2007 BMW M6 Convertible vs. 2007 Mercedes-Benz SL65 AMG

It's not easy to compare the 2007 BMW M6 Convertible and the 2007 Mercedes-Benz SL65 AMG. The whole idea teeters on the brink of the ridiculous.

How relevant is a comparison of a $108,795 BMW M6 to a $189,375 Mercedes SL? With about a thousand M6 convertibles and less than half as many SL65 AMGs expected to arrive this year, the pool of potential buyers won't get the soles of your feet wet.

The financing is certainly ridiculous. On a typical 60-month car loan, reduced by a generous $20,000 down payment, the monthly payment on a base SL65 (without any options) would be $3,812. By the end of those five years, the well-to-do SL65 AMG owner will have laid down $250,000 total.

And yet the 2007 BMW M6 Convertible and the 2007 Mercedes-Benz SL65 AMG offer such a sublime level of performance that they will fascinate anyone more likely to spend such money on real estate, not a car.

The age of reason?
For a little perspective, let's consult Thomas Paine, that great 18th-century American, who notes in The Age of Reason, "The sublime and the ridiculous are often so nearly related that it is difficult to class them separately. One step above the sublime makes the ridiculous, and one step above the ridiculous makes the sublime again."

So we couldn't pass up the opportunity to pit the 500-horsepower M6 soft top against the 604-hp retractable-hardtop SL.

These are power numbers that only a few short years ago would have seemed appropriate only for a racing car, and indeed a number of international racing series for sports cars restrict engine output to far less. But thanks to the miracles of electronics, such power has now become a proposition for the street.

If this isn't a tribute to the razor-thin margin between the sublime and the ridiculous, what is?

Bits of earth slightly rearranged to good effect
Consider for a moment that the engines powering the BMW M6 and Mercedes-Benz SL65 AMG are essentially made of minerals dug from the belly of the earth, smelted, cast, machined and assembled into the complex air pumps, no different from engines created by other automotive manufacturers.

But in detail, these engines are virtually hand-built by specialty divisions, the highest art produced by the most sophisticated engineering departments of their respective manufacturers. At BMW M and Mercedes-Benz AMG, performance is paramount and fuel economy be damned. It's no wonder M engines produce amazing amounts of horsepower and torque.

The M6 features a sublime, high-revving, 500-hp, 5.0-liter V10 engineered as a tribute to BMW's involvement in Formula 1 racing. We've written pages about this award-winning engine's elastic power delivery over a wide range of rpm, its elevated 8,250-rpm redline and its intoxicating exhaust note.

All the accolades still apply to the M6 convertible's V10, but now this engine is even more apt to kill your fuel economy because the unique yowl encourages full-throttle driving even more directly once the convertible's fabric top is peeled back. To illustrate, our average fuel consumption over two weeks was just 12.5 mpg, which explains this car's $3,000 gas-guzzler tax.

Since the 493-hp, 6.0-liter Mercedes-Benz V12 is merely sublime, AMG took it a step further in 2005 with a twin-turbo version that makes 604 hp. AMG actually detuned this engine from its peak of almost 900 pound-feet of unrestrained twist because Mercedes-Benz doesn't currently build a transmission that can contain that much torque, so the engine has been limited to just 738 lb-ft.

Even at this downgraded output, the SL65 AMG produces more twisting force than any current production car — or even turbodiesel truck, thank you. For a match, you'd have to look to the 6,000-pound Maybach 57S limousine from which the brutish SL65 borrows its powertrain.

Compared to the M6's frenetic V10, the SL65's V12 makes almost twice the torque at one-third the engine speed. Even at 1,000 rpm, the biturbo V12 makes 420 lb-ft of torque, which is more than the M6's maximum of 383 lb-ft at 6,100 rpm.

This is ridiculous power, but the SL65 is actually rated at 13 mpg city and 19 mpg highway by the EPA, or 1 mpg more fuel-efficient than the M6 convertible at 12 city, 19 highway. A $2,600 guzzler tax is levied against the SL65 and we earned a combined average of 11.5 mpg in our driving.

Gifted automatic vs. mental manual
The SL65's three-mode (comfort, sport, manual) five-speed automatic is specially tuned and aptly named the AMG SpeedShift transmission.

