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BMW chose the N47 20d & N57 30d as their diesel offerings for NA is simply due to convenience, cos those are the only 2 diesels (apart from the N57S tri-turbo) which can be had in EU6 compliant version. It would be far easier for them to adapt those 2 for EPA compliance cos the baseline (i.e. EU6) is quite close to Tier2 Bin5 wrt NOx emissions. Japan is another market where diesel emissions standards are extremely strict, & again they only offer the aforementioned diesels engines there (the 30d is badged as a 35d there).
The Mercedes OM651 engine u talked abt have sequential twin-turbos like the N47TT. In fact they offer that engine in 2 states of tune - 220D (170ps) & 250D (204ps).
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