The M6 Convertible makes history even before you unlock the doors. It is the most expensive car BMW has ever built, and by some margin the fastest soft-top. Interesting stuff, but so easy to forget once you're behind the wheel and living through a driving experience that you will not easily find in any other vehicle.You could, of course, find something very like it in the M6 Coupé, which is nearly £6000 cheaper, slightly faster (0.2 seconds to 62mph) and a shade more economical (0.5mpg combined); but Lord love you, child, these figures are the most tiresome of trifles in a car as expensive, rapid and thirsty as this one.The replacement of a proper roof with a soft top must have some effect on the structural stiffness of the M6, but even in hard action the M6 Convertible displays almost no sign whatever of scuttle shake. Just as well, too, because like the Coupé it uses the same five-litre V10 engine as the M5 and is therefore a Seriously Fast Car.Its performance potential can be altered at the touch of the MDynamic Mode button on the steering wheel, which alters the settings of the engine, the Dynamic Stability Control system and the change pattern of the electronically-controlled seven-speed Sequential Manual Gearbox. All of these things can be adjusted independently, using buttons mounted next to the gearlever, and the settings within MDynamic Mode can be set using the iDrive system, so there is plenty of scope for fiddling.For this test, BMW set up the car on its softest settings with MDynamic Mode switched off, and on its most fierce with it switched on. One of the most remarkable things about this entirely remarkable car is that it is perfectly usable on the road even in the latter configuration.The power boost, for example, involves an increase from an already sturdy 400bhp to a massive 507bhp, with a corresponding increase in throttle sensitivity, yet the M6 remains a pussycat in town even with the higher power output. For me, it passes the test of a true high-performance road car in that it is perfectly effective when being driven slowly.Trickle the M6 through urban areas or quiet country roads and you would barely know that the engine was there at all. The V10 burble isn't so much muffled as distant, almost as if it belonged to another car several yards away. But it turns into a glorious roar when you're on full throttle and watching the revcounter needle swing past 8000rpm.Better make that just a brief glance, though, because you'll also be experiencing acceleration which makes it a very good idea to have your eyes fixed on the road ahead. Quite apart from the normal hazards of motoring life, if you start using even a small part of the M6's performance potential you can be sure that a corner will be coming up very soon.Oh, yes. Corners. Designing a car which can blast along in a straight line is not the same as designing one which can deal with bends. Here the M6 is perhaps even more impressive; it beggars belief that any four-seat car transferring up to 507bhp to the tarmac on road-legal tyres could do so as smoothly as this one can.The Dynamic Stability Control provides a safety net, of course, but you would have to be a very clumsy sort of fellow to bring this into play. The real magic comes from the engineers who know their way round a good old-fashioned mechanical suspension system and can allow it to deal with the formidable power of an extraordinary engine. The M6 is so capable in this regard that it makes you wonder how the same company could have made such a pig's ear of the Z4 M Coupé.The rear of the M6 is so good that it makes the front seem disappointing. Actually, it's pretty good too, but the lack of turn-in sharpness and the slightly ponderous way it deals with the weight of the big V10 betrays the M6 as a Grand Tourer rather than the true sports car that everything else would lead you to expect. A Grand Tourer with dazzling overtaking ability, though - and if that's the kind of car you want, then here it is.Compared with the coupé-convertible arrangements that are now on the market, the M6's soft top seems quaintly passé, but its three layers (the middle one being made of polyurethane foam) do a fine job of deflecting exterior noise. When it's out of sight, it reduces the luggage capacity from 350 to 300 litres, and it's worth pointing out that the space that remains is partly down to the omission of a spare wheel.BMW instead supplies something called the M Mobility System, a get-you-home tyre repair system which someone somewhere will probably throw into the nearest ditch when they realise it will have no effect if the puncture has been caused by a damaged wheel.The M6 is one of an increasing number of cars to use a head-up display. This can be used simply to project the speed on to the windscreen, which is fine, but on another setting it also shows a multi-coloured digital revcounter. Too fussy, I thought.As David Morgan suggested in his road test of the Coupé, the M6 makes no sense at all when further down the same range the BMW 650i Sport is waiting for your attention. But in comparison with that car the M6 was never about sense in the first place. It's an extravagance, a wonderful thing to be able to say you own, and just occasionally a way of experiencing a motoring thrill which very few other people will even be able to guess at. Engine 4999 cc, 10 cylinders Power 507 bhp @7750 rpm Torque 384 ib/ft @6100 rpm Transmission 7 speed semi-auto Fuel/CO2 18.6 mpg / 366 g/km Acceleration 0-62mph: 4.8sec Top speed 155 mph Price From £87236.00 approx Release date 04/09/2006
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