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BMW has finally added a diesel to the 6-Series range. We compare it to the oil-burning CLS to see which makes the better grand tourer
In theory, fitting a luxury coupé with a diesel makes sense – so why has it taken upmarket brands so long to cotton on to this? After all, modern oil-burners offer excellent refinement, strong performance and good economy – ideal for cars designed to cover long distances.
More affordable coupés have had success breaking into this market, not least the Peugeot 407 Coupé and Alfa Romeo Brera, but for some reason BMW has resisted producing an oil-burning super-cruiser, despite having one of the best diesels in the business.
But now, four years after the 6-Series was introduced, it’s been fitted with the popular twin-turbo 3.0-litre straight-six. With 0-60mph in around six seconds, it has genuine performance credentials, plus there’s a claimed combined economy figure of over 40mpg.
However there’s more to a grand tourer than a great engine. Comfort and refinement are two key aspects, and that’s why we’re putting the BMW up against the Mercedes CLS 320 CDI. We’re big fans of the Merc, as it’s distinctively styled, comfortable and superbly engineered.
So which of these two superdiesels makes the better buy? Read on as we reveal all...
Mercedes CLS 320 CDI
At first glance, it’s not immediately apparent how the CLS fits into the Mercedes line-up.
Its four doors and E-Class underpinnings suggest it has plenty in common with the firm’s executive and luxury saloons, but the coupé bodystyle hints at an affinity with the CLK and CL models.
In fact, you may ask why we didn’t test the 635d against the latter, given that they are both luxury two-doors. The reason is simple – not only is the Merc considerably bigger and more expensive, but there’s no diesel engine available, so it’s difficult to see it as a genuine rival.
Compared to Mercedes’ latest offerings, such as the C and S-Class, the CLS doesn’t quite fit in. But in terms of dramatic styling, there’s little that can rival it. It’s not as overtly sporting as the 6-Series, but its low-slung, curvy profile gives it a classic feel. The optional multi-spoke alloy wheels fitted to our test car are a letdown, though. They’re too fussy and at £1,530 aren’t cheap, either.
It’s difficult to find fault with the cabin, though. As with the rest of the car, the cockpit isn’t quite as driver focused as the BMW’s, but it’s extremely comfortable, thanks to the sumptuous chairs – although the driver’s seat could do with being mounted slightly lower.
The cabin has more character than the 6-Series’, and although some of the switchgear doesn’t feel quite as solid, the general build quality is top-notch. Passengers in the back will find it much easier to get on with, too.
Admittedly, rear access isn’t completely straightforward due to the low doors and roofline, but in terms of legroom and space, it’s way ahead of the 635d. For cross-continent trips, the CLS is ideal.
Indeed, comfort is the Merc’s forte. It feels superbly refined, and the same can be said of the 3.0-litre diesel engine. The powerplant loses out to the BMW in terms of power and torque, but its delivery is silky smooth. It couldn’t match its rival at the test track, although a 0-60mph time of 7.0 seconds is still quick, especially considering the car’s extra kerbweight, and in kickdown it proved just as rapid. However, the Merc will hit you in the pocket when it comes to fuel and tax bills.
On the open road, the CLS gains speed quickly and quietly, while the seven-speed auto offers immaculately slick changes – although if you want the optional paddleshifters, you’ll have to shell out an extra £190. The V6 does without the slight gruffness that occasionally spoils the BMW’s air of refinement, while its lack of power means it’s not as responsive on country lanes.
Similarly it lacks the agility of its rival, but that’s not to say the CLS isn’t sharp and composed. Body control could be better, but the steering offers decent feel and the ride is among the best we’ve experienced. That makes the Merc a superb cruiser and contributes to a relaxed feel that allows you to effortlessly cover hundreds of miles – not something that can be said of the 635d with its firmer suspension.
And compared to the BMW, the CLS is relatively good value for money. It undercuts its rival by a hefty £12,000, which leaves you plenty spare to spend on options. But then you’ll need it if you want a similar standard of kit to the 635d. However, even with £9,000 worth of options, including sat-nav, xenon headlights, leather upholstery and Keyless Go, our test car still worked out cheaper than the pricey 6-Series.
