Were Rudolf Diesel to become significantly less dead, teleport to the 21st century and take a quick spin in the BMW 535d, he would surely give himself a pat upon his inventorly back. This engine might just be the greatest manifestation yet of Diesel's creation, a torque-stuffed terror, with a set of vital statistics that defy logic, science and probably quite a few other things too.
The 3.0-litre twin-turbo straight six, comprehensively re-engineered for the sixth-gen 5-Series, now produces 295bhp and a walloping 442lb ft of torque, good for a 0-62mph time of 5.7 seconds - only a second behind BMW's own M5 - but economy of 45mpg and CO2 emissions of just 162g/km. In a big, heavy, luxury car, that's... wrong. Either BMW is slipping a cheeky fiver to someone in the emissions regulation office, or one of its engineers has discovered some new law of physics.
It's a strange thing. At least with big, fast petrol engines, you get some aural warning of the speed piling on. But the 535d, with its eight-speed auto 'box - the only transmission available, and at least two more ratios than necessary - simply wafts along on a mighty bore of low-end torque, barely topping 2,000rpm, all the while flinging you towards triple figures and an awkward conversation with Her Majesty's law enforcement community. In real-world, real-road driving, the 535d would push the (now discontinued) M5 very close indeed: you have to work the M5's V10 hard to find its sweet spot, but the 535d's power is right where you want it, available in monstrous, effortless swathes. Supersaloon pace with 40mpg economy? Why ever not?
But this isn't, at least in the car we tried, quite an out-and-out sports saloon. Our 535d was fitted with BMW's optional adaptive dampers, which, even in their sharpest setting, still don't offer quite the purity of the best passive chassis. It's a problem compounded by the active steering, which - though weighty - feels a little artificial. We haven't tried a passively damped 535d yet: it'll be interesting to see if it offers a bit more honesty and agility.
That's about it from the ‘could do better' column. Yes, the £45,000 asking price for our test car is steep, especially as you can pick up the 530d - the 241bhp iteration of the same engine - for £3,500 less, working out at a hefty £64 per extra horsepower. But it's a 54bhp upgrade that transforms the 5-Series from a fine, fast car into... SuperDiesel. And, to sweeten the deal, the 535d manages the same emissions and economy as the 530d. If you're vacillating between the two, stump up the extra for one of the greatest engines in history. It's what Rudolph would have wanted.
We like: Supercar speed, diesel economy
We don't like: Electronic intervention
TopGear verdict: Not quite diesel's answer to the M5 (the car, not the road), but close. What an engine.
Performance: 0-62mph in 5.7secs, max 155mph, 46.3mpg
Tech: 2993cc, 6cyl, RWD, 295bhp, 442lb ft, 1790kg, 162g/km CO2
Tick this on the options list: 19-inch M-Sport alloys, £1,210
And avoid this: Lane departure warning, £470