3.0l omc swap to to a 2.5l omc problem

Busaghost

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I have a 1991 galaxy that had a 3.0l omc cobra set up but it had a cracked block so I swapped a 1976 2.5l omc I got for dirt cheap. I did this swap 4 years ago. The engine runs great but the problem I've been having ever since the swap is the outdrive. It feels really weak on take off and doesn't go as fast as it should. With the 3.0l I had a 18 pitch but that was not good so I ended working down to a 11 pitch and it running the. Eat it has since the swap but it's still lacking. Wot is running 3200 rpm at 27 mph and since the pitch is so low it looks like the drive is up a little while driving because it looks like it's spitting the water upward. My next assumption is that I have to change the gearing in the drive. Am I correct with assumption and if so what gearing do I need? Has anyone done this swap and have any information that would help? Thank you in advance.
 

Bondo

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Ayuh,.... Welcome Aboard,.... Yer drive should be geared at 'bout 2:1, or 1.98:1, or whatever OMC called it,....
Both those motors require the same gear ratio,...
That said, it's my guess the gear ratio has been wrong since ya bought it,....
I've also never seen an 11" pitch prop that fits an I/O,...
'n lastly, not sure what this quote means,....
and since the pitch is so low it looks like the drive is up a little while driving because it looks like it's spitting the water upward.
 

Lou C

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When you had the original 3.0 in it, do you recall if it reached the specified max RPM? In my '88 OMC manuals, 4200-4600 is the spec for the 3.0. The correct gear ratio for the 3.0 was 1.71:1. I don't have the specs for the 2.5 liter because at that time the other 4 cyl was the Ford 2.3 OHC also used in Pintos, Mustangs, Rangers etc.For that, the WOT was 5200-5600 and the drive ratio was 2:1. You might want to pull the drive and see what ratio you have. Just for reference, the 4.3 used 1.68:1 and the 5.0, 1.50:1. The 5.7 used a 1.41:1. If your boat got a drive from either of the V8s I could see how you could have this problem.

My boat had the correct ratio drive but the wrong prop on it when I got it. It came with 14.5x19 prop but it really needed a 15x17 to perform right.I have a 15.5x15 that I am going to experiment with this summer....
 

Busaghost

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With the original 3.0 in the boat it ran amazing. This is the first 4 cylinder boat I've ever owned. The boats and cars I first owned I always went for the bigger motors. I was always going for the fast lane. With that said I was pretty impressed at how the 3.0 cranked. I'm not sure of the original rpms but it had absolutely no problem planing and no problem reaching its top speed. I had got the boat for what I thought was a really great price and found out why on that first test run. Owner neglected to tell me he didn't winterize it and cracked the block. For the 11" pitch I had to bring my prop in for them to resize it. It runs a lot better now it it's still not correct. I'm not sure how to find out the gear ratio.I did bring it to a mechanic after I ran out of ideas and he supposedly took the drive apart and said everything was correct but I took a look at the out drive and I'm pretty sure he didn't touch it because I painted the outdrive with the drive bottom paint and unless he repainted the bolts and stuff he didn't touch it and got away with robbing me. I can't go any lower with the pitch of the prop. As of right now when the boat is in motion and looking back at the outdrive wake looks as if the tilt of the out drive halfway up but it's all the way down. It looks like it's not pushing enough water and the water it is pushing is going in an upward motion. I would say that's from the 11" pitch. That's the best I can explain that. Can you tell me how I can find out what the gear ratio is? Is the out drive stamped somewhere? Thanks again for the help
 

Scott Danforth

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Welcome aboard

I wouldnt have stepped backwards from a 3.0 to a 2.5, however that is just me. when you swapped the old 2.5 for the newer 3.0, did you use just the long block, or did you swap everything, including carb, etc from the 2.5?

your down about 20-25hp because of the smaller motor.

check the general health of your 2.5. check compression. if using the original 2.5 carb and linkage, check the bushings in the throttle linkage to make sure they are still there.
 

Busaghost

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I didn't want to down size from the 3.0 to the 2.5 but I didn't have money to buy a new motor and there where no used 3.0l for sale in my area at the time. The 2.5 looks exactly like the 3.0 and I got the engine for almost nothing. I swapped everything from the 3.0 to the 2.5 because it was in better condition. The engine runs great and sounds great. I took the exhaust manifold, carb, ignition, fuel pump and the alternator from the 3.0 and put it on the 2.5. Everything runs as it should, it just feels like I'm not getting the right enough power through the drive. Been going through the different prop sizes over the years and every time I decreased the pitch I had gotten more and more power but I'm at the lowest pitch I can go. I was just reading about the larger and the smaller lower end on the outdrive. Does anyone know the difference between them? Should I try changing the lower to the smaller hub? I found one on CL but don't wanna spend that much if I don't have to. The pictures I attached is what's on the boat now. Thanks again for all the help.
 

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southkogs

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Do you mean upper and lower gear case? And yes, they have gear ratios to them.

You wouldn't swap the hub on prop - you would use a different diameter (maybe) and/or pitch.
 

Bondo

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I was just reading about the larger and the smaller lower end on the outdrive.

Ayuh,... Those were the ole Stringer drives,....
The Stringer 800 is the larger, the Stringer 400 the smaller,....
 

Busaghost

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Ayuh,... Those were the ole Stringer drives,....
The Stringer 800 is the larger, the Stringer 400 the smaller,....

So the cobra outdrive don't have a smaller and larger lower just different gear ratios?
 

Scott Danforth

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the cobra drive and the stringer drive are completely different animals. only the name OMC ties them together. there is nothing common between them with the exception of a few bolts.
 

Lou C

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Well the answer is YES. The earlier Cobras (like 86-90 or so) used a smaller lower gearcase for the 4 cyl models. After that they used the same lower as the V6 and small block V8 models. In this case it is a non issue as long as you have the correct gear ratio to start with. You need to put the drive in gear, remove the spark plugs, then have someone watch the prop, and figure out a way to turn the engine over slowly. Have them tell you when the prop makes one full turn while you watch to see how many turns the crankshaft pulley makes for one turn of the prop. That is your gear ratio. Until you verify this you are just wasting time.
 

Busaghost

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Well the answer is YES. The earlier Cobras (like 86-90 or so) used a smaller lower gearcase for the 4 cyl models. After that they used the same lower as the V6 and small block V8 models. In this case it is a non issue as long as you have the correct gear ratio to start with. You need to put the drive in gear, remove the spark plugs, then have someone watch the prop, and figure out a way to turn the engine over slowly. Have them tell you when the prop makes one full turn while you watch to see how many turns the crankshaft pulley makes for one turn of the prop. That is your gear ratio. Until you verify this you are just wasting time.

As soon as I got some free time.... maybe Thursday or Friday I'll take a ride over to the boat and find out the ratio.
 
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