Behold the highly efficient BMW variable twin-turbo diesel engine. The not equal size turbos are there for a reason. The smaller turbo brings forth boost at low engine RPM and exhaust velocity. The larger turbo takes over at higher engine RPM, and handles forced induction chores up to the five-thousand RPM redline. Boost? Try 40 psi. Cleanup is handled by a catalyst and particulate scrubber after the turbos, and another catalyst after the urea injector. The not wee twenty liter urea tank needs filling only every 40K miles or so. The result is maximum diesel Bruce with a minimum of exhaust nastiness.