What gear ratio is my outdrive?

[MelLandry]

Mercruiser Pre-Alpha

Serial Number on port side 4530652

Starboard side says "H.P. 120/140"

The assembly on the transom has serial number 5235762

 

[MelLandry]

Re: What gear ratio is my outdrive?

I did a little digging.

I looks like it's a 1.98

Model: 120/140
Ratio: 1.98
Year: 1970 - 1982
Serial Number: 2791957-6225576
Complete Sterndrive: 01941017
Complete Upper: 9412A23
Upper Housing: 9412A18
Complete Lower: 8951A43
Lower Housing: 5356A 4

Thanks to everyone who took a look at it. Let me know if you come up with something different.

 

[vipzach]

Re: What gear ratio is my outdrive?

Mel, I believe you are correct. My alpha one has the 1.98 gear ratio and is attached to a 3.0.

 

[MelLandry]

Re: What gear ratio is my outdrive?

Mel, I believe you are correct. My alpha one has the 1.98 gear ratio and is attached to a 3.0.

That makes sense... Mine (at first glance) appears to be some-what of a miss-match. It was on my 5.0 (which has now been replaced with a 5.7).

But what do I know...?

 

[vipzach]

Re: What gear ratio is my outdrive?

Some people will over come the mismatch with a different prop. You can pull the top cap off the outdrive and count the "teeth" on the gear.

 

[CaptRon66]

Re: What gear ratio is my outdrive?

You can figure which ratio with the drive off the boat by turning the input shaft until the prop shaft makes one complete turn. The input shaft on the 1.98 will make almost 2 complete turns doing this. If you start your mark on the input shaft at 12:00 it will be very darn close to it when done checking. It will come up just a hair short.

 

[MelLandry]

Re: What gear ratio is my outdrive?

Some people will over come the mismatch with a different prop. You can pull the top cap off the outdrive and count the "teeth" on the gear.

I guess in the end, it really doesn't matter what the gear ratio is, as long as you have it proped to run in the proper WOT range (within some tolerance).

I'm sure gear ratio with respect to pitch of the prop plays some roll in prop slip which would effect hole-shot/mid-range. Maybe there is some "most efficient" prop pitch and you'd gear the outdrive to aim for that.

I'd also guess that the higher the ratio, the slower the gears inthe outdrive turn, the less ware there is on the outdrive.

All this is speculation on my part.

How does the mfg determint the proper gear ratio?

Just for reference: I was runing a 19" (with a cup) on the 305. I haven't broken the 350 in enough to run it at WOT to see how I should prop the new engine.

Thanks for the continued input. Feel free to call me out on any (or all) of my assumptions. It's just my nature to hypotosize and question. I don't claim to know much outside of my natural sciences background.

 

[Bondo]

Re: What gear ratio is my outdrive?

Well,........ My take Is;

Readily available Props to fit Outdrives run from 15" to 23",.....
Many propshops or stores only stock 17" to 21"..........

You Have to gear your outdrive to Use the available HorsePower you have on the other side,....
And,... Match the Requirements of your Hull Design...........

You'd have to go to Specialty or SSteel prop to find a prop that Fits.....
If you have the Wrong Ratio.....

 

[Don S]

Re: What gear ratio is my outdrive?

You need the gear ratio so you can keep your prop pitch in the 15" to 21" range. Less than that you will have a tug boat prop. You will be able to pull anything, but you won't be able to pull it very fast without going to the 10,000 rpm range and the engine ain't going to like that. Going the opposite side of the range, say 27" to 30" pitch, you would go 90 MPH but it would take you days to get to that speed because of all the cavitation.
That is why the same drive will have different gear ratio's depending on rather it's used on a 4 cyl, V6 or V8 and also different on big block and small block V8's
It's all about the available torque, cause they all run about the same rpm.

How does the mfg determint the proper gear ratio?

Now it's all done by computer programs. Engineering and trial and error (Heavy on the trial and error) was how it was done back in the 50's and 60's when outdrives first came on the scene.

Regarding your 350, if you have an hour or two on it, do a wide open throttle run on it and check the prop. It's really hard on an engine to be over propped, and really NOT the way to break one in.

 

[MelLandry]

Re: What gear ratio is my outdrive?

Thanks Don.

I went from a 27 year old 305 to a brand new 350 and I haven't chaged the prop yet so I hope I'm under propped. I was running a 19" on the 305, so I'll probably be toward the upper end of the scale when I get the new engine properly propped.

I've got about 6 hours on the new motor strictly following the break-in procedures you gave me in another thread. I haven't opened her up beyond 3/4 throttle yet. I might go back out this evening and see what she can do for a few seconds. From the way she feels... I know she's got a lot more beyond the 3250 I got her to.

 

[Don S]

Re: What gear ratio is my outdrive?

If you got the right rpm with the 305 and a 19" prop, you don't have a 1.98 drive on there, no matter what it says on the outside.
It isn't that hard to regear a drive to a different gear ratio.
If you REALLY need to know the ratio of what that drive is now, you either count the teath on the upper gears, or easier, just turn the engine over BY HAND and in forward gear and see how many turns of the crankshaft it take to make the prop turn 1 exact revolution.

 

[MelLandry]

Re: What gear ratio is my outdrive?

Thanks Don.

It's not imperative that I know. I just wanted to calculate the theorteical speed of potential props compared to what I have now. I knew something couldn't be right because my theoretical speed of my current prop was coming out WAY to close to my actual speed.

When I strip the interior this winter to replace the carpet, I'll be able to get to the engine better to turn it by hand.

Thanks agian for your help on everything.