Mercedes-AMG GT C Roadster

Both plenty of power and even more power have been crammed into the convertible version of Mercedes-AMG’s hand-built two-seat sports car.

Due to make its public debut at this month’s Paris motor show, the GT Roadster and GT C Roadster will both top out beyond 300km/h, despite their new folding cloth roofs.

But where the GT Roadster will follow the standard GT coupe’s powertrain with 350kW of power from the twin-turbo, 4.0-litre V8 engine, the GT C Roadster echoes the engineering of the just-launched GT R coupe and its power will jump up to 410kW.

The heavier hitter will follow the GT R in more ways than that, too, with the aluminium bodywork pushed out 57mm wider than the stock convertible, while it also uses bigger wheels and tyres, bigger front brakes and different gear ratios.

"With our two Roadster models, we are strategically expanding the AMG GT family by two exciting variants,” Mercedes-AMG boss Tobias Moers said.

“With the AMG GT C Roadster, we are also introducing a new model variant to which we have transferred main performance-related components from our top sports car, the AMG GT R. The result is a highly dynamic driving experience in a very exclusive ambience, coupled with that pure roadster feeling," he insisted.

While the stock GT Roadster won’t be slow, ripping through to 100km/h in 4.0sec and stretching out to a top speed of 302km/h, the GT C Roadster will cut the first figure back to 3.7 and lift the second one up to 316km/h.

To put that into perspective, the upcoming (and much cheaper) Audi TT RS will reach 100km/h at precisely the same time as the GT C Roadster, though it has the luxury of all-wheel drive, rather than the AMG’s rear-wheel drive, to help it get there.

The odd thing about the top speed figure is that the GT C Roadster gets its higher top speed even as the transaxle running shorter seventh gear and final-drive ratios for harder acceleration, though it runs a taller first gear than the stock unit.

The seven-speed dual-clutch transmission has also been heavily modified, though it still sits in the same rear-mounted transaxle unit on the rear axle to improve the car’s weight distribution. Both models retain Comfort, Sport and Sport+ modes, and the faster model also scores a Race mode for track work.

AMG has heavily modified the aluminum architecture and bodywork to accommodate the roofless forces it expects will punch through during cornering and acceleration.

The side sills have been thickened, with more triangular compartments added in, while there are more struts to brace the windscreen and the dash carrier. It also demanded a strut brace at the front and another one between the roof carrier and the fuel tank.

The GT C Roadster is undoubtedly the star of the show, but the stock GT Roadster is no slouch.

Its version of the 3982cc hot-vee engine delivers 630Nm from only 1700rpm, which it holds in a flat plateau until 5000rpm. Then the power peak of 350kW arrives at 6000 revs, which seems enough for a 1595kg (dry) convertible.

But with so much sheer power, the GT C Roadster needed even more mods for handling, stopping and going.

There is rear-wheel steering, which turns the rear wheels in the opposite direction to the front wheels at up to 100km/h, then switches to move in the same direction to improve high speed stability and handling.

It also upgrades the stock car’s mechanically locking differential with an electronically controlled unit, active damping on the AMG sport suspension layout and a performance exhaust plus it raises the size of the standard front brake discs from 360 to 390mm. It also uses a unique lithium-ion battery.

All this, plus the extra 57mm of bodywork, push the GT C Roadster’s weight up 65kg to 1660kg (dry), though the bump in power more than makes up for it.

Its 680Nm arrives from 1900rpm and stays on post until 5750rpm, then the 410kW takes over, peaking at exactly the same point in the rev range where the torque curve finally starts to fall, then the power hangs around in a flat plateau of its own for another 1000rpm.

Promising extremely flexible performance, the GT C Roadster needed to lift the front tyre size up from the stock 255/35 R19 rubber to 265/35 R19, then it raised the rear tyres from 295/35 R19 to 305/30 R20.

These tyres bolt to an entire system of forged aluminium components, including double wishbones, hub carriers and steering knuckles.

None of this accounts for its shocking jump in emissions, rising around 20 per cent from the standard car’s 219g/km to 259, while pushing the NEDC fuel consumption number from 9.4L/100km to 11.4.

Like the GT R coupe, both new convertible models receive the vertical louvres behind the grille, so the computer can decide whether to allow more cooling air in or to close the engine bay off for better aerodynamics and to allow the engine to heat up faster.

The new cloth roof itself can open and close at up to 50km/h, and the three-layer unit comes in three colours (the bodywork comes in 11 and the interior offers a choice of 10).