Mercruiser 3.7 Inboard Engine With Mercruise 3.0 Sterndrive

[wbelk7777]

I bought a boat today (Dynasty Triton 190) with a Mercruiser Inboard 3.7 engine in it. I didn?t notice until I got home with the boat that the Sterndrive has a 3.0 label on it. Is it possible that a 3.0 Sterndrive was installed on the 3.7 Mercruiser engine? Is this likely to cause any issues? Also, I didn?t pay much for the boat, so please don?t give me to hard of a time about not checking the boat out thoroughly. Thank you for your help.

 

[Lyle29464]

Re: Mercruiser 3.7 Inboard Engine With Mercruise 3.0 Sterndrive

No problem just take it easy the first trip out to make sure you are running in the proper RPM range. If the engine will easily exceed normal RPM"S you may have the wrong gears and need to increase your prop pitch or change the upper gears. If your not happy with it get back to the board with the pitch of your prop, the numbers on the drive unit and wot rpm's

 

[wbelk7777]

Re: Mercruiser 3.7 Inboard Engine With Mercruise 3.0 Sterndrive

Thank you for your reply.

1. Have you ever seen or heard of this type of engine/sterndrive miss match before?

2. Besides the engine running above the proper RPM is there any other issues this could cause (overheating (because the 3.7 is a closed system and uses the stern drive to cool), premature failure of the of the drive system, decrease in gas range/mileage)?

3. I did some research in to sterndrive ratios. The 3.0 uses a 1.98:1 and the 3.7 uses a 1.84:1. I take it that means the engine has to (in the case of the 3.0 1.98:1 system) turn 1.98 times for every 1 turn of the prop.

Thanks again for your help

 

[Don S]

Re: Mercruiser 3.7 Inboard Engine With Mercruise 3.0 Sterndrive

1. Have you ever seen or heard of this type of engine/sterndrive miss match before?

DIYers do it all the time, mostly because they don't realize there are different gear ratio's. What's worse is when they put a 1.50 drive from a V8 on a 3.0L and realize they can't get a prop low enough pitch to get full rpm, then they have a slug of a boat that is always lugging the engine.

The 3.7 should have a 1.84, which really isn't that far from a 1.98. Might see how it runs first, you may be able to change props and be fine.

PS: The previous owner may have already repropped it, or who knows, may have replaced the upper gears in the drive to the 1.84 and didn't bother the decals.

 

[wbelk7777]

Re: Mercruiser 3.7 Inboard Engine With Mercruise 3.0 Sterndrive

1. Does it say on the prop what pitch it is?

2. Where can I find out what the original props pitch was (the prop on the original 3.7 stern drive, before it was replaced)?

3. Does the pitch on the prop change the final drive ratio?

4. Do the sterndrive ratios like 1.84:1 and 1.98:1 take the prop pitch being used to come up with that number or is that number based purely on the gears in the housing.

5. What pitch prop should I need for this type of miss matched engine and sterndrive?

Thanks.

 

[Bondo]

Re: Mercruiser 3.7 Inboard Engine With Mercruise 3.0 Sterndrive

1. Does it say on the prop what pitch it is?

2. Where can I find out what the original props pitch was (the prop on the original 3.7 stern drive, before it was replaced)?

3. Does the pitch on the prop change the final drive ratio?

4. Do the sterndrive ratios like 1.84:1 and 1.98:1 take the prop pitch being used to come up with that number or is that number based purely on the gears in the housing.

5. What pitch prop should I need for this type of miss matched engine and sterndrive?

Thanks.

Ayuh,... 1) The prop's pitch should be stamped on it, Somewhere...
2) It don't matter, 'n is irrevelant...
3) Nope, that's the purpose of the Gears...
4) See #3...
5) It needs Whatever prop pitch that allows the motor to reach it's stated Rpms at Wot rating...

 

[HT32BSX115]

Re: Mercruiser 3.7 Inboard Engine With Mercruise 3.0 Sterndrive

is there any other issues this could cause (overheating (because the 3.7 is a closed system and uses the stern drive to cool), premature failure of the of the drive system, decrease in gas range/mileage)?

No...not really different than any other I/O.

Before you run it, you should just replace the raw water pump in the drive.

Also, since it is closed cooled, you might want to replace the coolant, and carefully search for raw water pump fragments that may have come from previous pump failures that the previous owner didn't remove.......That will absolutely cause overheats!

You now have an engine that is VERY sensitive to overheats. Make sure the gage works good and you check it often!!!

Like Don indicated, If the drive has been rebuilt, you may not know what the ratio is......the previous owner/installer may have already fixed the ratio/prop "stuff"!!....

Just run it and see if it turns near the top of the correct(for that engine) WOT RPM range with the boat lightly loaded...... If it doesn't, or over speeds, try a different prop.


Cheers,


Rick