With the VW emissions scandal still fresh in mind, potential bans for diesel cars on the horizon in European cities and improvements being made to compact, efficient petrol engines, it's fair to say diesel passenger cars are facing a few challenges. BMW hasn't given up hope, however, putting its faith in the new M550d xDrive as flagship of the 5 Series range. With four turbochargers boosting its six-cylinder engine, the 550d is the most powerful production six-cylinder diesel car ever built.
We didn't mistype that, BMW really has fitted four turbos to the inline-six in the M550d xDrive. The last M550d wasn't exactly a slouch, but dropping its tri-turbo engine in favor of the new quad has freed up another 14 kW (19 hp) of power and 20 Nm of torque. That brings peak power to 294 kW (394 hp), and peak torque to 760 Nm.
One of the biggest criticisms of heavily boosted engines is always lag, but clever multi-stage turbocharging technology in the M550d means drivers should have peak pull on tap almost everywhere in the rev range. The engine is producing 450 Nm of twist at just above idle, and peak torque is on tap between 2,000 and 3,000 RPM. Meanwhile, peak power is pegged at 4,400 RPM. The upshot of all this is a car that should absolutely hammer toward the horizon from almost any speed, in any of the eight gears on offer.
Zero to 100 km/h (62 mph) flies by in 4.4 seconds in the sedan, and takes 4.6 seconds in the ever-practical Touring, while both cars share the same electronically-limited 250 km/h (155 mph) top speed. In spite of this 'bahn-storming performance, BMW says the 550d will sip between 5.9 and 6.2 l/100km (40 and 38 MPG) on the combined cycle, an 11 percent improvement on the outgoing car.
With so much torque, running with rear-wheel drive was never really a viable option in the M550d. To stop its rear-rubber being turned into smoke, BMW has fitted the car with xDrive all-wheel drive for year-round traction. The system is rear-biased under normal driving, but can shuffle power to the front axle if it detects slip.
Diesel and all-wheel drive isn't a combination you'll find in many (if any) memorable sports sedans of the past century, but BMW has still spent some time making sure the 550d xDrive can live up the the M badge on the boot. Rear-wheel steering, designed to make the car feel pointy at low speeds and stable on the highway, is standard, and the 10-mm (0.4-in) drop in ride height should help keep the car flat in the corners. When it's all said and done, this is still a big luxury cruiser. Anyone desperate to take their 5 Series to the track will likely be better served by the upcoming M5 when it launches.
On the tech front, the M550d xDrive will be available with the full suite of semi-autonomous goodies you get on the standard 5 Series. With the right options boxes ticked, the car will steer, accelerate and brake at speeds of up to 210 km/h (130 mph), before pulling into a tight parking space while driver watches from the outside.
Speaking of the outside, the styling of the M550d xDrive is suitably restrained. There are a few unique chrome trim pieces and some chunky alloy wheels to set the diesel range topper apart from its garden-variety siblings, but the basic package looks just like every other 5 Series. It's a similar story inside, where quilted leather seats and a sports steering wheel are the only changes of substance to the base BMW formula.
BMW is yet to release pricing details for the M550d xDrive.