Taking pride of place under the bonnet of the M division fettled four-door is the N57S, as BMW M division insiders like to refer to it. The highly complex and expensive to produce tri-turbocharged 3.0-litre in-line six-cylinder diesel engine – also destined for the upcoming X5 M50d and X6 M50d – provides BMW’s first dedicated performance diesel with the sort of instant-on performance and inherent entertainment value you usually only ever encounter from a high capacity petrol powerplant.
The aluminium block engine is a true technological tour de force, but despite BMW’s best efforts it still weighs 24kg more than the twin-turbocharged 3.0-litre unit it is based on. This places even greater weight over the front axle than the 535d – a car which it pips in overall output by 81bhp and 103lb ft at 376bhp and 545lb ft – the latter developed between 2000 and 3000rpm. And I never remember thinking the 535d lacks for punch.
What these bald figures fail to convey is the omnipresent energy. There’s no off-boost lethargy or waiting for it to come on song as engine speed builds. With the latest in high pressure, piezo valve guided direct injection together with a small, low interia turbocharger working in concert with a larger unit at low engine speeds to enhance the induction process, it delivers enormous shove and tremendous flexibility, and that’s just in the first couple of thousand revs.
Above 2700rpm, a second small turbocharger is engaged, providing additional boost pressure (up a maximum of 3.5 bar) and truly monumental in-gear thrust – the kind to make even the M5 feel, dare I say, a tad weak by way of direct comparison. Despite the M550d xDrive’s 1895kg kerb weight, it is heroically, epically quick.
Diesel engines aren’t supposed to respond to throttle inputs like this. At any revs, in any gear, the M550d xDrive simply flies. Pegged hard, its accelerative nature is nothing less than brutal. Nor are oilburners meant to accept revs with such unbridled enthusiasm. The new BMW engine feels totally at home operating beyond 4000rpm. In fact, it encourages you to. Peak revs are limited to 5500rpm.
What’s more it sounds terrific. Unlike the M5, which receives a synthetic soundtrack, the M550d xDrive’s aural traits are genuine – although in a process that is gaining popularity right across the automotive spectrum, BMW does rely on the speakers to enhance their effect. The combination of engine and exhaust sounds is not unlike the original Audi quattro, with a deep warble at low revs and a wonderfully exuberant baritone wail up high.
For all its undoubted enthusiasm, epical thrust and aural delights, though, it’s easy to overlook perhaps the M550d xDrive’s most convincing traits – it’s superbly refined nature, relative economy and crushing long distance qualities. Mated to an upgraded version of BMW’s superb eight-speed automatic gearbox, the engine is not totally free of vibration but is smoother than any other high performance oilburner throughout its entire rev range, returns real world consumption not too far from BMW’s own claims and requires just 3000rpm at a heady 120mph cruise.
It’s big and heavy, but with the latest version of BMW’s four-wheel drive xDrive system apportioning drive to each corner it handles well, even in tricky weather conditions. It can’t quite match the M5 for overall dynamic ability: the electro-mechanical steering system lacks for consistent weighting and the ride is a little brittle (even in comfort mode) on certain surfaces. But with one determined stab of the throttle any deficiency is forgotten.