BMW M 6 Convertible, Mercedes SL 63 AMG, Front view 31 Photos Zoom

BMW M6 Convertible vs. Mercedes SL 63 AMG: Open-air turbos with 567 and 577 HP

Both cars love fresh air cruises on country roads and low altitude flights on the motorway at 300 km/h. But what can the BMW M6 Convertible and Mercedes SL 63 AMG muster on the race track?

Theory and practice are sometimes as close together as Untertürkheim and Shanghai. "Which test is up next?" The Mercedes SL 63 AMG with 577 HP lines up against the BMW M6 Convertible with the 567 HP Competition Package. The result of the photo op: surely a front page image with burnout. So much for the theory.

The BMW M6 Convertible fights to resist burnout

Two hours later, this is put into practice on a deserted country road. First attempt in tye BMW M6 Convertible, disable the DSC. And while the Bavarian supposedly sets its electronic limitation to the side, the photographer gets into position. Hold the brakes, give it full throttle and slowly release the brakes – this is the typical formula for success for smoking tyres.

And in the BMW M6 Convertible? Even with the DSC disabled, its electronics resist. Driving with the brakes applied and thus causing the rear wheels to spin: impossible. And without the brakes? Even when ruthlessly accelerating, the mechanical grip is so great that the rear tyres hardly slip at all. The result: an unspectacular plume of smoke.

As our photographer crouches bewildered in the roadside ditch, the driver climbs frustrated from the BMW M6 and into the Mercedes SL 63 AMG. Here too, with the "ESP off" setting applied, the transmission electronics know just "either" and "or" – either braking or driving. Wild, smoking burnout orgies, à la Mustang Shelby – no chance. The sad, modern age of electronics.

Mercedes SL 63 AMG paints a black 50-metre autograph on the asphalt

So back to the editorial office without a burnout image? No, luckily numerous Youtube videos reveal a button combination by means of which the Mercedes SL 63 AMG can be switched to test stand mode, via a hidden menu. Klick, klick, klick, confirm "roll test" in the instrument cluster – ESP and ABS are now completely disabled. The 63 mutates into an unfiltered muscle car.

Hold the brakes, full throttle, slowly release the brakes – wafts of smoke finally bellow from the wheel housings, suddenly the Conti Sport Contact aromais in the air and the Mercedes SL 63 AMG paints its black 50-metre autograph on the asphalt. Be careful, dear boys amongst men, the menu was not actually designed for this type of thing! It goes without saying that we shot the burnout right at the end of our measurement and test procedure. No other test this year has been as long-winded as the comparison between the BMW M6 Convertible and the Mercedes SL 63 AMG Roadster. Which brings us back to the subject of "theory and practice" mentioned at the start.

The two open-top sports cars originally lined up for the comparison test back in July. With an external temperature of 27 degrees Celsius, we initially took to our test airfield in Lahr to determine the standard measurement values. First it was up to the BMW M6 Convertible to flex its muscles. The optional Competition Package (additional charge of 8,500 Euros), in addition to the power increase of 14 HP, also includes chassis modifications with tighter springs, dampers and stabilisers. In addition to this, the M-differential with electronically controlled multi-disc locks in conjunction with the Competition Package receives modified control tuning. Furthermore, the characteristic curve of the hydraulic power steering has been more directly tuned.

A power boost for the BMW M6 Convertible and SL 63

Even though the Competition Package is actually intended to improve primarily the drive dynamics, M GmbH also promises improved sprint qualities. The BMW M6 Convertible is reported to manage the sprint from 0 to 100 and 200 km/h 0.1 seconds and 0.2 seconds faster respectively. At 4.3 and 13.3 seconds, the reinforced M6 Convertible flew from 0 to 100 two tenths faster than the already tested M6 Convertible without the optional sport package (sport auto8/2012). In the sprint to 200 km/h the Competition version even managed an improvement of 0.9 seconds.

