Can you use different gear ratio?

[busatom]

I have a87 454 magnum, and need a new lower unit, but cant find an aftermarket one, as they dont make them in 1.32, as they will blow up-hence the change to the bravo drive in 88. I was told i could put a 1.47 gear ratio lower unit in but would have to up the pitch of my prop. Can this be done? What are the consequences? Thanks,

Tom

 

[TilliamWe]

Re: Can you use different gear ratio?

Yep, you can change the ratio, and you should only need to reprop. The idea behind the 1.32 ratio was to make the one gear larger in relation to the other, with the thought that it would hold up better.
The only problem I forsee (but I am not all seeing!) is that if you are currently using something like a 16 or 15 pitch prop, you are not going to be able to reduce your prop pitch. I don't know of any 14 or 13 pitch props for Alpha drives. If you are currently using a 19 pitch, then you should be okay, as a change to a 17 should be all you'd need.

 

[wca_tim]

Re: Can you use different gear ratio?

Just to add to the coments from our friend on the Illinois river, The other thing that the lower gear (less reduction) does for you is to put less torque on the lower gearcase... at the expense of more on the upper gearcase...

on Props. a good place to start is to plug your numbers into a prop slip calculator for your current set up. take note of your current speed. Now change the gear ratio to the new ratio you're considering. Then change the prop pitch and have it recalculate the speed until you get the same speeed. The pitch you have to enter to get the same speed with the new gear ratio will be about what you're looking at in terms of repropping. Won't necessarily be exact, but will let you know what kind of a change it is going to take.... I get that you'll probably wind up going 2 -3 inches up in pitch...

 

[180shabah]

Re: Can you use different gear ratio?

...I get that you'll probably wind up going 2 -3 inches up in pitch...

Correct. Increase in prop pitch will be needed.

 

[busatom]

Re: Can you use different gear ratio?

well i had a 19 pitch 3 blade prop but rpms were to high 52-5300, need to be at 42-4600, so Im gonna go with a 23 pitch 4 blade, hopefully all will work out. It is a 24' Baja with the 454 but im not killing it and trying to go balls to the wall. I hope it will be ok. I ordered it from SEI. Any more comments are appreciated. Thanks,

Tom

 

[Bt Doctur]

Re: Can you use different gear ratio?

If were talking Alpha type drive (Not GEN II`s)the gear ratio is done in the upper ,lowers are all the same

 

[wca_tim]

Re: Can you use different gear ratio?

I'm assuming that you ordered the whole outdrive including both upper and lower.

and the prop you're talking about sounds like a really good place to start...