Cummins 6BTA Problem

[Doug]

Our 1997 32+2 CB has a Cummins 6BTA 370 HP Turbo. The engine has approximately 1300 hours on it.

We recently took this boat to the Bahamas. In the process, we put about 135 hours on it.
The boat used 8 quarts of oil during this trip. There was a lot of “smoke” buildup on the transom of the boat. The turbocharger was rebuilt last year, so my mechanic thought it was best to do a compression test. Here is what he found:

Cylinder #1 285 PSI
Cylinder #2 361.5 PSI
Cylinder #3 348 PSI
Cylinder #4 270.5 PSI
Cylinder #5 345 PSI
Cylinder #6 345.5 PSI

Based on this test, my mechanic, which worked for Cummins for 10 years, feels this engine needs to be overhauled, which means removing the engine and installing a rebuilt long block. Anyone on this forum had simular problems? Are there other tests we should be doing before making the decision to replace the engine?

Thanks for your help,

[Russell]

I do not know how low the compression gets before a rebuild is necessary but two cylinders are definitely low. I have over 2000 hours and burn no oil, so 8 quarts in 135 hours sounds bad to me. Unfortunately your mechanic is probably correct. Hopefully Leonard will post on this. He seems to have a lot of experience with Cummins engines.

[jleonard]

If you listen to or read the experts on Boatdiesel, a compresion test is absolutely useless. Meaningless.
A blowby pressure test is what to use for determining if a rebuild is necessary. That is per Cummins.
I have info on that at work and if you are interested in that test procedure I can send tomorrow. Just say the word.

How was this compression test performed? With the engine not running like you would test a gasoline engine?
I have witnessed "running" compression tests on a diesel where each cyl was measured but while the engine was at idle and warm.
I might be inclined to believe that kind of test more than the above type.

That said IF you do need to to something, consider repairing only the bad cylinder(s). With a Cummins you can bore .020 over and replace 2 pistons (or perhaps just rings) and that is an authorized repair in Cummins eyes. I know several who have done that. That should be a consideration.

[filer]

Check the document library for blowby measurement procedures.

[ScubaPete]

I can't find any documentation on how to check blowby. ( I have Cummins 6BTA 330's). I understand that the blowby is a good indicator of engine wear and tear, and that you leave the oil fill cap loose, and see if it shoots through the roof or not!! I would appreciate more details on exactly how to do this test and what I am looking for. I tried searching the document library, but didn't find anything.

Thanks in advance.

[jleonard]

I just looked and it IS in the library. In the Cummins section.
Don't know how else to help you.

[ScubaPete]

I found it all, thanks. I didn't realize there was a difference between a "member" and a "supporting member". None of the document library is accessible to regular members, so that's why I couldn't find anything. I am now a supporting member.

[DougSea]

I found it all, thanks. I didn't realize there was a difference between a "member" and a "supporting member". None of the document library is accessible to regular members, so that's why I couldn't find anything. I am now a supporting member.

And, as fellow members, we all appreciate your support!

(I think both AOG and BoatDiesel should be required for any Albin owner. Two great sites. And our "community" here is just awesome!)

[jleonard]

JOHN COLLINS......
In my opinion the library should be available to all members.
That was MY intention when I submitted materials to the library.
I understand you can run the site how you see fit, howver I must make my thoughts on this subject known.

Jay Leonard

[nan1rik1]

I have a 2001 32+2 Command Bridge with Cummins 6TBA Turbo, 370 hspr. The sweet spot for cruising seems to be around 2200=2400 RPM which will turn 14 kts. or so. This seems like low RPM to me and the previous owner touted 17-20 kts. but was not sure @ what RPM. Has anyone any suggestions as to appropriate cruising RPM for this engine? Is 2700-2800 reasonable. As I'm sure we all are, I'm looking for that magical balance between performance & economy without straining the engine.

[jleonard]

I believe rated HP is at 3000 rpm for the 370.. According to boatdiesel you should be propped to get an actual 3100 + at WOT.
If you are there then theoretically you could cruise at 2800 but that is running right on the edge of that engine's capacity. Like running with the pedal to the metal in a vehicle for 4 or 6 hours. I don't think you want to be there for longevity reasons.

A wiser choice is to run like you have been.

When I had my 6BTA 270 I was told by the boatdiesel gurus to run at approx 10-14 psi of boost as a max for cruising to get longevity out of the engine. That was 2000/2100 rpm for me on a 2600 rpm rated engine or 500/600 off the top. If you had a boost gage you would be able to do that. Add a pyrometer and you could monitor EGT as well and know where your max cruise should be.

[Pitou]

I have a 2001 32+2 Command Bridge with Cummins 6TBA Turbo, 370 hspr. The sweet spot for cruising seems to be around 2200=2400 RPM which will turn 14 kts. or so. This seems like low RPM to me and the previous owner touted 17-20 kts. but was not sure _AT_ what RPM. Has anyone any suggestions as to appropriate cruising RPM for this engine? Is 2700-2800 reasonable. As I'm sure we all are, I'm looking for that magical balance between performance & economy without straining the engine.

I've found my sweet spot to be 2200 to 2400 RPM as well. Just completed a Maine trip cruising at 2400 RPM for the ride up and back ... avg. 17 kts and 10gph / I have a little less displacement than the 32+2. Dropping down to 2200 ... 14 +/- kts and 8 gph. As I recall from Cummins data ... when you hit 2600 ... 14gph, 2800 .... 22 gph, 3000 .... 30gph. You can see the efficiency drop pretty quickly at the higher rpm's. Not only do you blow more fuel at the higher rpm, but the enjoyment of the ride diminishes too. Just one Capts. opinion.