TURNING a hairpin corner, we depress the accelerator and the BMW X6 picks up its skirts and sprints away up a straight section of road, quickly changing up through its eight gears.

That would all seem par for the course except that the particular piece of road is one of the steepest chunks of bitumen you could find within easy reach of Melbourne, up Mount Dandenong.

Another, more average SUV – especially one as bulky as this 2.2-tonne behemoth – would have perhaps held second or third gear for much of the climb, protesting a little along the way, but not the Black Caviar of diesel 4x4s – the BMW X6 M50d.

The effortless acceleration makes molehills out of mountains, nicely paved ones at least.

BMW has bestowed 750Nm of torque on its new M50d, which, from its name and performance, you might expect to have a 5.0-litre diesel engine.

In fact, this beast has just 3.0-litres in a six-cylinder package, but under the derailed BMW nomenclature of late, qualifies for the ‘50’ badge because – well, it has more turbos or power or something than the standard diesel engine that rates only a 30d name.

The difference comes down to an extra turbo – three in all – and some tender loving care and bibs and bobs from the folks at M GmbH – BMW’s make-them-go-fast unit in Bavaria.

Along with the matching X5 M50d, the X6 M50d is the first BMW model to get a light working over from the M division, in a new sub-brand called M Performance Vehicles.

It’s the M vehicle you have when an extra $30k seems rather a stretch for the real thing.

But the only light thing about the X6 M50d is the effort required for forward progress, regardless of road elevation.

The deep well of torque starts at just 1500rpm, and thanks to the way the three turbos come on stream, keeps the mumbo going all the way to 5000rpm and beyond, adding 280kW of power along the way. A tractor diesel this is not.

However, the performance of this world-best diesel six is all well ordered, without the white-knuckle, neck-bending excitement of, say, an M3.

We expected the chassis to struggle a little with the jumbo-jet torque, but even on sharp, wet corners with the throttle maxed out, the M50d kept its composure.

The diesel engine is beautifully matched to the ZF eight-speed automatic, zipping up and down through the cogs neatly, in either auto or flappy-paddle manual mode.

The driver can select a transmission ‘sport’ setting, but when you have 750Nm of torque under the right foot from 1500rpm, it seems a bit unnecessary. Who needs revs?

The X6 can’t disguise its bulk, though, and even with some M magic in the chassis – including special tuning for the suspension and steering and bigger brakes – this is more Mike Tyson than Usain Bolt.

But the fuel economy is more Cadel Evans. BMW claims an excellent 7.7 litres per 100km, and we got a damned reasonable 9.3L/100km in a taxing drive over some sharp hills.

This effort underscores the beauty of smallish capacity engines with big blowers to achieve barrel-chested acceleration with levels of fuel economy that once would have flattered a Corolla.

And quiet, too. Boy, can BMW do diesels. No wonder Toyota has partnered up with them.

Inside, the M-enhanced sports seats look the part with their double-stitched seams and leather-Alcantara combo trim, matching well with the chunky M leather steering wheel.

But we can’t say the same for the plastic-looking trim on the dash of the test car. It is probably an expensive and exotic material, but it looks cheap.

While your road tester might be considered vertically challenged by some mean-spirited people, the head-up display speedo reflected on the windscreen was hard to see, even with the driver’s seat jacked up.

In the back seat, we have space for three people now on a bench seat, which might not sound like a breakthrough, but many people worried about the two-seat restriction of the original model. After all, the X6 is likely to be the family car.

Finally, we come to the styling of the X6 M50d, which retains the polarising sloping back. If you like it, good. If you don’t, the X5 also gets the big diesel.

The X6 gets solid, purposeful-looking 20-inch alloy wheels, big-bore exhausts and a bunch of minor exterior changes that come with the mild mid-model facelift for the X6 in its fourth year.

Oh, and it gets an M badge on the back, which will be worth plenty for some, even at a $157,000 asking price.