1:50 to 1:84 - Any problems?

alwhite00

alwhite00

Active Member
May 31, 2010
1,809
Michigan
Boat Info
1997 250 Sundancer
Engines
5.7 EFI/ BIII
Hello:

I have a 86 Sea Ray Seville 21MC - Great boat - Love it. The boat aparently had a 4.3 originally - The PO bought it with a cracked block. He trans planted a merc 5.7 / 260 in place of the 4.3 - Boat runs great and tops out at just under 40 mph on the GPS.
It has a Alpha one first gen outdrive that I believe was original to the boat. I was looking at the outdrive and it has stamped into it 1:84 - I am asumeing that this is the gear ratio. I believe the 5.7 / 260 engines had a 1:50 gear ratio. I'm running a 14.25 / 21 3 blade aluminum prop.
Will this cause any problems? I am familar with car gear ratios but this boat thing is all greek to me. RPM's top out right about 4,400 - Everything seems to work corectly.
Can the GR be changed? Should it be changed?

thanks

LK
 
Al, check out sea ray's website for what size prop would have come in yoru boat as if it was ordered w/ the 5.7l. My guess is you will need to drop down to a 17 pitch if you re-gear. Your higher pitch prop is counteracting the taller gears right now.

Can it be changed? Yes.
Should it be changed? I'm currious to hear what others say. But I believe things are designed a certain way for a reason.

You will only need to change the gears in the upper housing - or buy a new upper housing.


From iBoats: "The ratio is an issue because you are jumping up in HP and the prop will be different. You cannot effectively reprop to compensate for that much of a change in HP without blowing something up. While they both spin at 4500 RPM's The torque is completely different, the drive for the 4.3 is set up to maximize the available HP and Torque of the 4.3L not the V8. You will over rev the V8 and stuff will break. You will want to regear. Not a big deal but it is the proper way to do the conversion." http://forums.iboats.com/showthread.php?t=255503
 
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thanks for the info - I don't think my boat was available with the merc 260 - only the 4.3 & the 470 4 cyl. (I think that is what it is) I have a lot to learn on this boating stuff and just stumbled across the GR differences poking around and figured it wouldn't hurt to ask.
If I just swap the gears is there a "kit" to do it? Do they have to be set up like a ring & pinion in a car or do they just bolt in?

Thx.


LK
 
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I had an old OMC once that the upper gearcase exploded. I couldn't find the right set but found a similar one, can't remember the ratios. I wanted the boat back in the water asap so I just did it as a temporary fix. I adjusted the prop to get the right WOT rpm's and then drove that boat for many years without changing back to the spec. I think if you're getting proper rpm's and 40mph in a 1986 21' boat, you're doing just fine.
Just my 2 cents...
 
Alibuma said:
Here you go. http://www.sterndrive.cc/mm5/merchant.mvc?Screen=CTGY&Category_Code=MercruiserAlpha

I would not attempt the gears unless you have the tools and knowledge of things like adjusting backlash. Just a minute mistake will be disastrous.
Click to expand...


By looking at the gears I do not see anything that would be adjustable. Or is there different shims that adjust the settings on the gears, The look like pretty simple gears (90 degrees) Just curious.
I am happy with the performance, The boat with trailer is around 5,500 lbs so it's no light weight by any means. I am more or less curious what I could expect with a GR & prop change or if I would be just swapping for the sake of being "correct" If my final ratios work out to be the same (prop & GR) would I gain anything by swapping gears?
As I mentioned, The WOT RPM's are right around 4,400 so it seems to be pretty decent as far as that goes. It will cruise on plane at around 3,000 and around 22-24mph.

I appreceiate all of the responses so far, Very good info.

LK
 
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Yes, I don't know what advantage you will get, like you said other than being correct. The whole torque curve might be an advantage if it will get you out of the hole faster. Your current WOT and Cruise RPMs sound about right - same as my boat with the 1.5 and a 15.25x15 prop.
 
I'll have to time how long it takes to get on plane next time I am out & see if it's decent compared to others. Like I said, I do not mind changing it if it will make it a better boat, I just don't want to go thru the work & $$$ if I end up the same performance.

thanks.

LK
 
I personally don't think you need to mess with it. I doubt there is any significant difference in strength between the 1.84 gearset and the 1.50 gearset. The 1.50 gearset may be just barely stronger due to gear thickness and tooth contact area.

I don't think your older 260hp 350 is overstressing the 1.84 drive. Also, 4400 rpm is right in the middle of the recommended WOT range of 4200-4600 on the older 260.

There's a remote chance a 1.50 drive and a 17 or 19 pitch alum prop might perform just a little bit better due to better prop rpm efficiency, but I doubt it. Cruising and WOT speed numbers would likely be the same.

I do think an SS prop would help you. Since you have a fairly tall pitch prop (21), an SS prop will slip less and give you a bit more speed at a given engine RPM. The improvements from SS props are generally a little less on lower pitches like 15 or 17. If you could find a used 21p Merc Vengeance or Silverado prop, I bet you would be pretty happy with the result.
 
EricW said:
I do think an SS prop would help you. Since you have a fairly tall pitch prop (21), an SS prop will slip less and give you a bit more speed at a given engine RPM. The improvements from SS props are generally a little less on lower pitches like 15 or 17. If you could find a used 21p Merc Vengeance or Silverado prop, I bet you would be pretty happy with the result.
Click to expand...

i was thinking of doing just that, Kind of rocky in certain areas arund me but where I have been hanging around it's all sand.

LK
 
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