Four turbos are better than three: BMW debuts new quad-turbocharged diesel engine.

PHOTO BY BMW

QUAD-TURBO IS THE NEW TRI-TURBO

   The latest broadside fired by BMW in the German horsepower wars -- or rather the German torque wars -- is a diesel 3.0-liter inline-six breathed on by four turbochargers.

  Specs for the new unit, codenamed B57, were published just recently after the engine was announced at the 37th International Vienna Motor Symposium.

   Munich's newest diesel pumps out 394 hp and 561 lb-ft of torque, with all of that torque on tap at just over 2,000 rpm. All of this is achieved through multistage turbocharging; two high-pressure turbos and two low-pressure turbos take care of low-end torque and high-end torque, also taking advantage of better charge-air cooling.

PHOTO BY BMW

   The figures themselves represent a gradual step up from the tri-turbo N57S engine offered in the BMW 750d, 750Ld, 550d, X5 M50d and X6 M50d; that tri-turbo churned out 381 hp and 546 lb-ft of torque.

   Under the hood of which model might we see this quad-turbo? The B57 will appear in the upcoming 2016 750d xDrive where it will be paired with a ZF 8HP 8-speed automatic transmission, which will allow for sprints from 0 to 60 in just 4.5 seconds.

   Don't rush out to order one just yet, and not just because gasoline is cheap enough to make 1990s Ford Expeditions sensible commuter cars: the 750d xDrive has not been confirmed for the U.S. But as the 7-Series is currently offered with just two different gasoline engines, we have to believe that we'll see more versions as the 7-Series rolls on. That tiny four-cylinder that BMW offers in the 7-Series in some other markets probably won't be among them, though.