Most high-end cars combine impressive refinement with power, style and charisma. But fewer add to this a unique build quality, and a frugality not unworthy of a compact diesel saloon. The diesel-engined version of the 6-series Cabrio achieves all this, while still perfectly maintaining the sound, feel and responsiveness of a true thoroughbred luxury sports car.

Hans Knol ten Bensel

Sliding behind the wheel of the 6-series Cabrio is a unique experience. The design and layout of the dashboard, the touch and "feel" of the steering wheel, levers, knobs and pedals, all this puts you in the unique and typical ambiente of a top-of-the-range BMW. Although the 6-series is a bigger car, it never feels massive or obtrusive. The driver is transported by the splendid cockpit-like atmosphere, and feels the precision and the superb build quality of the car.

Starting the twin-turbocharged diesel is with the now obligatory button, and the engine bursts smoothly into life, without a trace of the typical diesel clatter. Indeed, the engine has the third generation common rail fuel injection system, and this shows.

There is not the slightest trace of vibration, nor any hint of mechanical stress when one puts the car through its paces. The engine revs ever so smoothly, and offers even some acoustic pleasure in the mid-range rev band, culminating in a purposeful roar when the throttle is floored.

The beauty is that all this smoothness is masterfully combined with plenty of power, as the engine delivers 286hp and 580Nm of torque. The characteristics of this engine combine very well with the six speed automatic gearbox, which can be controlled manually by the opulently chromed paddles at the steering wheel. This is the new sport automatic gearbox introduced on the new 6 series, and is fitted as standard to 635d models.

One can also leave the central gearlever in the "D" position and let the box do all the the work. But the driver interface is enhanced by steering wheel-mounted gearshift paddles and a redesigned ergonomic centre-console-mounted gearlever. In addition, with enhanced gearbox electronics and hydraulics, one press of the Sport button located just behind the gearlever delivers faster shift times and sportier driving characteristics. This, in turn, also adjusts the car’s throttle mapping and Servotronic steering controls to deliver the best conditions for a more dynamic drive.

Quite frankly, we hardly felt the need to influence or assist the gearbox in its work, and just enjoyed the stream of smooth power this engine/gearbox combination delivered.

Be amazed with us about the technical refinement the engine has to offer. We can only quote what the factory has to say about it: "developments in diesel engine refinement and performance have led engineers to install BMW’s benchmark diesel engines in models previously considered sacrosanct." We can only agree here…

Autobahnmeister?

The car has wonderful all-round qualities, feeling quite in its element in endless city crawls, but truly comes into its own on the open road. The dynamic qualities of suspension, the impeccable stability and responsiveness make the BMW a delight to drive on winding roads, where the bigger car

never shows a hint of heaviness. Of course, fast bends are preferred, but the sensitive and precise steering let you also drive easily through hairpins and sharper turns. Choosing the "sport" mode still enhances this beaviour. Where the car also comes very much into its own is of course on motorways, where the car can really show its (grand) touring qualities. Wind and road noise are virtually absent, and also the engine note is very subdued up to top speed. What is truly impressive is the frugality of the car at legal cruising speeds. At a pace between 120 and 130 kph it is not rare to see the fuel consumption needle flirting with the 5 litre per 100 km mark!

The performance leaves little to be desired. The car will accelerate from 0 to 100 in a mere 6.6 seconds, and the top speed is electronically limited to 250 kilometers. The beauty lies in the effortless manner these figures are reached, adding to, or rather, being the very essence of BMW driving pleasure. As we pointed out, the power under the bonnet doesn’t compromise economy with the new 635d Convertible delivering a frugal 39.2mpg on the combined cycle. This ensures that the car can travel a theoretical continent-crushing 601 miles between refuelling The 635d emits a very responsible 190g/km.

State of the art "dynamic" technology…

This balance of performance and economy is made possible by BMW’s variable twin turbo technology which features two different-sized turbochargers operating in sequential stages depending on driver inputs.

Consistency is the key to the engine’s smooth power delivery. Despite the mechanics of two turbochargers providing variable power, they deliver a consistent 2.85bar pressure to the intercooler at all times. This ensures optimum performance delivery from just above idle through to the red line.

With the smaller turbocharger operating at low engine speeds and a larger unit working at higher speeds, BMW’s variable turbo system differentiates progressively between three operating conditions. At low engine speeds, exhaust gasses pass directly through the vanes of the small turbocharger, spinning both ends of the turbo and compressing the intake air. This is the key to eliminating turbo-lag. The result is 95 per cent of maximum torque available at just 1,500rpm.

With increasing engine speed, the small turbocharger operates at its optimum efficiency. To continue on the same performance curve as revs increase, the larger turbocharger is brought into play via a gate in the exhaust system. As throttle and speed increases, the larger unit works in tandem with the smaller one, compressing and driving more inlet air to the intercooler.

