BMW got out of the business of building V-8 diesels several years ago, as did Mercedes-Benz. But the Volkswagen Group didn’t, and that’s one reason why BMW has continued to squeeze the most out of its inline six-cylinder turbo-diesels. Currently, the Bavarians offer a triple-turbo diesel engine, rated at 376 horsepower. (That’s it pictured above.) But that’s not enough to compete with Audi, which is continually pushing the limits of its V-8 diesel, and so BMW engineers were sent back to the drawing board with a clear assignment: Create a “V-8 fighter.”
And here it is. BMW’s top-of-the-line turbo-diesel remains an inline six, but it gets four turbochargers, two low-pressure units and two high-pressure units. In effect, BMW engineers replaced the single low-pressure charger of the triple-turbo setup with two smaller ones. Under low load, one high-pressure charger is boosting. With higher load and rpm, the two low-pressure turbos add boost to the high-pressure charger. From around 2700 rpm, a second high-pressure charger comes on and all four chargers provide boost. Fitted in the upcoming, Europe-only 750d xDrive, the new engine makes 394 horsepower from 4000 to 4400 rpm; maximum torque is rated at 561 lb-ft and is available from 2000 to 3000 rpm. That’s up from the triple-turbo’s 376 horsepower and 546 lb-ft. On the road, the new engine’s capabilities translate into a zero-to-60-mph time of around 4.5 second...