BMW M3 Cabrio 2008 Review

Notch this down as a test of two BMW firsts: the first M-car with a retractable hardtop roof and the first to adopt a double-clutch seven-speed gearbox. Other models that embrace the technology will surely follow.

The sooner the new Getrag-sourced gearbox (M-DCT) replaces BMW's inadequate SMG 'box in the M5 and M6 the better. Unfortunately, there are no plans to use the seven-speeder during the life cycle of the current cars.

Next year's much-revised Z4 implements a folding steel roof, while the M version of the two-seater goes V8 and receives the M-DCT. The convertible 3 Series has been covered previously in Wheels (March 2007, more here), so we'll concentrate on the new trannie.

Best of all, you can forget the old SMG, as the M-DCT is a transformation: it's the best dual-clutch gearbox in my experience (though I've not driven the new 911, and the VW Group's new seven-speeder runs it close).

Only in offering six manual and five automatic modes does the new 'box follow SMG. But the M-DCT slurs between ratios when ambling and punches with a jolt in the faster speeds, if the driver exploits the M3's considerable rev range.

There is a hint, but only a hint, of clunkiness when manoeuvring in first and second gears during stop-start driving, though because the transmission allows the creep, this is mostly camouflaged. Left in drive, the 'box is virtually seamless and in most circumstances (steep hills are an exception) can be treated like a torque-converter auto.

Swap to manual and the paddles are quick and responsive and would normally be my choice. But the gear lever itself is so short you can hold it in the palm of the hand and flick from ratio to ratio as if it were an open-wheel racer. Roof down, 309kW V8 exploring the upper reaches, you change gears just to hear the engine. Stirring stuff.

The Drivelogic button on the console even alters the strategy of the changes, reducing the torque in the more comfortable modes to further smooth out shifts. At the other extreme, disengage the DSC stability control system and a full-bore launch control can shred the rear tyres. Yes, the M-DCT is about three generations ahead of the SMG system.

Because any M3 offers layers of tuning of the chassis, transmission and engine, the convertible takes time to learn through the ubiquitous iDrive. It's a process any real enthusiast will relish. Happily, once found, your favourite settings can be stored and accessed via a button on the steering wheel.

There are penalties (beyond the circa-$13,000 price premium) in choosing the convertible over the coupe, though for those who must drive alfresco, these will be unimportant. Roof erect, the car's profile is more three-box than the shapely, fastback coupe.

More importantly, the complex roof adds 230kg, taking the M3 convertible to a robust 1810kg, thus blunting the performance by 0.5sec in the rush to 100km/h. That's if you nominate the conventional six-speed gearbox - opting for the $6900 M-DCT saves 0.2secs for a 5.1sec sprint.

The bulk also notably intrudes on rear seat accommodation: the seat back is more vertical than the coupe, while boot space is reduced by 80 litres to 350 litres. That's with the roof up - the boot shrinks to a tiny and poorly packaged 210 litres when it's stored. Still, when the three-piece roof is in place, the refinement is such that you might as well be in the coupe.

Serious drivers will always choose the M3 coupe for its performance and handling, but the immense appeal of the convertible is obvious. And we'd tick the M-DCT option box every time.

BOXES OF TRICKS
It is no coincidence that BMW should launch the M3 convertible with the M-DCT just months before Ferrari's all-new California, which also gets a seven-speed twin-clutch gearbox, hits the streets.

Both are high-performance, V8-powered front engine convertibles with folding metal roofs. In fact, German gearbox manufacturer Getrag supplies both BMW and Ferrari with the new gearboxes - although Ferrari is yet to confirm that Getrag is the supplier, or to provide transmission ratios.

From this you could assume that these 'boxes are the same. Not so: the M3 uses a longitudinal transmission mounted immediately behind the engine, while the Ferrari's is a longitudinal transaxle, mounted at the rear. The M3 uses two wet clutches: one acting on the even-numbered gears, while the other engages the odd gears and reverse.

How long does the gearbox take to shift? A 430 Scuderia-matching 60 milliseconds? BMW says only, "Without the slightest interruption" in power, while Ferrari claims theirs, "cuts the gear shifting time to zero."

Getrag currently makes three six-speed twin-clutch gearboxes for transverse, front- and all-wheel drive models (for, among others, Ford, Volvo and Chrysler), and two for longitudinal applications. A couple of weeks after Ferrari's announcement, Porsche's revealed the new 911 has a similar system, supplied by Getrag's great rival ZF.

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