Through the gears in milliseconds

Little luggage space is lost when the hood of the M6 comes down.

BMW is showing its fastest-ever soft-top at the British Motor Show - the M6 convertible, the first M version of the 6 Series drop-top.

Due to arrive here next year, this car is powered by a 5.0-litre V10 engine that revs to over 8000rpm, its 378kW and 519Nm propelling it from 0 to 100 in just 4.8 seconds.

The engine was voted International Engine of the Year for the second consecutive year. Here it's mated to a seven-speed sequential manual transmission as used in BMW's M5 that - at its fastest setting - can make gear changes in just 50 milliseconds.

It must be efficient to make the most of the engine, for at 2005kg this car is heavy. Its power-to-weight ratio pits it against the Mercedes CLS55 AMG, or the Jaguar XKR, also revealed at the British show.

Like those cars it offers visual clues to the monster within, most obvious from the rear, with its four chromed muffler tips and an air dam that makes this car longer than the standard 6 Series convertible.

But of course this M6 isn't just about speed. A drop-top must be stylish too, and part of the car's appeal is the unique design of its roof.

That hood is a triple-layered construction, with a polyurethane foam insulation layer between the inner and outer skins to keep heat in, and cold and noise out. It also features what BMW calls a fin roof, to create the same dynamic contours as the coupe when the roof is in place.

The rear window can be raised or lowered electronically, whether the hood is up or down.

The hood folds at the touch of a button, or the remote, without losing a lot of luggage space - from 350 litres roof-up, it drops to 300 roof-down.

This car uses a range of high-tech solutions to cut weight without compromising strength. Body panels of thermoplastic, carbon fibre and sheet moulding compound, as well as judicious use of aluminium, have ensured a 50-50 weight distribution.

There's the latest generation of dynamic stability control, which features a dynamic mode for the sporting driver.

Dramatic styling and 19-inch alloy wheels with low-profile sporting tyres complete the picture.

Too sporty for you? A 3 Series coupe will arrive in September using a 2.5-litre six-cylinder twin-turbo petrol engine to deliver 225kW and 250Nm of torque, for a 0-100 time of 6.9 seconds. Or there's the 335 with a 3.0-litre engine and 225kW and 400Nm to take you from 0 to 100 in 5.5 seconds.

Prices have yet to be confirmed but you can expect to pay around $270,000 for the M6.