This version of the company's once ubiquitous five-speed is used because the newer seven-speed isn't robust enough to handle the SL65's prodigious output. Yet so imperceptible are the five-speed's shifts (especially in "Sport" at full throttle) that we could not discern the shift points when we took a close look at the acceleration graph from our testing.

The SpeedShift insinuates upshifts almost as indistinguishably as a continuously variable transmission, and only the exhaust note and wavering tach needle let on that gear ratios are changing. Brilliant stuff.

In contrast, the M6's electrohydraulically actuated, sequential-shift seven-speed manual gearbox frequently seems like an exercise in the engineering ridiculous. It offers six modes for driver-activated shifting and five modes of automated shifting, so the SMG (sequential manual gearbox) obliges you to preselect your driving mood. Even the complex software (especially in auto-shift modes) can't always be in synch with the driver, the engine and the road.

SMG's defenders cite its infallibly smooth downshifts and impossibly quick upshifts, but we think you'll notice this only when you're driving at full speed. And we've discovered that it's actually possible for a driver to beat the SMG's performance on the dragstrip with a simple manual transmission.

Making tracks
In the relative safety of the test track's environs, we programmed the M6's steering-wheel-mounted M button for maximum engine, transmission and suspension performance, obviated both stability and traction controls, and probed traction limits of the M6 convertible.

If you've read a proper road test of an SMG-equipped M car, you could probably recite the acceleration test protocol: wheelspin, pedal out, grip and go. By the third run, we had matched BMW's claim of 4.6 seconds to 60 mph. The M6 went on to cover the quarter-mile in 12.9 seconds at 113 mph. A few more attempts brought similar results. With some more practice, our experience tells us we might shave another tenth, but no more. Entertaining, involving and decent results, yet not as quick as we expected.

On the other hand, the M6 brakes, while not piling up asphalt under its nose like the SL65 did, still halted the 4,400-pound convertible from 60 mph in just 114 feet. The feel of the pedal is nearly as good as it gets: nicely weighted and easy to modulate.

Hold your horses
After driving the 604-horse Mercedes SL65 responsibly in public and witnessing the ESP traction-control light blinking like a little yellow strobe, we decided to first record an ESP-aided quarter-mile run. Just as AMG claims, the maximum-strength SL easily dispatched the M6's best with a 4.2-second 0-60-mph time and a resulting quarter-mile of 12.2 seconds at 121 mph.

With these ridiculous baseline numbers in the bag, we turned off the ESP. After a couple aborted runs where excess wheelspin caused the transmission to automatically upshift to 2nd gear, we finally found the slip-angle sweet spot. We managed to deduct 3/10ths from the 0-60-mph time to a mind-blowing 3.9 seconds, although the time to the quarter-mile was just 0.1 second quicker. By then turbocharger heat had sapped power from the engine, so our trap speed was a relatively ordinary 119.4 mph.

The manner in which the SL65 accelerates makes you believe that it might not be powered by an internal-combustion engine at all. The tidal wave of torque bathes the accelerative experience as if a jet turbine were involved. It feels like acceleration piled atop acceleration at an exponential rate. Several experienced drivers on the staff reported moments of I've-never-felt-anything-like-it.

As stupidly fast as the SL65 is, we cannot hide our enthusiasm for the M6 convertible on the drag strip. The surround-sound experience was uncommonly visceral. With the optional head-up display doing its part to make you feel like a fighter pilot, each pull of the shift paddle makes you feel as if you've triggered a guided missile as the M6 barks its tires and lunges ahead with every upshift.

Be warned, though. When driven in this manner, the much touted SMG is about as smooth and subtle as a hockey hip check. Hope you like rough sports.

Sure it's fast, but can it corner?
Any sixth-grader could've guessed the 604-hp SL would be quicker than the 500-hp M6, but even we didn't see what was coming next.

Mercedes-Benz AMG cars are notoriously rapid, but they don't have a reputation for grip or poise. The SL65 is equipped with an AMG-massaged suspension that features "active body control" (ABC), which has three dynamic settings plus ride-height regulation, as well. Each corner of the car's multilink suspension is supplemented by hydraulic rams that effectively preload the entire suspension column in anticipation of turning, accelerating or braking.

In its most dynamic mode, ABC contradicts expected body motions, and the overall effect is a little unsettling. The car doesn't squat while accelerating or even dive while braking, and it actually leans into a corner rather than away from it. The results seem to defy the SL65's 4,518-pound weight. It's an experience that seems to challenge the laws of physics.

