual exhaust unit seems to perform just a tad better so far.
