Tri-turbos are sooo last season. Aiming to keep up with fashion BMW is reportedly working on quad-turbo engines, according to Autocar.
The company already fielded a tri-turbo design in the X5 xDrive50d M Performance and the M550d, adding an electrically driven turbocharger for extra low-end boost pressure. The quad-turbo design could follow that with another electrically driven turbo, or BMW could opt to go the conventional route and simply add a second bi-turbo setup to the diesel inline-six.
The quad-turbo inline-six diesel engine is expected to debut in the M750d 7-series sedan in 2016, and it could also make a cameo appearance in the xDrive50d M Performance versions of the X5 and the X6 in the near term.