Gear Ratio Question / Swaping Lower Units

[Sponge Bob]

Hi everyone. I'm not exactly new here, but I don't come out of my hole very often. Mainly I lurk, read, and absorb all I can from those who are more knowledgeable than I. <br /><br />Last year my lower unit went out on me at the end of the season. I know I should be shot, but I’m just getting around to replacing/repairing it. I have a 1980 Citation with an IL6, 165hp. The upper unit has a decal on it that says 165 also. From everything that I’ve gathered the gear ratio is 1.65:1. I’ve come across a lower unit from a 1971 120hp in good condition that I’ve been tempted to buy. My questions are…<br /><br />1. What kind of performance loss can I expect? <br />2. Is it even feasible to use this lower?<br />3. Would I have to change props?<br />4. Are the gear ratios too different? (I’ve heard that the upper unit determines the ratio, but I’m not sure I believe it, considering my source.)<br />5. What are the pros and cons of going this route? (other than the fact that it’s a used unit)<br /><br />Sorry for asking so many questions. I couldn’t think of a better place to ask where I would get an honest answer. Thanks.<br /><br />SB

 

[jlshields]

Re: Gear Ratio Question / Swaping Lower Units

You heard right. The top end determines the gear ratio. It can be determined by putting it in gear, turning the input staft and noting how many turns it takes to turn the propshaft once.

 

[Bt Doctur]

Re: Gear Ratio Question / Swaping Lower Units

the lowers are basically the same,with the exception of a very old Series 1 drive and the newer GenII drives

 

[Sponge Bob]

Re: Gear Ratio Question / Swaping Lower Units

Thanks for the confirmation. I guess my source isn't completely full of it. Thank you.<br /><br />So, a lower unit is a lower unit is a lower unit. Right? A lower from a 120hp is the same as a lower from a 165hp except for the prop. (Given that they are the same model) Is that correct?

 

[Sponge Bob]

Re: Gear Ratio Question / Swaping Lower Units

Thanks, BT. You were faster on the button than I was.