BMW 635d Sport
Given that BMW expects the 635d to account for nearly three-quarters of 6-Series sales in the UK, it’s a real surprise it has taken the firm so long to develop this variant.
But with the M3, X5, 1-Series coupé, facelifted 5-Series and several other models launched in the last 12 months, the company has certainly had its hands full.
Yet even among that line-up, the 635d stands out. Our test car was a Sport model fitted with optional alloy wheels, which give it a hint of M6, while the Deep Sea Blue paintwork looks fantastic. There was plenty of controversy when the model was first launched, but now the novelty has worn off, it’s easy to appreciate what a well styled car it is. It’s not exactly pretty, but the aggressive and distinctive look of the two-door gives the CLS a run for its money in terms of visual impact. It’s also more likely to appeal to younger buyers than the Mercedes.
However, customers with a family in tow will struggle when it comes to practicality. The two-door body means access to the rear is tricky and the seats themselves are cramped, with both passengers locked in place by the bulky transmission tunnel and limited headroom. The BMW is only 100mm shorter than the CLS, yet from the back seats it feels much smaller.
The 635d is much better suited to accommodating only two people – up front the cabin is spacious and the electrically adjustable, figure-hugging seats, which are standard on the high-spec Sport, offer excellent support and comfort. Finding a good driving position is simple.
As we’ve come to expect on BMW models, build quality is immaculate and the interior is neatly laid out – but there are gripes. There’s a lack of stowage, especially on the centre console, and the iDrive cabin control system often turns a straightforward operation into a long-winded process.
However, eight programmable buttons on the dash make things easier.
The 3.0-litre straight-six is hard to fault, though. Thanks to its twin turbochargers, it offers even more torque than the 507bhp V10 in the M6, and has an impressive combination of power and economy – although during its stint with us it returned a little over 30mpg, which is not as high as we’d hoped.
There was no arguing with its performance at the test track, as the 635d completed the 0-60mph sprint in a mightily impressive 5.7 seconds. Out on the road, it feels the quicker car too, especially in kickdown. However, it’s the effortless nature with which it gains speed that really impresses, while the engine remains unruffled even when pushed hard.
Through corners, the BMW continues to impress. There’s lots of grip, the steering has good feedback and the 6-Series is composed. It’s more involving than the CLS, but the pay-off is a firm ride and a surprising amount of road noise. This lets the coupé down and it’s not comfortable enough to be a true GT – this could be down to the run-flat tyres, which are standard on all BMWs.
The price is also likely to sting. At more than £56,000, the 635d is expensive, especially when compared to the CLS. The BMW is well equipped, though, with parking sensors, leather and xenon lights as standard – these are all options on the Merc. But our test car still had £8,500 of extras fitted, including the £2,000 navigation system and active cruise control at £1,265.
VERDICT
The last time we put these two cars head-to-head was in petrol form – and it was the Mercedes that came out on top then. So, do the diesel engines have a bearing on the outcome this time? Well, it’s hard not to be impressed by the 635d – it’s certainly the best engine in the model line-up, delivering an addictive blend of power and economy.
That’s not to say the V6 unit in the Merc isn’t just as impressive. It’s not as potent, but it’s superbly refined. The four-door set-up means that the CLS is also more practical and has the bigger boot, while the ride quality is nothing short of astonishing.
However, although grand tourers are all about luxury and comfort, there also needs to be an element of driver appeal – and that’s where the 635d wins back some ground. The ride is firm, yet the car offers endless grip and an agility that is missing in its rival.
Ultimately, it boils down to what you want from your coupé – performance, economy and involving handling, or outstanding comfort. Both are excellent choices – but the Merc’s big price advantage gives it the nod.
1st Mercedes CLS 320 CDI
2nd BMW 635d Sport
Mercedes CLS vs BMW 6-Series | Car Group Tests | Car Reviews | Auto Express
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