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BMW 550d review

Time:2018-02-13 15:34Turbochargers information Click:

motoring First Drives

Designed to fit between the warmed-over M Sport and full-blown M models, BMW’s new M50d engine is the most powerful diesel ever put into production by the Munich engine makers. It is notable for its array of three turbochargers, which, with commendable numerical illiteracy, the press pack refers to as an extension of “the company’s TwinPower Turbo technology”.

But is it absolutely necessary to bolt three, highly expensive compressors on to a six-cylinder oil burner? Here things get complicated. The three turbos have different characteristics, which boost at different engine speeds with the aim of plumping up the torque curve from low engine speeds. This in turn minimises the throttle delay that can occur with exhaust-driven forced-induction engines – aka turbo lag.

The first, tiny high-pressure turbocharger with a variable-geometry inlet starts to boost at just above idling speed, which means an almost instantaneous response to the accelerator. The second, larger, low-pressure turbo boosts from about 1,500rpm, which means the massive 546lb ft peak torque figure is maintained from 2,000rpm to 3,000rpm. Then, at 2,700rpm, an exhaust valve opens to spin up a third, small, high-pressure turbo to increase the charge pressure even further and maintain maximum power of 376bhp from 4,000rpm to 4,400rpm. A traditional wastegate valve prevents the larger turbo from over-pressuring the system at very high speeds.

The idea is that you get a consistent level of power and torque across the whole rev range, but even so this engine’s torque curve is not exactly a railroad, resembling more the Cape’s Table Mountain, with the flat bit between 2,000rpm and 3,000rpm. That’s partly why the engine is only available with BMW’s eight-speed automatic transmission and four-wheel drive, which contain the phenomenal torque peak and increases the CO2 output in an M550d XDrive 5-series saloon (the only version of the engine installation available to drive) by between three and four per cent, as well as increasing the kerb weight (in our estimation) by about 60lb. But the combination of 4x4 and an auto 'box can hide a lot of other things, too.

"We programme the eight-speed to avoid transition modes where efficiency is low," explains Dr Nikolai Ardey, BMW's director of diesel engine development. I ask him if he also avoids engine periods where noise vibration and harshness peak and he grins. "Of course!"

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That's as maybe, but there were times during the test drive when this phenomenal engine spiked enough vibration through the 5-series' frame to have my torso rattling like a dead-man's chest.

Out on the open road the combination of loco power and slick auto works well, and it’s certainly quick. On part throttle openings it sounds occasionally like you’ve dropped a couple of cylinders in the firing order, but at steady speeds this is the unstoppable force, with a significant roar. Stand on it and you’d swear there was a V8 under the hood and the big mill gets in the groove like a Motown horn section.

But this engine is really all about the mid range and gets a bit thrashy at peak revs, although, by the time you’re up near the red line, the police helicopter gunships will be gathering.

For the most part this remarkable engine is fast, refined and doesn’t feel like the agricultural beast that so many of its rivals resemble. With the unit out of the car you can see the Heath Robinson exhaust manifolding, which sits on the left-hand side of the front of the block as you look at it. And it is this which makes right-hand drive an impossibility for this 5-series saloon version and why we will only see the unit in BMW's SUV models, the £60,325 X5 M50d and £62,260 X6 M50d. Or does it? When pressed, the engineers admitted that it is the four-wheel drive system that makes RHD impossible and that a rear-driven, right-hand-drive M550d would be possible at least.

While we understand why they haven’t done it for the tiny UK market for such a car, it is a shame. That’s a shame for those after a saloon, as this diesel M5 is a super car; fast, capable, with a soundtrack that mostly beguiles and half decent fuel economy as well. If you opt for an X5 or X6 instead, it should do equally good service.


BMW 550d

Tested: 2,993cc, straight-six-cylinder tri-turbo diesel, eight-speed automatic transmission, four-wheel drive

Price/on sale: £60,325/June 16

Power/torque: 376bhp @ 4,000rpm/546lb ft @ 2,000rpm

Acceleration: 0-62mph 5.4sec

Top speed: 155mph

Fuel economy: 37.7mpg (EU Combined)

CO2 emissions: 199g/km

VED band: J (£445 for first year, £245 thereafter)

Verdict: Powerful and economical, this is a useful addition to BMW's engine range. It's just a pity the UK isn't going to get the 5-series with this lump

Telegraph rating: Three out of five stars

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