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I call it the alibi button. And so does my grinning German host. It goes like this. When the monstrously powerful 911 Turbo cabriolet hits 75mph, that clamshell rear wing deploys dramatically skyward. That’s to generate more downforce and keep the back of the car stuck to the road.

But it’s also a handy way for a well-informed cop to tell that you’re doing 75mph or more. Which could, depending on the circumstances, range from being just slightly illegal to commanding a jail sentence.

That’s where this dash-mounted button comes in. It allows the driver to deploy the wing manually at any speed below 75mph, meaning a clever-dick cop cannot use it to prove you’re speeding. As my now chortling Porsche host points out: “You can just say that you were showing off your wing!” Indeed the new turbo cabriolet raises the art of showing off to a new level. Flared wheel arches cloak massive 18in wheels, and along with that extrovert wing the car sports huge front airdams and enough air intakes and strakes along its flanks to ventilate a Holiday Inn.

Central to its show-off persona, though, is that extraordinary roof. It’s a marvellous piece of engineering, folding in a Z-pattern into a boot behind the rear seats. All you do to raise or lower it is press a button and within 20 seconds it is folded away under a hard cover or locked in place and ready for bad weather.

Even better, you can do the raising or lowering at speeds up to 30mph. I tried going quicker than that and the roof just stops folding while the dash display gives you angry warnings. Very embarrassing.

The other great thing about the roof is that the car’s looks aren’t messed up by it, an aesthetic feat that past convertible 911s have failed to achieve. The rear window is also made of solid glass with a built-in defroster, which when added to the car’s excellent insulation means you benefit from a level of protection and noise refinement that rivals the turbo coupé.

And get this: Porsche says the cabriolet has been wind-tunnel-tested to 209mph with the roof up, and no problems reported.

Which brings us neatly to the performance of the new cabriolet. When you cut the roof off you weaken a car’s structure, so to correct this as much as possible the cabriolet’s body is reinforced. This makes it heavier than the coupé, which would normally mean you’d expect it to be slightly slower.

Compromised performance, then? Well, yes, but only if you’re the sort of lunatic who worries that the 0 to 62mph sprint is slowed by a couple of tenths to 4.3sec.

For the vast majority of us, though, the cabriolet’s performance remains quite sensational, the twin-turbo 420bhp flat-six delivering colossal waves of acceleration from just about any speed.

Porsche claim a top speed of 190mph, earning it the joint honour of being the fastest soft top production car on the market (alongside the similarly quick Dodge Viper SRT-10). It’s a characterful engine, too, with that signature 911 bass that hardens into the most glorious heavy metal thrash ever. It’s not a pretty noise, just a truly inspirational one.

And in spite of the big macho numbers the car generates, it is no beast to drive. The clutch is reasonably light and the six-speed manual gearbox is a marvellous match for the engine and a great pleasure to use. So the car has a bit of a dual personality: you can waft regally the way you would in a luxury grand touring car or hoon around making lots of noise on your way to the 6800rpm limit.

For those who want it, Porsche offers Tiptronic S, a five-speed automatic transmission that can be shifted up and down with buttons on the steering wheel. Or you can just drive along in normal automatic mode. If poncing around with your wing up in the air is your bag, then this will do you, but I don’t think it does a great engine justice.

The turbo cabriolet would not be your first choice as a track-day car but don’t get the idea this car is a lounge-lizard boulevardier.

It has four-wheel drive that distributes power to the front wheels when it detects the rear wheels are struggling. You can feel the system work as it slingshots you out of corners with the front wheels gamely scrabbling for purchase.

It does make you feel a bit of a hero, but for those who overestimate their talent there is an electronic stability programme that helps rein things in if either end of the car begins to lose the plot. Amusingly, though, Porsche’s own literature points out that its system “cannot override the laws of physics”.

Which you might just expect it to do, given that when the 911 Turbo cabriolet goes on sale about November time, its huge performance will come with a similarly huge £96,130 price tag.

VITAL STATISTICS

Model Porsche 911 Turbo Cabriolet
Engine type Six cylinders, 3600cc
Power/Torque 420bhp @ 6000rpm / 413 lb ft @ 2700-4600rpm
Transmission Six-speed manual
Suspension (front) MacPherson struts, dampers, coil springs (rear) multi-link independent, dampers, coil springs
Tyres (front) 225/40 ZR18, (rear) 295/30 ZR18
Fuel/CO2 21.9mpg combined / 309g/km
Acceleration 0 to 62mph: 4.3sec
Top speed 190mph
Price £96,130
Verdict Beautifully engineered, sensational performance, but price