BMW Built the Only Quad-Turbo Diesel Engine Ever - and It Was a Wild Ride

Before the diesel emissions scandal upended the auto industry. Audi was the king of compression ignition. The brand had eight Le Mans wins with diesel prototypes and even built a V12 oil-burner destined for the R8, though it ended up in a Q7 instead. But BMW wasn’t about to sit back.

They had already mastered tri-turbo straight-six diesels, producing 375 horsepower and 546 lb-ft of torque in models since 2012. So when Audi kept pushing. BMW responded with something no one had ever attempted: a quad-turbocharged diesel engine.

In 2016. 0-liter straight-six with four turbochargers. The setup used two small variable-vane turbos for instant low-end torque and two larger units that kicked in above 2,000 rpm. Clever bypass flaps let the engine send exhaust directly to the big turbos when needed, virtually eliminating turbo lag.

The result was 394 horsepower, 516 lb-ft of torque, and an impressive 41 mpg. This engine found its way into the 5 Series, 7 Series, X5, X6, and X7. On paper, the power increase over the tri-turbo was modest, but the real-world drivability and throttle response were transformative.

Unfortunately, timing was everything, and BMW’s timing was terrible. The B57D30S0 debuted just after Dieselgate, when diesel’s reputation was in freefall. Audi’s SQ7 TDI, with a V8 and one fewer turbo, still outran the BMW to 60 mph by half a second. The quad-turbo engine was expensive to develop, complex to service, and burdened with emissions hardware like particulate filters and EGR valves.

When Euro 6d regulations took effect in 2021, requiring real-world emissions testing and banning defeat devices, the writing was on the wall. BMW announced the engine’s retirement in January 2020. It lasted barely four years. For all its brilliance, the B57D30S0 arrived five years too late and left far too soon, a monument to a moment when automakers were willing to try anything, even four turbos on a diesel.