By 1,750rpm, the engine reaches its peak torque of 580Nm. The 635d’s maximum output of 286hp is achieved at 4,400rpm. In this higher rev range, the larger turbocharger is doing all the work with an induction control valve closing off the route to the smaller turbocharger, thus bypassing the potential bottleneck of the smaller unit as the volume of air increases.

We drove the car in still rather cold weather, but were surprised to see how often we could drive this thoroughbred with the hood down. The car then offers a clear glass rear windshield to prevent air turbulence from the rear, and it is very effective indeed.

Electronic assistance… keeps you on the straight and narrow.

All models in the new 6 Series range feature, as standard, a three-stage traction control system. BMW’s well-proven Dynamic Stability Control is supplemented by five additional safety features to create DSC+:

1. Brake drying gently applies the pads to the discs in wet conditions to clear a film of water and prepare the car for more effective braking.

2. Brake pre-tensioning recognises the driver’s foot coming off the accelerator in preparation for an emergency stop and readies the brakes.

3. Soft stop releases brake pressure just before the car reaches a halt in order to avoid a jolt on stopping.

4. Hill-start assistant applies the brakes for a short time when starting off on a gradient to prevent the car rolling back.

5. Brake fade compensation recognises brake temperature rises and automatically increases brake pressure to prevent any fading.

On the lever of the cruise control, one can also set the distances for the radar controlled distance safety control..  

We appreciated very much some electronic accessories installed on our test car. There is first of all the Lane Departure Warning System for enhanced driver safety. The system works at over 40mph by using a camera fitted in the front of the interior rear view mirror on the windscreen, a control unit and a new signalling system. This innovative driver assistance system monitors the distance from one or both lines on either side of the road to identify any deviations from the lane of travel. As soon as the car detects the driver is steering out of the lane without the indicator being used, it feeds the driver a discreet vibration through the steering wheel as an alert to make a correction.

Active Cruise Control with Stop & Go…

We also appreciated the Active Cruise Control with "Stop & Go." The system uses the latest radar sensors with an enlarged area of vision, and offers a choice of four distance settings to keep the car at a specific distance in front of traffic ahead, irrespective of vehicle speed. Once the speed of the car is set with the ergonomic cruise control lever at the left of the steering column, the radar sensors monitor the road. When the car detects another vehicle ahead, the system applies the brakes where necessary, even in stop-and-go traffic at very low speeds or where it reaches a standstill. Once the road is free again, Active Cruise Control with Stop & Go will re-accelerate the car to the pre-set desired speed.

Thanks to "thinking" or "intelligent" leather, which reflects the sunrays, the interior never heats up even when tha car stands in broad sunlight all day...  

The maximum braking force applied by Active Cruise Control with Stop & Go is a quite heavy four metres/sec2. Should the deceleration required to avoid the car ahead be greater, the driver is warned to intervene manually by visual and acoustic signals.

Ultimately, the driver still retains control of the car: after a stop of more than three seconds, he/she is required to apply the accelerator or press the resume button on the cruise control stalk to re-accelerate. We found this "active Stop and Go" control a true delight to use in urban traffic in early mornings and last but not least after a long day’s work…

The I-control dial lest you set up the car to your very personal wishes...  

Head-up Display: reducing driver fatigue

Our test car came also with the HUD. With this device, the driver receives information and data directly into his line of sight without the need to refocus his eyes, a common cause of fatigue when driving. For example, the driver is able to see the current vehicle speed, navigation information or Check Control messages without taking his eyes off the road, and we found it also quite useful.

‘Intelligent’ leather

As would be expected in a luxury Grand Tourer, leather coverings to the seats, centre console and door panels are standard. The convertible features BMW’s innovative SunReflective Technology as standard. First seen on the new 3 Series Convertible in Spring 2007, SunReflective leather prevents excessive heating of the seat surface when the roof is left down on a sunny day. BMW’s SunReflective technology counteracts this by reflecting the infra-red radiation in the sunlight and the roof can therefore remain retracted all day in the sun.

This unique leather covering (on seat and armrest surfaces) features ‘cool pigments’ which are embedded in the material during the production process.

The typical shape of the hood in closed position adds to the personality of the 6 series Cabrio.  

Appeal

The 6-series convertible is a magnificent GT also in this diesel version. It is very versatile, offering sublime creature comfort combined with excellent finish, that unique BMW appeal and driving pleasure. Add to this the excellent economy and you agree with us that this car has to be on you short list when you consider buying a 2+2 open Grand Tourer…

Hans Knol ten Bensel

The obligatory starting knob, and the chromed gearchange paddles at the steering wheel for those instant changes...

The "d" on the back of the car is here also synonymous for refinement...

All photos here were taken with the Sony Cyber-Shot, but good help and adjustments from Photoshop were needed...