4.3L mercruiser - lower unit ratio change- having issues …. please help

[CRicer1982]

hi guys, we did a big boat overhaul while it was in storage last winter and we had some issues with bearings inside of the lower unit so I decided to just buy a whole lower unit from Mercury and when I called in they did not have the 1.81 ratio in stock so we decided to go with a 1.64( alpha one gen 2)
Didn’t think it was much of a change. It wouldn’t be much issue to give us a little more top and speed when we make the big trips to Kingston, etc..
2010 tahoe q5 ( 19 ft bowrider)
Well, I am totally unhappy with the change and can’t seem to get the boat working as good as it used to
With the old drive, we had a four blade 23 pitch and it came out of the hole well and topped out at about 40 mile an hour
also used a 25 pitch for sometime still came out of the hole well with a little more top and speed up to 45miles an hour

Now I will say that the engine in his boat is not old. It is brand new in 2022.
Bought it as a crate motor from performance engines in Kingston and have had no problems with it, but it does seem to be a little extra snappy ( has 40 hrs now )carb tks

So after putting the original 4 Blade on and going for a water test I was thoroughly disappointed. I didn’t necessarily come out of the whole terrible, but it didn’t have any power to pull the front end of the boat out of the water when trimming
like not even an inch or two
so we’re ploughing around super disappointed.

So after doing much reading, I decide to order in a 14 1/4 -17 pitch four blade figuring out if I went down this far at least it would get me what I needed for power to be able to trim the boat up
Once again, disappointed as I’m spinning the prop on launch and then topping out speed at about 30 mile an hour

so after that disappointment, I pulled out a 21 page 3 blade that had been in the boat as a spare, and it actually works better than any of the other two but still a little bit of spin on launch but once up on plane, everything seemed very normal to trim the boat over the water and top speed of about 36 miles an hour

Just didn’t know if anybody had done this before, and if there was a perfect prop
that would make all these stupid issues go away

We have now ordered parts for the old drive and my Marine mechanic is now rebuilding that one as I think I’m just going to switch back…
What an expensive mistake I hope somebody can shed a little light on this issue and maybe warrant others not to do the same thing

Thanks for your time and any help is appreciated

Chris

 

[kghost]

One- gear ratio of an outdrive (1.5:1, 1.65:1, 1:84:1, 1.94:1 are over all outdrive ratios. The lower alone is only a part of that ratio. 90%+ lowers have the same ratio, some have different like 2:1 so you need to know the pinion and forward gear tooth count.

The upper generally determines the gear ratio except when elevation or oversized Underpowered boats are in the mix then they use a different lower gear ratio.

 

[CRicer1982]

upper gear was a 1.84
changed to a 1.64

 

[Droid]

Later gen2 drives had 2 different lower gears that work in conjunction with the upper to give final ratio.
I don't remember the exact specifications....
You dropped 3 prop sizes. I'd have guessed 1, maybe 2.

 

[kghost]

In the morning I will look at a manual and see if I can clarify.
.i do believe the 1.84/1:1 is a final ratio.
Lower and upper combined..

So if you Only purchased a lower and specified a ratio then that may be an issue. You should have bought one of thousands for sale on ebay and FB narket place...

 

[Droid]

The Alpha gen1 & earlier only had one lower set. The lower ratio was 1.65:1
So on a final ratio 1.65 drive only (and the final ratio is what MerCruiser lists on its brochures & manuals) the upper drive & driven gears have equal # of teeth (24 & 24). So this is why on a 24x24 gear set it needs to be indexed & reassembled the same. All others do not matter because the teeth all mesh together differently with each revolution.

Now on optional high altitude ratio drives I'm not certain. Maybe there is an optional lower set, IDK.

Again, on the Alpha gen2 there are 2 different lower ratios. I believe 1 was stock on 3 litre only. This twerking of the lower gears is why we have 1.47R instead of 1.5R, 1.81R vs 1.84R, 1.64 vs 1.65, ect compared with the 1st 30+ years.