Correcting potential bad gear ratio

[bp1221]

Ok first of all thank you to everyone who's posed answers in this forum it's absolutely the best resource I've found and have top to bottomed many threads for various problems. Here's my predicament:

I have an 89 sea ray Seville cuddy cabin 21' with a 4.3L Chevy engine. I shredded the input shaft gear (pretty sure that's it, basically the one coming in from the engine drive shaft that makes the right angle turn in the upper unit)

No idea what gear ratio it was, no possibility to count teeth, can't find documentation, and can't seem to find any stamped anything on it or anywhere handy (maybe I didn't look in the right place? )

Anyhow a complete upper and lower unit fell into my lap (SE106) with the 1.65 gear ratio. I did a bunch of reading and took an estimate that mine might have been that one, lucky days.

Anyways fast forward to today, back on the water, RPMs topping out early (3100ish). Noticably lower top speed 45km/h compared to 65ish, gets on plane but slowly.

Just saw on another post on here that it's likely im meant to have 1.85 gear ratio not 1.65 which given the feel of it today seems reasonable.

My question is would decreasing the pitch of the prop to 17 make a difference (from 19) as that's about the same ratio of the error in gear ratio?

Obviously getting the correct upper unit would be ideal, and might be the answer but wondering if this is worth a try first with an aluminum? Not sure what else I may have damaged on the old leg during the failure so would like to keep the new unit if possible

-B

 

[airshot]

One inch of pitch equals about a 200 rpm change, one inch on diameter equals about 100 rpm change. These are general calculations that folks use to tune their props to their engine. So...based on your 3100 rpm with a 19" pitch......you would have to really reduce your pitch a bunch....better off getting the correct gear ratio !!

 

[bp1221]

One inch of pitch equals about a 200 rpm change, one inch on diameter equals about 100 rpm change. These are general calculations that folks use to tune their props to their engine. So...based on your 3100 rpm with a 19" pitch......you would have to really reduce your pitch a bunch....better off getting the correct gear ratio !!

Hmm yeah I was looking at a 16 as a test. Am I understanding the problem tho?

By over gearing I've bottomed out the engine before it really gets to its max power range. I've traded low end power for top end speed (and it feels like that too, very snappy at 1200) but it's just too heavy a pull for the engine before it reaches max? I'm not sure how much of a job replacing the upper shaft would be tho trying to avoid it if poss

 

[Chris1956]

When your boat was made, the Alpha I Gen I gear ratios were pretty standard for each engine. I had an '88 19' SeaRay Sorrento with a 4.3. I ran a 21P prop, on a lighter, smaller boat than yours. So your 19Pprop likely gave you the proper performance.

Provided your tach is accurate, your engine is lugging. The rule of thumb is 400RPM loss or gain for every 2" in pitch. change up or down. I would think you will get more as your engine gets into it's higher power band.

Probably someone on this forum knows what the std gear ratio for 4.3 motors was in '89. If your original gear ratio was 1.85 and the new one is 1.65, mathematically a 2 inch drop in pitch should get you back to proper max RPM, however, the low PMs you are seeing makes me doubt it.

Usually prop diameters are paired with pitch, so you cannot control both dimensions. So a 17" pitch prop should get you close to 3500 RPM, a 15" pitch 3900RPM. Both should get some more PM because the engine is making more power.

Modular aluminum props are pretty cheap, and they can share hubs, saving some $. I would expect new gears or components with different gear ratios not so.

See if you can borrow a 15" pitch prop and try it. I think you should aim for close to 4400RPM at max throttle.

 

[Scott Danforth]

your original ratio was most likely 1.81:1
with your new radio of 1.65:1, you may not find a prop small enough

additionally, is your motor healthy? compression between 140 and 150 psi ?

 

[Scott06]

Ok first of all thank you to everyone who's posed answers in this forum it's absolutely the best resource I've found and have top to bottomed many threads for various problems. Here's my predicament:

I have an 89 sea ray Seville cuddy cabin 21' with a 4.3L Chevy engine. I shredded the input shaft gear (pretty sure that's it, basically the one coming in from the engine drive shaft that makes the right angle turn in the upper unit)

No idea what gear ratio it was, no possibility to count teeth, can't find documentation, and can't seem to find any stamped anything on it or anywhere handy (maybe I didn't look in the right place? )

Anyhow a complete upper and lower unit fell into my lap (SE106) with the 1.65 gear ratio. I did a bunch of reading and took an estimate that mine might have been that one, lucky days.

Anyways fast forward to today, back on the water, RPMs topping out early (3100ish). Noticably lower top speed 45km/h compared to 65ish, gets on plane but slowly.

Just saw on another post on here that it's likely im meant to have 1.85 gear ratio not 1.65 which given the feel of it today seems reasonable.

My question is would decreasing the pitch of the prop to 17 make a difference (from 19) as that's about the same ratio of the error in gear ratio?

Obviously getting the correct upper unit would be ideal, and might be the answer but wondering if this is worth a try first with an aluminum? Not sure what else I may have damaged on the old leg during the failure so would like to keep the new unit if possible

-B

yes drop 2" of pitch should put you at the same place. I did a repower where I went the other way- swapped a 4.3 for a 5.0. Kept the 1.81 V6 drive ratio vs getting a 1.62 drive and went from 19" to 21" pitch. Works great

 

[bp1221]

am changing the plugs tomorrow will check the compression while im there. Thanks to everyone for the answers, i'll post an update with the new prop or if anything else presents itself to be of use to anyone else.