Turbocharger issue

In the days following our sea trial we did some additional tests to see if we could isolate the problem with the variance in rpm and boost between the port and starboard engines.  We ran both engines at wide open throttle at the dock with no load, and they both achieved 2950 rpm, as they should.  During this test, although turbocharger boost doesn't rise so much without a load on the engines, the port engine showed 2 pounds of boost while starboard showed 5.  We then switched boost gauges in the panel and repeated the test.  The results were the same, eliminating a faulty gauge.

Our suspicions were still with the port turbocharger.  Not only was it not on the engine during its dynamometer test, it also has a different assembly number than the turbocharger on the starboard engine.  I made a couple of phone calls to Cummins Northwest in Renton, and eventually determined that the starboard turbocharger was correct for our 250hp engine.  They could not tell me what engine the port turbocharger was for - it didn't match any of the numbers in their cross references.  I shopped around some on the Internet, and eventually found a listing where this "orphan" turbocharger was offered for a 300/330hp model Cummins.  These models develop their maximum horsepower at a higher rpm, and my theory was that this turbocharger may be engineered to develop its usable boost for an engine that is delivering more cubic feet per minute of exhaust gases than we are doing with our 250hp model.

I also posted our boost pressure and rpm difference issue on the forum at boatdiesel.com, but didn't get answers, at least answers I wanted.  The experts are focused on the rpm difference and suggest the first order of business would be to adjust the propellers so that both engines can achieve 2600+ rpm.  They also pointed out many other issues that could cause a difference in boost, like differences in exhaust back pressure, fuel supply, clogged intercooler and so forth.  Those are all possibilities, but what I know for sure is that the turbochargers are not a matched set.  I think getting back to symmetry first before addressing propeller pitch and the other issues is the way to go.  I've checked with Ron at Seamaster, and he still has the original turbocharger that we returned.  He's going to send that back, minus the bad exhaust housing, and we'll swap it with the one there now.  One thing we'll have to double check is if the exhaust housing is the right size.  It can't be too small or the turbine wheel won't fit or will contact the sides.  It can't be too big, or some exhaust will escape past the turbine without contributing.  It has to be 'mama bear' just right, and Ray has all sorts of calipers that we can use to assure that it is.

I did receive assurances on boatdiesel.com that as long as we don't exceed 2000 rpm we can cruise along indefinitely on our current propeller configuration without doing damage to the engines.  This is quite the comfort, as we haven't exceeded that rpm in all our years of cruising. 

When we get the other turbocharger back, checked out and installed, we'll be making another trial run to see if it makes any difference with more symmetry in our equipment.