FRANKFURT MOTOR SHOW: Brabus unveils world's fastest drop-top

The world’s fastest convertible doesn’t come from Ferrari, McLaren or Porsche. Right now, it comes from Brabus.

The Mercedes-Benz tuning specialist, based in Ruhr Valley, has turned the S 65 AMG into the Rocket 900 Cabrio, the most powerful four-seat convertible on the planet.

Brabus has pushed AMG's twin-turbo V12 out from 6.0 to 6.3 litres and it now crunches out an astonishing 662kW of power and has a -- detuned! -- peak torque of 1500Nm.

So, Brabus claims a top speed beyond 350km/h and zero to 100km/h in 3.9 seconds.

You can purchase the Rocket 900 complete from Brabus, or, if the V12 biturbo S 65 Cabriolet sitting in your garage now feels inadequate, the company can re-fettle it.

Brabus claims its Rocket 900 has taken the fastest cabrio crown from another Brabus, the outgoing 850 6.0 Biturbo Cabrio, based on the Mercedes S 63 4MATIC.

With both the engine’s stroke and bore enlarged to push it out to 6.3 litres (384ci), the V12 gets matching forged pistons and billet-steel conrods. The turbochargers are also larger than the standard AMG units, while the downpipes have been pushed out to 80mm.

The 900 Rocket uses a custom intake module, drawing air through a new duct in the bonnet, while the new intake manifold uses a much larger air-filter box. There is even glittering gold reflection foil on the intake pipes to maintain lower air temperatures.

Which isn't to suggest Brabus has ignored the environmental niceties completely -- the 900 Rocket does maintain the original donor car’s idle-stop system. So that’s something. Of course, it can be switched off.

Brabus claims an NEDC fuel consumption figure of 13.9L/100km for combined CO2 emissions of 323g/km.

The 900 Rocket runs another kind of idle-stop system, too. The V12 drop-top cleverly remembers its last driving-mode setting and maintains it when restarted.

Peak torque arrives at 4200rpm, though it’s pulled back to 1200rpm in real-world use to protect the powertrain.

Meanwhile, the peak power -- which equates to 900 horsepower, hence the Rocket 900's name -- arrives at just 5500rpm.

Frighteningly, all of the gristle becomes speed only through the rear wheels, and only through an astonishingly brave seven-speed automatic transmission, then via a limited-slip differential.

You can shift ratios manually using alloy paddle shifters or just leave the transmission in 'Drive'.

Brabus claims extensive testing in the wind tunnel and on-road to verify the worth of its mild aerodynamic flourishes, including the front spoiler, carbon-fibre diffuser and the carbon-fibre boot lid.

The big convertible runs on 21-inch monobloc alloy wheels, with 255/35 ZR21 front tyres and 295/30 ZR21 rears. A set of 22-inch units is available, which pushes the tyre size out to 255/30 ZR22 up front and 295/25 ZR22 at the back.

Ride height is about 15mm lower than the standard AMG, which is what you need when staring at a 400km/h speedometer.