BMW’s quad-turbo diesel was too complicated (and expensive) to survive – Jalopnik

In 2016, BMW unveiled its 3.0-liter inline-six quad-turbo diesel engine. The story behind it isn’t as important as where BMW got its brand name, but the B57S (or B57D30S0, as it was known internally) was very ambitious for its time. Being able to give large sedans and SUVs performance comparable to sports cars while also returning 40 mpg in some applications is commendable, to say the least. This was thanks to a complex multi-stage arrangement, using two sets of turbochargers: low-pressure versions for immediate response at low engine speeds, and high-pressure versions to provide sustained pulling power at high speeds. The result was an engine that could produce 400 horsepower and 560 pound-feet of torque.

Technically, the engine did exactly what BMW intended. In some cases, these heavy luxury cars reached a speed of 0-100 km/h in less than five seconds. For a brief period, the quad-turbo diesel even powered the fastest diesel sedan in the world.

It may have had some impressive engineering feats to brag about, but the quad-turbo had some inherent drawbacks that doomed it. First, it was expensive, even by BMW standards. Models with the engine were definitely not among the most affordable BMWs available. Four turbochargers, state-of-the-art injection systems and a host of emissions hardware made design and manufacturing expensive. Inevitably, buyers had to deal with complex maintenance, and this also reduced the resale value of the cars that used it.

From BMW’s perspective, the business case became weaker over time. Sales numbers weren’t that bad, but then COVID hit and material and production costs skyrocketed. Continuing with the engine was simply not justified from a financial point of view. At the same time, the quad-turbo setup proved relatively unrewarding for aftermarket tuning. Despite the hardware, gains over factory production were modest, largely because emissions controls limited the engine at every turn.

The final blow came from the regulations. Euro 6d emissions standards imposed strict limits on carbon dioxide emissions, which the quad-turbo diesel found difficult to meet. Although early versions of the engine were praised for being more efficient compared to its triple-turbo cousins, real-world emissions struggled to meet the new regulations. Although there is growing sentiment that emissions standards mean nothing for the environment, compliance is still mandatory. If emissions fell below compliant thresholds, performance would have declined and costs would have increased, putting major question marks over the engines’ continued existence.

In 2019 and 2020, BMW began phasing out the quad-turbo diesel from its range. Production ended after just a few years, with Final Edition versions of the X5 M50d and X7 M50d serving as a silent breakaway from the engine in select European markets. BMW proved that a diesel engine could deliver sports car performance and impressive efficiency at the same time. What it couldn’t prove was that such complexity could survive in a world increasingly shaped by emissions cuts, rising costs and shifting market priorities.