Test date 12 October 2007 Price when new £56,110

What is it?
BMW’s big coupe, now fitted with the 3.0-litre twin-turbo diesel engine. Given the praise that has been heaped upon each and every model to receive this excellent engine, it hardly comes as a surprise to discover that the award-winning unit has now found its way into the subtly facelifted 6-series.

What's it like?
As you slot the electronic key into the dashboard for the first time and hear the raw (if distant) chatter of a diesel, you can’t help but wonder whether the new 635d is such a good idea. Not for very long, though. One determined stab of the throttle is all it takes to wipe away any doubts.
The 635d comprehensively outguns the petrol-engined 630i and can scare even the V8 650i on occasion. On top of this, its combined average fuel consumption of 41mpg allows you to enjoy the performance without it threatening to empty your wallet, while giving it a range of more than 600 miles.
It is all achieved through a complex induction system that employs two turbochargers of differing size. The result is a specific output of 96bhp per litre, giving the 635d a generous 286bhp at 4400rpm. But it’s the torque that really steals the show; there’s 427lb ft at just 1750rpm, and 369lb ft of that can be tapped from as low as 1250rpm.
BMW’s six-speed manual gearbox can’t cope with all the twist action. That leaves the latest six-speed automatic gearbox – with a fiddly joystick-style shift lever, remote shift paddles that move with the steering wheel and a different set of ratios to those found in petrol-powered versions of the 6-series – to channel it all back to the rear wheels.
BMW claims 0-62mph in 6.3sec, a good 0.4sec inside the time quoted for the 630i. However, the standing-start performance pales next to the 635d’s stomping in-gear ability. It gathers speed in truly impressive fashion; response is even better in the mid-range than down low, at which point the muscular-looking coupé is delivering the sort of acceleration to convince even the most ardent petrol fan that diesel motoring really has come of age.
Subjectively the 635d feels faster than the six-speed manual 650i’s 50-75mph fourth-gear split of 4.9sec. Much faster.
There’s more to the way the 635d drives than just its vast urge, however. The engine provides a wonderfully linear delivery of power from idle all the way to the red line. In this respect it is quite entertaining, the revs rising and falling with greater urgency than many comparable diesels.
While lacking the inherent refinement of its petrol-engined siblings, the diesel’s growl is also particularly well suited to the 6-series’ bullish character.

Should I buy one?
Yes. The 635d, more than any other car, is the latest example of the potential of diesel. Endowed with an unparalleled blend of performance and frugality, it very nearly renders the BMW coupé’s more traditional petrol engines obsolete, without quite managing it. But it does provide the facelifted 6-series with a worthy new addition that is sure to extend its appeal.
First drive data

How much?
Price when new £56,110
Price as tested £53,910
How fast?
0-62 mph 6.3 sec
Max speed 155 mph
How big?
Weight 1580 kg
How thirsty?
Combined 40.9 mpg
CO2 emissions 183 g/km
Engine
Layout 6 , 2993 cc
Max power 290 bhp
Max torque 428 lb ft at 1750 rpm

Greg Kable
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Sounds almost like a match made in heaven.

A very positive review for the 635d!! That's great...diesel engines become more and more popular.

?

What? small turbo for low revs , big turbo for higher revs...what else is new?

i have the same engine in one of my cars(E90 335d) i had it remapped and its unbelievable. i think it will defo boost 6 series sales.

Nothing, I just thought they were of the same size.

That must be the petrol 335i then.

It's the same, klier...

For the sake of clarity, the twin-turbo installations in the six cylinder 3.0i and 3.0d engines differ as follows.

In the 3.0i Petrol the turbos are identical, light pressure units mounted in parallel, meaning that one turbo is fed independently from 3 cylinders and the other is fed, also via a separate exhaust manifold, by the other three cylinders. Neither cylinder has any influence on the level of boost created by the other. It is a charmingly simple approach with devastating results.



In the 3.0d Diesel powered engines, the turbo's are mounted sequentially off a single exhaust manifold fed by all six cylinders. The arrangement is complex and requires some deft electronic control. At low rpm, the engine only produces enough volumetric flow to speed the turbine of the low pressure turbo. This turbo, as it spools up, supplies pressurised air to the engine which then produces greater volumetric flow. This causes the larger turbo to spool up and deliver the desired amount of boost pressure at higher rpm. The interaction between the two turbos is electronically controlled. This enables this 3.0d engine to push out a superlative 200 kW for its displacement and enables it to rev to an unheard-of-for-a-diesel 5000 rpm.

Here is a cut-away image of the sequential turbo installation.

Thanks a bunch, Martin for clarifying this. I was wrong obviously...