Full Test: 2008 BMW M3 Convertible With M DCT

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We're on final approach into a tight corner, and our left index finger stretches for the minus paddle behind the 2008 BMW M3 Convertible's small, fat-rimmed steering wheel. All we want is a quick, rev-matched, 3-2 downshift from the car's seven-speed dual-clutch transmission.

Our downshift arrives in a violent, noisy spasm, a come-on to every sport biker in the canyon. Yet it still feels like one of the most precise gearchanges we've ever been able to take credit for. It also makes the BMW M3's 4.0-liter V8 very happy. Back down in 2nd gear, the free-revving engine howls to the bikers, and even through gusts of wind and gnats, we hear it tell us how much it hates neutral throttle. If you're not working this V8, you're killing it softly.

Messy emotions are inevitable when you're driving a hardtop-convertible version of a legendary performance car, as the thrill of a sunburn on a summer day has a way of obscuring unfortunate realities like excess curb weight and reduced structural rigidity.

But there's nothing messy or emotional about BMW's new dual-clutch gearbox (abbreviated as M DCT, or M DKG if you're German), which is all business in all of its 11 shift modes. It's also such an effective replacement for the 2008 BMW M3's conventional manual transmission that we wonder if it's not a greater threat to M-division purists than a bulky retractable hardtop.

Let's Doppelkupplung
Perhaps "threat" is too strong a word. But unlike the E46 M3's SMG transmission, the dual-clutch transmission in the 2008 BMW M3 Convertible weans you off the third pedal with disturbing ease.

It works like the dual-clutch gearboxes in the Nissan GT-R, Mitsubishi Lancer Evolution MR and Volkswagen R32. But BMW's transmission is as smooth as a real automatic in city traffic, and with five settings for shift speed available in "D," the transition from one gear to another doesn't have to be aggressive unless you want it to be. The M DCT even allows you to creep a bit while parking your M3.

And while the M3's dual-clutch gearbox separates you from the physical involvement that comes with shifting a true manual transmission, it denies you none of the drivetrain performance. Shifts are superhumanly quick and sledgehammer firm in the sequential M5 and M6 modes, and the ergonomically designed shift paddles (upshifts on the right and downshifts on the left) are always in reach if you keep your hands near nine and three on the steering wheel.

The M DCT is even geared like a real manual transmission, with a short-ratio 4.78 1st gear (compared to the 4.055 gear in the six-speed manual) and closely spaced ratios for 2nd through 6th gears. Seventh is a direct 1.00:1 gear — there's no overdrive for this dual-clutch gearbox. At the same time, the DCT car has a taller final-drive ratio than the manual-transmission car (3.154 versus 3.846), so the effective rpm at cruising speed is much the same.

These numbers tell us that an M3 with the dual-clutch transmission should be quick. But the DCT won't be available on the coupe or sedan until 2009, and this 2008 BMW M3 Convertible outweighs those cars by an average of 500 pounds. With its complete plate of options, our $81,970 test car also takes M3 pricing to a whole new level. The M5, you'll note, starts at $83,900.

It Gets Respect; Does It Deserve It?
Evidently, word hasn't yet spread about the hardtop M3 convertible's weight problem, because in two weeks of driving, we get only two challenges to our accelerative authority and both come from drivers in supercars — a Ford GT and a Ferrari F430.

We can keep the ruse going in freeway traffic, but it's a good thing we don't have to face them on the drag strip. Here, the DCT M3 convertible hangs with the manual-shift M3 sedan to 60 mph with a 4.8-second run (or 4.6 seconds with 1 foot of rollout like on a drag strip). The M3 coupe was 0.2 second quicker.

Our M3 convertible doesn't lose much ground through the quarter-mile, which it dispatches in 13.0 seconds compared to 12.9 in the sedan and 12.7 in the coupe. The convertible's 107.9-mph quarter-mile speed is much slower, though, as the M3 sedan hits 111 mph while the coupe is at 112 mph. Were this hypothetical drag race to continue beyond the quarter-mile, the M3 convertible would eventually fall farther behind. Even less flattering is the realization that we've timed a BMW 335i coupe with an automatic transmission at 13.3 seconds at 105.9 mph.

The drivetrain is not the problem here, as the convertible's 4.0-liter V8 is tuned identically to the other M3s and offers 414 horsepower at 8,300 rpm and 295 pound-feet of torque at 3,900 rpm. Further, BMW's internal performance estimates suggest a DCT-equipped M3 convertible has a couple tenths of a second advantage over a manual-shift version. So we think our test car's 4,140-pound curb weight is the issue here, which also drags fuel economy down to a 15-mpg average over 800 miles.