And what can the Mercedes SL 63 AMG bring to the comparison test? Since June 2014, the 5.5-litre biturbo in all SL 63 models, known internally as the M157 has produced a standardised 577 HP. The version with 530 HP has been discontinued, as has the enhanced version with the Performance Package (556 HP). Dynamics fans should find not only the power increase to be more interesting than the new 2Look Edition equipment line, with more contrasting paintwork than our test car in Designo Cashmere White Magno, but also the limited slip differential, which is also now part of the standard equipment.

In the acceleraetion measurement, the additional 20 HP compared to the last tested R231 series Mercedes SL 63 AMG (sport auto9/2012) was only marginally evident: the current Power-SL accelerated to 100 km/h one tenth of a second faster and was 0.3 seconds quicker when accelerating to 200 km/h.

Deceleration on a similar level

The SL braking system showed considerable improvement. While the last test car, which was fitted with a steel braking system, weakened slightly when decelerating from 100 km/h (braking distance of 39.4 metres), the current test rocket with optional ceramic brakes (added cost of 8,270 Euros) impressed with respectable values (36.7 m). This time there could be no talk of fading or other symptoms of failure. The BMW M6, likewise fitted with the M-Carbon Ceramic Braking System, also subject to charge (8,800 Euros) with the Competition Package delivered similar deceleration values (36.90 m).

Back to the present, on our country road. In the comparison test, the BMW M6 Convertible electronically lowered its fabric hat in 19 seconds, while the Mercedes SL 63 AMG Roadster opens its electro-hydraulic folding roof with panoramic glass surface in the same time (added cost of 2,550 Euros). Long, drawn out country road bends mingled with straights await – a menu that is to the tastes of both power convertibles.

Roof down, enjoy the sound: while the V8 biturbo from BMW hums with somewhat artificial bass, its counterpart from AMG sounds more robust. However, the biturbos do not celebrate the same naturally aspirated carnival as the early M6 and SL-63 engines.

The ESP light flickers in the BMW M6 Convertible

Whether the sound is to your liking or not, even the fresh-air sports cars today confuse all country road straights with the Döttinger Höhe. The BMW M6 Convertible, in the fastest of the three gearshift programmes, changes between the gears of its seven-speed dual clutch transmission more snappily and responds more nimbly to the gear change commands of the steering wheel rocker switch than the seven-speed automatic transmission referred to as the AMG Speedshift MCT in the Mercedes SL 63 AMG.

In the comparison test, the maximum 900 Nm in the Mercedes line up against the maximum of 680 Nm in the BMW. With activated driving assistant, the SL 63 is more chivalrous as it hammers its hefty torque out on the country asphalt. Or to put it another way: on bumps the SL control system is not quite as obvious as the BMW M6 Convertible.

It is unclear how often the SL actually unleashes its full power, but nevertheless, the nervous, yellow blinking ESP light only irritates you on rare occasions. Regardless of whether on the ateral joints of the motorway, or bumpy country roads: the ESP light in the BMW M6 Convertible flickers on uneven road surfaces like an advertising board in New York's Time Square. In so doing, the BMW noticeably holds back with regard to power.

Now, from the early Winter country road mess-about, back to the hard facts. On 23 Juli the BMW M6 Convertible with Competition Package and the SL 63 AMG met for the first time in Hockenheim. At 2,027 Kilo (M6) and 1,847 kilos (SL), both the BMW (minus 20 kg) and the Mercedes (minus 28 kg) weigh less than both previous test cars, but with these weight specifications one thing is immediately clear: both convertibles will meet much more often in race tracks' VIP car parks than on the track itself.

The BMW M6 Convertible rounds the course in 1:14.7 minutes

Even if the weight is present at all times in the limit range, both big fish do very well on the race track. Important to mention: on 23 July the outdoor temperatures were similar to the climate inside the hot stone oven of a Hockenheim pizzeria. The instrument cluster of the BMW M6 registered 35 degrees Celsius in the comparison test, whereas the temperatures on the asphalt were in excess of 50 degrees.

