GENEVA MOTOR SHOW: Mercedes-AMG GT R Roadster revealed

Mercedes-Benz has lifted the lid on its astonishingly good Mercedes-AMG GT R coupe to create a tantalisingly fast convertible.

The Mercedes-AMG GT R Roadster will be on sale in Europe directly after the Geneva motor show this week, complete with the coupe’s thumping 430kW twin-turbo V8 sitting up front.

The arrival of the GT R Roadster brings AMG’s GT range up to 16, including five (real) coupes, four roadsters, two track-pack racers and five four-door (marketing) coupes.

Nestled just behind the front axle line, the 4.0-litre biturbo V8 also delivers 700Nm of torque, hurling the 1635kg two-seater to 100km/h in 3.6 seconds.

But while straight-line performance seems like the key part of a 317km/h roadster, the key tricks of the coupe have been carried over into the soft-top.

Firstly, it scores the GT R’s active aerodynamics, including its automatically extending undertray, made of carbon-fibre, which extends 40mm downwards to change the underbody airflow.

AMG claims the venturi-effect from dropping the 2kg “profile” reduces front axle lift by 40kg at 250km/h.

It extends in the car’s performance profiles, but retracts again when the driver chooses more relaxed driving modes.

The newest AMG roadster also scores the GT R’s rear-wheel steering, plus its radiator air outlet can open and close to direct more or less air beneath the double rear diffuser to improve its handling.

AMG has not made a point of admitting how much weight it has cost the GT R to lop off its roof and replace it with three layers of acoustic mat and fabric and a bunch of underbody stiffening. The coupe, though, weighs 1555kg, so AMG has added 80kg with the roadster.

The push-button roof is supported by a folding structure of steel, aluminium and magnesium, but most of the rest of the reinforcement is already there on the coupe.

AMG insists that’s because the GT donor car was designed as a coupe and a roadster right from its earliest sketches, so the reinforcement does double duty.

On the GT R Roadster, that includes a carbon-fibre torque tube for the transaxle powertrain that, at 13.9kg, is 40 per cent lighter than the aluminium version on the lesser GT models.

The transmission tunnel is also braced with carbon-fibre, which AMG claims is worth a 7.5 per cent step up in the body’s torsional rigidity, plus there are two more carbon-fibre diagonal braces in the engine bay.

As well as being offered in a new matte grey paint scheme on top of 13 other colours, it has more of the same hard-core interior as on the coupe.

The obvious new feature is the inscription ‘1 of 750’ on the centre console – a seemingly lazy piece of sequential numbering that Audi Sport was pilloried for with its R8 V10 RWS.

It retains the hard-hitting sibling’s 12.3-inch digital instrument cluster and its centrally located 10.25-inch infotainment screen. There are seven different driving modes in the GT R Roadster, plus three display styles for the instruments.

And there’s the adjustable traction-control knob that the GT R coupe made notable, with the driver able to adjust the car’s drift angle, even midway through a drift.

It supports that with Slippery, Comfort, Sport, Sport Plus, Race and Individual modes, with the Race mode tightening up the dual-clutch seven-speed transmission to deliver short, brutal gear changes.

For all that, though, people usually go to AMG for the sound system, and then usually for the one under the bonnet.

Its torque peak is available from 2100 to 5500rpm, thanks in part to a hot-vee layout with the turbochargers inside the vee angle of the V8.

Its boost pressure rises, like the coupe, to 1.35 bar, its compression ratio is higher and its exhaust ports are different. It uses air-to-water intercoolers, and the intercoolers themselves have two-stage, low-temperature water circuits.

The bottom of the engine features a sand-case aluminium crankcase, plus the super-slidey Nanoslide coatings on the cylinder liners reduce friction and wear on the forged aluminium pistons.

The loud-ish exhaust system is 6kg lighter than it is on the stock cars, with a titanium rear silencer and thin-wall stainless steel on the front part of the exhaust system.

The handling is helped by adding 46mm of width to even the GT S Roadster at the extremities of the carbon-fibre front quarter panels, while it’s 57mm wider at the rear.

The wheel-arches are filled with forged matt-black wheels, with 10.0J x 19 ET56 wheels at the front and 12.0 x 20 ET42 alloys at the rear.

They’re wrapped in 275/35 ZR19 Michelin Pilot Sport Cup 2 tyres at the front and 325/30 ZR 20s at the rear.

But the unsung heroes of the handling package on the coupe, and probably on the Roadster, are its dynamic engine and transmission mounts, which run soft for comfort but stiffen whenever they need to so they can minimise weight transfer under hard cornering.

The core chassis is also different to the next-hottest AMG Roadster, with new wishbones, hub carriers and steering knuckles from forged aluminium and no-play uniball bearings on the rear axle’s lower wishbones. The rear-end also has a thicker (but still hollow) anti-roll bar.

It uses a unique specification for its coil-over suspension set-up, with the continuously variable adaptive dampers receiving their own mapping.

While the 390mm front brake discs and 360mm rears are strong enough, AMG prefers track drivers to upgrade to a carbon-ceramic version that’s 15kg lighter, even though the front discs grow to 402mm and the rears stay at 360mm.