Launch Control
Depending on the weather and your own level of patience, you can also blame the performance on the dual-clutch transmission's launch control feature. Launch control is the key to the quickest acceleration numbers, but we discovered the learning curve is steep.

Here's what you do. First, make sure the stability control is completely off, as this unlocks the fastest M6 shift mode. With your left foot on the brake pedal, press the console shift lever forward. Hold it there and a checkered flag appears in the instrument panel, indicating launch control is active. Continue holding the shift lever and press the gas pedal to the floor, which revs the engine to 5,500 rpm. Now with your left hand, adjust the launch speed in increments of 100 rpm by pressing the cruise control stalk forward or back. You can't launch any lower than 5,000 rpm or any higher than 6,000.

Now comes the delicate part: Release the brake, wait a couple nanoseconds, and then release the shift lever. This sounds easy, but the combination of foot and hand motions is counterintuitive, and there's an initial tendency to pull both feet out, cancelling the launch. Get it right, though, and you feel like you've been fired from a gun turret. Keep the gas pedal pinned and the transmission upshifts automatically (and abusively) at the 8,400-rpm redline.

Cool, huh? But as discovered in our testing, hot weather seems to make the launch control behave erratically, so sometimes the 265/35ZR19 Michelin Pilot Sport PS2 rear meats spin until they melt, and sometimes the gearbox fails to upshift. And sometimes, particularly if you've already launched the M3 a couple times, it won't work at all.

This is normal, BMW officials tell us, as company engineers set very specific operating thresholds to protect the driveline against damage. If any of the engine, transmission and differential fluids are deemed too hot, the DCT locks you out — or does its best to discourage you. This isn't wholly unreasonable, but in practice, this is the most finicky of all the launch-control setups we've tested.

And Then We Cut Loose on the Back Roads
Fortunately, there are other things you can do with a 2008 BMW M3 Convertible. In contrast to other M3s, none of the electronic damper settings on the convertible are too firm for public-road use, so it's easier to talk people into riding along with you — an indication of our baser human nature, but why buy a four-place convertible if you're going to drive around by yourself?

Start adding some speed through corners and you shouldn't get any complaints from your passenger. The M3 convertible is as balanced and athletic as you'd ever expect a 2-ton BMW without a top to be. It doesn't feel quite as structurally rigid as we'd like, but overall, it's quite pleasant to hustle through turns.

And since the folding metal top distributes some 54 percent of the M3 convertible's weight over the rear wheels, you're rarely more than seconds away from a powerslide. Given free range on a road course, the M3 convertible spends most of its time sideways. If you have $82K to spend on a drift car, it should be perfect.

But if slalom and skid pad numbers are important to you, the 2008 BMW M3 Convertible will seem highly imperfect. Its 67.8-mph slalom speed and 0.86g on the skid pad fall well short of the M3 sedan (71.8 mph; 0.93g) and M3 coupe (73.3 mph; 0.95g), even though it wears the same tire model and size. Worse, it's also significantly less agile than the civilian-spec 335i coupe (68.4 mph; 0.88g). Curb weight and softer suspension tuning (compared to the other M3s) are in play here, and you either accept the sacrifice or you don't.

Braking, at least, forces less of a compromise. Our M3 convertible's 108-foot stopping distance from 60 mph is a bit longer than the others (104 feet for the M3 sedan; 100 feet even for the coupe), but pedal feel is excellent and the brakes work well when they're hot.

The M3 You'll Love To Hate and Hate To Love
On a gut level, you want to accept the 2008 BMW M3 Convertible's limitations, because it delivers so many of the right sensations — the insistent tone of the workaholic V8, the exhaust pop on downshifts, the beads of sweat that form on your sunburned forehead.

Yet, for all the benefits of its retractable hardtop, this M3 is too heavy for its own good, so its handling performance numbers simply aren't those of a serious performance car. And when the asking price balloons past $80,000, they really need to be.

But we suspect BMW knows exactly how to play this hand. Hard-core types like us will stick to the M3 coupe and sedan, while this M3 convertible draws a slightly less demanding crowd. And that slick dual-clutch transmission will bring us together and eventually eradicate those evil three-pedal cars altogether.

The manufacturer provided Edmunds this vehicle for the purposes of evaluation.