But even after the fast lap of the short circuit, there are many positive notes on the M6 test car card: very well-balanced, great grip at the front and rear axles, astonishingly neutral handling, steering in Sport Plus has honest feedback and strong holding forces, ABS regulation okay, transmission shifts quickly and engages gears without switching delays. With a lap time of 1:14.7 minutes, the Competition M6 is 0.7 seconds faster than the "normal" convertible with 552 HP.

Whereas the V8 biturbo in the BMW could cope well with the extreme temperatures, the SL engine somewhat ran out of breath on the track. As the data records later showed in the lap time comparison, the thrust at 150 km/h was not quite as strong as in cooler conditions. Did it experience thermal problems and thus reduce the engine power as a precaution? From a subjective perspective, it would appear so. After a lap that the Mercedes SL 63 AMG didn't manage to complete in under 1:14 minutes, we broke short our trip to Hockenheim and sent the V8 biturbo back to Affalterbach for a technical check. According to AMG, however, the diagnostic device did not report any problems.

Bad luck for the BMW M6 Convertible

A second test date for the lap times was scheduled for the end of August. For a fair comparison, both vehicles should once again have the chance to drive a fast lap in somewhat cooler conditions. While the SL 63 reached the Hockenheim Ring free of damage, the BMW M6 Convertible, through no fault of its own, landed with a radiator defect. A piece of debris lying on the motorway, which was cast off by the vehicle travelling in front, unfortunately bored into the grille of the BMW. And a simultaneous lap time dual was now off the cards. There it is again, the matter of "theory and practice"...

The SL 63 AMG ran its laps of the short circuit alone. At a temperature of 26 degrees, the V8 biturbo now approached the task with a great deal more dedication. Not only is the SL sitting position deeper than in the M6, but the centre of gravity of the Swabian two-seater also appears to be lower down. The Mercedes SL 63 AMG makes very good use of its weight advantage of 180 kilos. With the optional AMG Performance Chassis, including 30 percent firmer dampers, it moves more light-footed over the race track (if you actually dare to use the term light-footed in referred to a car weighing 1,847 kg), steers more directly, does not push as much when braking and scores points with its surprisingly good traction under load.

The feedback from the steering is precise, but a little too smooth. In direct comparison with the taut M6 steering, the SL steering seems a little synthetic. While the ceramic braking system impressed in Hockenheim with solid deceleration values of up to 11.5 m/s2, the Conti tyres limited the drive in the limit range. The SL 63 put down its fastest lap time of 1:13.1 minutes in the first timed lap. After that the level of grip over the next three laps of the short circuit noticeably reduced in the comparison test. Not to be forgotten: at 26 degrees Celsius the external temperatures were still hot.

No more chances for the M6 and SL 63 AMG

The feeling is that in cooler weather, faster lap times would be possible in both vehicles. The test ambition, to drive both vehicles in comparable temperatures in Hockenheim, resulted in new test cars being ordered . On 27 October, at 14 degrees, it seemed like the perfect moment had come for a lap time dual between the Mercedes SL 63 and BMW M6.

But then there is also the issue of "track availability at the Hockenheim Ring". An external events agnecy organised a one-week training event on the Baden Formula 1 track for BMW Motorsport at the same time as the third M6 and SL 63 test car trial. Normally we could use the one-hour lunch break to record the lap times, but this time the training event organisers were stubborn. Both the SL 63 and the M6 Convertible were turned away and did not have the chance to improve their lap times.

This is why we spoke so lengthily about "theory and practice" in everydaysport autotest driving and why we took such great pride in capturing the perfect burnout shot in the photo shoot shortly before the end of the test.

Christian Gebhardt

Photo

Achim Hartmann

Date

6 May 2015
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