We can presume that the dinosaurs kept evolving right up to the point when the meteor arrived and high-performance diesels have managed the same trick. The sky hasn't quite turned to fire quite yet, but the medium-term prospects for expensive oil burners are looking pretty bleak; some of the forthcoming wave of Low Emissions Zones look set to treat even Euro 6 oilers as if they were 300,000-mile 1995 Mondeo 1.8 TDs. With a wave of fast petrol hybrids approaching it's no surprise buyers are already wary about models that might find themselves locked out of towns and cities in a few years. Cars like the BMW M550d will almost certainly be the last of their genus.
I drove the M550i in the U.S. last year and didn't gel with it at all, reckoning it was pretty much the opposite of what an M car was until recently. Now I've driven the diesel fired M550d Touring in Spain, a car that seems even further from Motorsport Division's one-time core values, and have to admit that I like it a whole lot more.
While it's no sharper than its petrol sister to drive hard, the M550d doesn't feel like an understudy. In the places where both models are sold alongside each other the M550i has to live in the long shadow cast by the M5. The 50i is significantly cheaper, and has a V8 that provides plenty of pace and character, but it lacks almost all the athleticism of its single-digit sister. The M550d isn't a dynamic scalpel either, but nor does it have the not-quite feel of the M550i, and its brawny quad-turbocharged diesel is pretty much perfectly matched to the chassis's laid-back character.
That's right, four turbochargers; a turbine-tally to match that of the Bugatti Chiron. The 3.0-litre six that powered the previous generation F10 M550d (which didn't come to the UK) as well as the X5 and Z6 M50ds (which did) used two small turbos low down which were then supplemented by a single large one. The new 394hp B57S motor switches the large turbo for two medium-sized ones to improve response, with one of these permanently boosting and the second coming on stream above 2500rpm. A clever system briefly bypasses the two smaller turbos when a big load is demanded at lower speeds, spooling the bigger blower more quickly.
So while the overall increase in power and torque over the old triple-boosted motor have been modest - 19hp and 15lb ft - it delivers more lower down. At 1000rpm BMW claims the engine is already making 330 lb ft, and by 2000rpm it is delivering its peak 561 lb ft. That's a way short of the 664lb ft produced by the V8 TDI in the Audi SQ7 and Bentley Bentayga Diesel, but still mightily impressive, the 50d being the most powerful six-cylinder passenger car diesel engine in the world.
Transmission is as in the M550i, with the ubiquitous eight-speed ZF auto which seems to have unrolled its beach towel across the entire upper-premium segment, with drive sent to each corner through an xDrive all-wheel drive system. This is capable of a rear biased torque delivery, but lacks the M5's party piece of going purely rear drive.
In a straight race with the M550i the M550d would be almost certain to lose. The D has 56hp less and its 4.4-second official 0-62mph time is half a second adrift, with the heavier Touring (unavailable with the petrol engine) adding another two tenths to that. But, barring an unlikely trip to a drag strip, that's unlikely to matter in the slightest, because when it comes to real world pace the diesel M has pretty much everything beaten.
Speed is delivered with an almost total lack of effort. Left in Drive the M550d responds to even a light throttle application with solid, no-nonsense urge. Press harder and the rate increases with almost perfect linearity; by the time the pedal gets close to the bulkhead the sensation has turned into 'seats for takeoff', with no sense of acceleration tailing off as the numbers on the speedometer get increasingly silly, even with the gearbox still delivering upshifts between 3500rpm and 4000rpm. Push further to kickdown and the transmission holds ratios, although never goes to the 5500rpm redline marked on the tacho. Switching to the punchier Sport mode adds a noticeable shift-shock bump to the upchanges, but it never feels like it's breaking a sweat.
It lacks the aural excitement of the M550i, and indeed the V8-sounding Audi V8 TDI. There's a muscular, distant hum which doesn't grow significantly louder with more revs, and which is fades into the background as road and wind noise increase with speed. If you set out to find it - by using the gearbox's manual mode - then there is a hint of lag low down. But even locked into 8th at an indicated 60mph with 1200rpm showing on the rev counter it still pulls strongly after just a couple of beats of pause.
In short, it's a car that's clearly been designed for the Autobahn, where I'm certain it feels absolutely mighty; capable of carrying four people and a boot full of stuff at triple-digit averages and without the irksome need to worry about filling stations. But I've not got an Autobahn...
In further proof of the German sense of humour, my test route consisted of some of the tightest and twistiest mountain roads that Andalusia has to offer. In the baking heat of the far south of Spain the M550d felt some way from its comfort zone. Active dampers do a good job of maintaining discipline over the frequently broken surfaces, but even in their firmest Sport setting the car never feels anything other than big and heavy. And also surprisingly soft for a car wearing an M badge, despite the claim of active anti-roll bars, heavy cornering lots produced lots of lean and the brake pedal soon started to soften when asked to repeatedly deal with the speeds the car can reach on even short straights. Blame Newtonian physics and a 1955kg kerbweight.
As with the M550i the variable ratio steering is accurate and yields good initial responses, the M550d scythes its way through fast sweepers with just a few degrees of input. But in slower turns there is a fair amount of understeer and, despite the claimed rear-biased power delivery, even big throttle applications don't produce any sense of the rear end doing more than digging in to deliver total traction. Steering feel is disappointing, too with any meaningful messages hidden behind gloopy assistance with the different dynamic modes altering the weight without adding any feel.
It was the M550d's misfortune that I drove it back to back with the new M2 Competition, which felt almighty on the same roads. The fact it's not a mountain pass weapon probably won't come as much of a surprise, but its eight-tenth performance is still deeply impressive. Compared to its most obvious uber-diesel rival, the Audi SQ7, I suspect it would feel considerably more lithe and fleet of foot.
I got flamed for saying I didn't think the M550i deserved an M badge. I have to report feeling the same way about the M550d. It's a properly serious bit of kit and a hugely appealing all-rounder. As a Touring it's also a very stylish riposte to the default choice of a premium SUV. Yet while it will probably be remembered as one of the high watermarks of diesel technology in years to come, but beyond the speed and chunky bodykit I'm struggling to nominate anything particularly M-ish about it.
It's a moot point anyway because, unless you like outside the UK, there's very little chance of being able to buy one. BMW GB says that, as with the M550i, the market for the 50d is just too narrow to make sense at the moment given the size of the supplement it would carry over the 530d xDrive. As the war on diesel gathers pace, that's unlikely to change.
SPECIFICATION - BMW M550D
Engine: 2993cc straight six, diesel, quad turbocharged
Power (hp): 394@4400rpm
Torque (lb ft): 561@2000 - 3000rpm
0-62mph: 4.6-sec
Top speed: 155mph (limited)
Weight: 1955kg
MPG: 45.5mpg
CO2: 163g/km
Price: €89,200 (Germany)
(The eagle-eyed among you may have spotted that the model pictured above isn't actually an M550d. Sadly its maker declined the opportunity to take any photographs of the car in Spain - or indeed anywhere else for that matter. A sign of its increasingly marginal status in the lineup? Perhaps. Either way the only official 'photographs' that exist are the ones below. Apologies.)
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