More thoughts...

.5 Bar sounds a tad on the light side since 617s are supposed to run something like .8 Bar. But, that difference could simply be your gauge calibration. Try to find another air pressure gauge and plumb it in temporarily to compare the readings.

It's possible that, given the genesis of your engine (a stationary engine in a tech school?) the wastegate has been monkeyed with at some point. Or, it may have been 'detuned' for enhanced reliability but that doesn't really make much sense because the MB is so darned reliable anyway.

If the whine is objectionably loud, maybe you could add a tractor muffler to the stack? I know also that sometimes the stacks on marine engines wind up being lenghened or shortened to eliminate objectionable harmonics...you may be experiencing the same sort of thing with that long, straight stack.

I do think if that Mog was mine I'd just run it...if there was anything wrong with that turbo or the oil supply, it would have died long ago. If the turbo ist kaput, go get another off of a junker.

Just my .02
R Leo

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Never a dull moment at Berry Hill Farm.