What is it?
BMW’s big coupe, now fitted with the 3.0-litre twin-turbo diesel engine. Given the praise that has been heaped upon each and every model to receive this excellent engine, it hardly comes as a surprise to discover that the award-winning unit has now found its way into the subtly facelifted 6-series.
What's it like?
As you slot the electronic key into the dashboard for the first time and hear the raw (if distant) chatter of a diesel, you can’t help but wonder whether the new 635d is such a good idea. Not for very long, though. One determined stab of the throttle is all it takes to wipe away any doubts.
The 635d comprehensively outguns the petrol-engined 630i and can scare even the V8 650i on occasion. On top of this, its combined average fuel consumption of 41mpg allows you to enjoy the performance without it threatening to empty your wallet, while giving it a range of more than 600 miles.
It is all achieved through a complex induction system that employs two turbochargers of differing size. The result is a specific output of 96bhp per litre, giving the 635d a generous 286bhp at 4400rpm. But it’s the torque that really steals the show; there’s 427lb ft at just 1750rpm, and 369lb ft of that can be tapped from as low as 1250rpm.
BMW’s six-speed manual gearbox can’t cope with all the twist action. That leaves the latest six-speed automatic gearbox – with a fiddly joystick-style shift lever, remote shift paddles that move with the steering wheel and a different set of ratios to those found in petrol-powered versions of the 6-series – to channel it all back to the rear wheels.
BMW claims 0-62mph in 6.3sec, a good 0.4sec inside the time quoted for the 630i. However, the standing-start performance pales next to the 635d’s stomping in-gear ability. It gathers speed in truly impressive fashion; response is even better in the mid-range than down low, at which point the muscular-looking coupé is delivering the sort of acceleration to convince even the most ardent petrol fan that diesel motoring really has come of age.
Subjectively the 635d feels faster than the six-speed manual 650i’s 50-75mph fourth-gear split of 4.9sec. Much faster.
There’s more to the way the 635d drives than just its vast urge, however. The engine provides a wonderfully linear delivery of power from idle all the way to the red line. In this respect it is quite entertaining, the revs rising and falling with greater urgency than many comparable diesels.
While lacking the inherent refinement of its petrol-engined siblings, the diesel’s growl is also particularly well suited to the 6-series’ bullish character.
Should I buy one?
Yes. The 635d, more than any other car, is the latest example of the potential of diesel. Endowed with an unparalleled blend of performance and frugality, it very nearly renders the BMW coupé’s more traditional petrol engines obsolete, without quite managing it. But it does provide the facelifted 6-series with a worthy new addition that is sure to extend its appeal.