The C-15 "bridge" engine(MBN) has "smart" wastegate system, but also has DOC(diesel oxidation catalyst) in the OEM muffler, thus more exhaust backpressure.

I've read on the web, that some guys have replaced that OEM (DOC) muffler, with a std muffler and got up to .4 MPG increase=less exh. backpressure. I know if I owned one of those, I too, would change the muffler for that kind of increase.

This was the last of the "single turbo" CAT C-15 engines.