Confused

[Marcel1952]

Hello all,

After reading a lot in the web about gear ratio in omc cobra sterndrives, I think I know less than before I started.
I'm absolute confused but I hope there is an answer from one of you out there.

I have a Bayliner with a 3.0 liter engine from 1989, marked 302AMRMED.
On the transom shield there is a model number saying 985288
The outdrive gears are shot in the lower part of the unit and I knew that when I bought it.

I was looking for a replacement with a mercruiser drive.
I don't have any documentation on the gear ratio so I counted the tooth on all the gearwheels.
In the top part it's 21 and 16 and in the lower one is it 13 and 26.

Here is the part that confuses me, This gear setup should be for a 5.7 liter engine not a 3.0
Is this a correct assumption or is there something I missed?
If yes maybe this is the reason why the gears are damaged in the first place.

Other question, If i want to replace the cobra drive for a mercruiser, what gear ratio do I need for the 3.0 OMC?
I can get my hands on a 1:62 ratio but I don't want to burn my fingers if it's the wrong one.

Marcel

 

[Bondo]

Other question, If i want to replace the cobra drive for a mercruiser,

Ayuh,..... Merc uses the 1.98:1 or 2:1 gear ratio for the 3.0l motors,....

A Merc drive won't fit onto an OMC powered boat,.....

You'd have to swap Everything, from the pulleys to the prop,.....

 

[Marcel1952]

Are you sure Bondo_
I found this conversion kit and I thought that would make it possible.
http://www.sterndrive.cc/product/Fits-Mercruiser-Gen-II-Cobra-Conversion-Kit.html

 

[wrench 3]

Very interesting!!
Let us know how it turns out.

 

[Bondo]

• Designed to work with our SE116 drive.
• If the drive is ordered at the same time as the Conversion kit, we will modify the drive at no charge.

Ayuh,.... I guess yer right,.... Kinda forgot 'bout the SEI kits,.....

1.98:1 is what ya want,.....

 

[Marcel1952]

Great, 1.98:1
However that means that the current broken outdrive is the wrong ratio for the engine and most probably changed by a previous owner.
The combination 21 16 and 13 26 on gears should be for a 5.7 liter, I think.

Does anyone know what will happen if you boat with a combination like that?
High fuel consumption? excesive wear on the drive? lower speed only?

 

[HT32BSX115]

Great, 1.98:1
However that means that the current broken outdrive is the wrong ratio for the engine and most probably changed by a previous owner.
The combination 21 16 and 13 26 on gears should be for a 5.7 liter, I think.

Does anyone know what will happen if you boat with a combination like that?
High fuel consumption? excesive wear on the drive? lower speed only?

The 5.7L drive ratio is usually around 1.5:1 or so. A 3.0L engine wants around 2.00:1 ratio. You can use a 1.5:1 ratio but the prop has to be really low pitch to compensate. That makes everything LESS efficient, BECAUSE recreational boat propellers are most efficient in a rather small (prop) RPM range.

It's been done, but all I/O (and OB) manufacturers use particular ratios for a reason.

In either case, you select a prop pitch to produce "rated" engine RPM at wide-open-throttle. (WOT)

When you use the wrong ratio, and then use a prop that results in "right" WOT RPM, you will lose either holeshot, cruising speed or maximum speed performance, or all three........

Regards,


Rick

 

[Marcel1952]

Thanks for this clear answer, Rick.

 

[jerryjerry05]

Post a pic of the lower unit on the Cobra drive.
​The 3.0 used a much small lower unit.
The upper gear housing in the early years was the same on the 2.3 through 7.4
Just the insides were changed.

 

[Marcel1952]

No Title

Well I just stopped with even thinking about refurbish as the housing itself is also very bad damaged where the reversed gearwheel is located.
As the gearwheels are so jammed and stuck, that without any brute force and more damage it's not possible to get it out.
It's not worth the trouble in my opinion.
Better to salvage the good parts and sell it online to have some money for a new SEI.

I still try to get my head around the gear ratio.The reply from Rick and Bondo are clear on the 1.98:1 but the OMC sterndrive manual is not clear.
The model number is 985288 but is not in the manual.
Also the ratio of the 3.0 liter of '89 is in the list 1.71:1 (see attachment)

Does anyone recognize the 985288 model and the corresponding ratio?

 

[jerryjerry05]

That chart is for the Cobra.
​If your changing to Mercruiser then you need the drive that hooks up to the 3.0
That's the 1.98
http://www.sterndrivereplacement.com/

Go there and they have a list of the ratios.
Any of the drives probably will mount but the performance will not be good.