With its meaty Bridgestone tires, the SL stopped from 60 mph in a mere 110 feet and orbited the skid pad at 0.89 g. What's more, the SL blitzed the slalom course at a sports-carlike 67.4 mph, no doubt aided by its mechanical limited-slip differential.

Yet the SL doesn't feel sporting, as the combination of wooden steering feel, artificial body motions (or lack thereof) and its numb electrohydraulic brakes don't make for a rewarding experience. Even so, the SL65 does feel well-mannered and even docile on public roads.

The M6 convertible achieved a slightly slower but more enthralling 67.1-mph run through the slalom. Its variable-ratio steering is more communicative, the brakes are more easily modulated, and despite less-than-perfect suspension tuning, it does what it's told in a way that's entirely expected.

We know the M6 convertible is 482 pounds heavier and has slightly different damper settings from the carbon-top M6 we previously tested, but the convertible's center of gravity is reportedly lower. Be that as it may, the topless M6 circled our skid pad at 0.83 g to the M6 coupe's 0.89 g and only tied the slalom speed of the coupe.

The M6's three-mode electronic damper control (EDC) provides noticeably different ride characteristics by varying the shock valving, but two of the three settings do their best work only on perfectly smooth pavement. We suspect that's because the BMW varies only damping, while the Mercedes suspension maintains the relative relationship between springing and damping.

In the softest setting, the M6's EDC transforms low-frequency impacts into a bouncy and even floaty body response, as if the dampers have taken the day off and only springs were at work. In the firmest setting, the wheels sometime skip over rough pavement. We found compromise by setting EDC right in the middle, with one light on, for both everyday driving and track testing.

So while the M6 feels confident at the track, its ride isn't the best suited to cope with varying types of pavement.

Top test
Common sense would suggest a retractable-hardtop convertible should be better suited to resisting extreme temperatures, annoying roadside noise and guys with knives. True, but you might be a little surprised to learn that the SL65's top up/down operation is also quicker to accomplish than that of the M6 by about 3 seconds.

Even so, the M6 ragtop was nearly as quiet as the SL hardtop, although the blind spots were large with the M6's top in place.

The M6 convertible's rear glass is power-operated and it can be raised and lowered to function as a wind blocker, while the Mercedes wind blocker of plastic mesh must be raised by hand.

Ah, but the SL also has a trick up its sleeve. When the hardtop is put away in the trunk, a push of a button tilts the whole folded apparatus just enough to allow access to the luggage tub.

At the tape
Until we put numbers to paper, we all thought the SL65 was a one-trick pony with warp drive. And believe us, it's one trick that never gets old. Every time we gave into our devilish speed demon, we'd always be rewarded with an "Oooooh-my-gaaaaaahd!" burst of acceleration that ruptures time.

Only later did we discover that not only is the SL65 ridiculously swift, but it can also stop and corner like we expected the M6 would.

The M6's high-revving engine might have less output than the turbocharged SL's, but the BMW V10 displays a wider range of talent. The effective fun zone begins at 2,000 rpm and extends to 8,250 rpm, and there are seven gear ratios with which to play. The SL's 6,000-rpm limit means there's but 4,000 rpm in which to have fun with just five gears. In a canyon duel, we found the Mercedes shifting more often and at inopportune moments, where the BMW could hold a gear a little longer when we needed it.

When we focused our attention on features we also discovered that the less expensive BMW actually came out on top. See the "Top 10 Features" for specifics. The M6 offers more of the stuff we think an automobile of this caliber should have.

Our editors all agreed they'd prefer the SL65 AMG to the M6 convertible for their personal use, but were split on which they'd recommend to a person shopping in this elite segment. It all depends on what you're looking for.

On the one hand, the M6 invites intense involvement, with a soundtrack that'll make your knees wobble. On the other, the SL asks no more of you than $3,800 each month and an occasionally prudent right foot.

The staff was as divided as the points total indicates. In the end, the 2007 BMW M6 Convertible ekes out a 1.4-point victory over the 2007 Mercedes-Benz SL65 AMG. Hardly a resounding win for the Bavarian, but its price and content beat outright performance and our recommendations. Both cars are talented, charismatic, sublime and ridiculous.

The manufacturers provided Edmunds these vehicles for the purposes of evaluation.