Mercury tower of power gear ratio

[archbuilder]

I have an 87 115 L6. I was looking through the specifications the other day and notices it has a 2:1 gear ratio in the lower unit. I also noticed that some of the other years came with at 1.78:1 ratio. Does the ratio increase the top in or bottom end? I'm not familiar with the relationship of the two gears and the effect on prop RPM. I was a little surprised there weren't more ratios offered.

 

[75TowerOfPower]

Re: Mercury tower of power gear ratio

The ratio's are just like in a truck, the lower numerically (1.78) will have a higher top speed and the higher (1:2) will have a better hole shot, all this is assuming same boat and same prop.

 

[Chris1956]

Re: Mercury tower of power gear ratio

Tower, You forgot to add that the higher gear ratio will give higher top speed, if the motor has the power for it.

The real reason for the different gear ratios is to allow the motor to reach the max recommended RPM with a standard prop on a standard boat. More powerful motors has the higher gear ratios ie 1.75::1 versus 2::1.

 

[archbuilder]

Re: Mercury tower of power gear ratio

that is what I thought, so a 1.78 will have a higher top end, assuming the motor has the power to push the boat with that gear ratio.

 

[dave4881]

Re: Mercury tower of power gear ratio

that is what I thought, so a 1.78 will have a higher top end, assuming the motor has the power to push the boat with that gear ratio.

I'm certain that the 1.78 gear ratio is standard. The 2.0 gear ratio is considered for "high altitude" use.

 

[Chris1956]

Re: Mercury tower of power gear ratio

Dave, The inline sixes normally have a 2::1 gear ratio, as do the 135-150 HP V6 Mercs.

 

[archbuilder]

Re: Mercury tower of power gear ratio

Thanks for the info. Will the 1.78:1 gear set up fit in a 2:1 housing?

 

[jimmbo]

I have an 87 115 L6. I was looking through the specifications the other day and notices it has a 2:1 gear ratio in the lower unit. I also noticed that some of the other years came with at 1.78:1 ratio. Does the ratio increase the top in or bottom end? I'm not familiar with the relationship of the two gears and the effect on prop RPM. I was a little surprised there weren't more ratios offered.

The 2:1 ratio has the standard for the inline 6s since 1973, the last time a 1.78:1 was used was in the 1972 115hp. As for more speed with the 1.78 vs. the 2:1, highly unlikely, as you will have to drop to a lower pitch to compensate for the higher load on the engine. In theory, the 1.78 gears may fit in some of the gearcases prior to 1979.


Dmacelbird... start your own thread. Your question goes off on a tangent from the OPs