BMW got out of the business of building V-8 diesels several years ago, as did Mercedes-Benz. But the VW 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 turbodiesel remains an inline-six, but it gets no fewer than 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 are providing 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 0-to-60-mph time of around 4.5 seconds in the 750d xDrive; top speed will be governed at 155 mph. As a welcome side effect, fuel consumption is lower by 5 per cent.
Boasting a bore of 84 millimeters and a stroke of 94 millimeters, the new diesel is part of BMW’s modular engine family. Like before, BMW was looking to achieve “exceptional revving capability,” and the briefing we attended spoke of a significant improvement in response. A new fuel-injection system operates at more than 36,000 psi, compared to the outgoing engine’s 32,000. Cylinder pressure was increased to a maximum of 3046 psi, with the increase in both injection pressure and cylinder pressure being given the most credit for the new engine’s power boost over the old one.
BMW said that maximum torque is limited by the transmission, not by the engine. To which we reply: Get a stronger transmission. This engine promises to be awesome, and we hope it eventually will be offered in the U.S., even as we know the likelihood is slim.
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