BMW 635d Twin-Turbo Diesel Coupé and Cabrio

**Horsepower** · March 17th, 2008 03:10 AM

What is it?

The BMW 6 Series is nearly four years old and has just been given the most imperceptible mid-life face-lift imaginable.

Headlamp-spotters might clock the minor revisions to the front lights, but even keen students of the current car will struggle to suss the new "light-reflecting" lower edges of the front and rear valances (which apparently help make the car look wider).

Far more significant is the addition of an engine that at first glance seems utterly wrong - a diesel. How does the rakish look and playboy persona of the big coupe fit with a fuel associated with economy? Being frugal has never been cool. "Who on earth will want a diesel 6 Series?" we asked. "Everyone", they said.

The 635d uses, and get your God-voice ready, The World's Most Powerful 3.0 Diesel Engine. Good, that helps save some face down the pub. It produces 286hp, which interestingly enough is exactly the same amount of power that the mid-80s M635i produced. Except TWMPDE has roughly twice the economy. Granted, 'Twenty-year old performance with half the bills' may not be the ***iest strapline, but it's a very tempting one, and a sensible package.

In fact, it makes so much sense that it's almost more of a wonder what's taken BMW so long. The German firm estimates that the 635d is likely to account for around two-thirds of 6-Series sales, single-handedly virtually sweeping the entry-level 630 out of the showroom altogether in the process. At the moment, the 630 accounts for over half of all sales, but that's likely to drop below 20 per cent, and the only reason it will survive is that it's effectively the only way to buy a 6 Series with a manual gearbox.

What's more, despite the seemingly straightforward logic of such a car, it's hard to think of any direct rivals for the big BMW. Other coupes such as the Mercedes SL and Jaguar XK aren't available with a diesel engine, while perhaps the closest is the Mercedes CLS 320CDI which isn't as fast or, strictly speaking, a coupe.

Does it do the job?

To put it bluntly, yes, it does the job, and then some. This car is urgent. Taut. Responsive. In no way does it feel like a traditional diesel - there's no agricultural soundtrack, and it never runs out of revs. It's quick, too: going from a standstill to 62mph takes 6.3 seconds, and like most modern BMWs, TWMPDE is limited to 155mph.

One of the joys of this engine is the absence of any turbo lag, and that's thanks to cleverly deployed twin turbochargers. One turbo is small and quick to spool up and gives the car its oomph at lower revs. The second turbo is larger, and seamlessly kicks in at higher speeds to give the car some more impetus. It's a cute trick and works incredibly effectively.

The huge amount of torque this generates - 580Nm, all of which is available from just 1750rpm - gives the car its eagerness to accelerate and means things never feel harsh. As a result, the 635d is a lovely thing to drive quickly. Get into a rhythm on a swift country road and the 635d is staggeringly good. Short of your surname being Schumacher, it's hard to imagine a faster way to cross country in any other 6-Series, including the V10-engined M6. Praise indeed.

Then, just as you've finished being amazed, you notice there's a "Sport" button. Punch it and the revs climb about 400rpm as you enter a more aggressive throttle map. Owners will probably never turn it off.

The surging engine is well matched to the firmly suspended chassis, and the 635d comes as standard with a smooth-as-silk six-speed automatic gearbox. You can use the steering-wheel paddles to change gear by pulling with fingers or pushing with thumbs, but after two seconds, when the novelty has worn off, you realise your fingers and thumbs are much better employed twirling the i-Drive controller, so just stick the gearbox in D and let it sort itself out.
Then, once you've made like a safe-cracker to find that i-Drive hides the average mpg reading under the "Navigation" menu, you can see whether the diesel engine makes for any benefits at the pumps.

BMW's favourite argument for making a diesel GT car is its range - if the engine lets you get on with long-haul grand touring rather than asking to be brimmed up every five minutes, then continent-crushing becomes a reality, not a cliche.

With a quoted combined average of 40.9mpg, theoretically TWMPDE can go for 629 miles between fills. That's London to Switzerland on a tank - not just continent-crushing, but bladder-bursting too. Assume you average 50mph: that's 12 hours in the saddle, which would be uncomfortable, unsafe and, err, unwise.

Luckily, that situation would never arise. The 635d is such an enjoyable thing to thrash that you'll probably average nearer 20mpg, not 40mpg. (Of course, this brings its own problems: now you'll only get from London to Belgium, and therefore run the risk of dying from boredom before finding a garage in which to replenish.)

Nevertheless, that 40.9mpg average fuel economy is mighty impressive, more than the petrol-engined 630i (at 35.8mpg) and double that of the flagship M6 (19.1mpg). Better yet, the 635d is faster, cleaner and more economical than the Mercedes CLS 320CDI and, aside from being just pipped in the 0 to 60mph sprint (at 6.2 seconds), the same applies against the Jaguar XK. Driving the 635d isn't just a case of having your cake and eating it, it's like supermarket sweep in the whole bakery.

Should I give it garage space?

Forget the irrelevancies about range, lose your diesel snobbery, and appreciate this car as a handsome (and possibly wider-looking) coupe with a charismatic engine. You may find the similarly performing 630i petrol, which is only about 5mpg thirstier and £4,600 cheaper, a tempting package. But, the characteristics of TWMPDE make the 635d a more entertaining car to drive, and therefore a much better proposition.
Plus, without any current rivals, the latest addition to the 6 Series range may be alone, but it surely won't be long before others are following suit when they see the likely sales rise as a result. A staggeringly good all-rounder, it's our favourite BMW 6-Series.

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