IT was very nearly worth the imminent onset of hypothermia to hear the rasp of this BMW's exhausts with roof lowered and throttle pressed with conviction.

Eventually the wind chill called a halt for roof erection (electrically, in seconds) but not before the car had registered a beguiling mix of brawn and sophistication.

Many people choose a drop top for its looks and the chance to hear the birds chirping and people mowing their lawns on a sunny summer's day.

Not so many want a near limitless stream of power that turns this ragtop into a genuine performance machine. For those that do, BMW will happily supply a new M240i, in coupe or cabriolet shape.

Sitting at the top of the BMW 2 Series range (itself based closely on the hatchback 1 Series), the twosome gain a more powerful engine than before, up by 14 horses to a not inconsiderable 340 horsepower.

That shaves 0.3 seconds off the zero to 62mph time - now 4.7 seconds if you choose an automatic gearbox - and quite enough to provide all the thrills you'll ever need on the way to work.

Or on some deserted stretch of smooth continental mountain pass, where you could knock the auto 'box (eight speeds and £1,600 extra) into manual and control the changes from the paddles behind the chunky leather wrapped steering wheel.

Best to do this with the roof folded away, the better to hear the exhaust note's angry bark as each change goes through and the horizon fast forwards towards you.

Top speeds in all versions of the M140i and M240i are electronically limited to 155mph, which ought to be sufficient even on a clear stretch of unrestricted autobahn.

The twin turbocharged six-cylinder engines powering these mid-sized fun machines have been worked on by BMW's engineers so that there's improved economy and emissions along with the power lift.

It means a reduction of up to seven per cent in the official average fuel consumption, with an impressive (although unlikely in the real world) 39.8mpg with auto gears in the M140i and M240i Coupe and 38.2mpg for the heavier Convertible.

CO2 figures are 163 and 169g/km for auto boxed cars and a bit worse (179/189) for models fitted with the six-speed manual gearbox.

Cleverly, BMW points out that prices for these more powerful newcomers remain unchanged (M140i £31,875, M240i Coupe £35,090, M240i Convertible £38,535) but running costs for anyone paying company car tax are down, thanks to the cleaner tailpipe emissions.

The M Sport versions talked about here all have firmer suspension but the Convertible has its own settings, a little softer to help cut to a minimum the near inevitable shimmy on bad roads caused when you chop the metal roof off a car and replace it with canvas.

Even on poor roads you hardly notice and the delights of open top motoring (if you're so inclined) more than compensate for the modest extra vibration felt through the wheel.

Whichever version you choose there's a typical BMW cockpit awaiting you, where function wins over extravagant looks every time.

It results in a cabin lacking the obvious glamour of a recent Mercedes, for instance, but one where the driver is very much king of his powerful empire.

And it all works nicely, from clear digital speed readout to a big central screen, which can connect you to the outside world via a built-in SIM card and for £900 through a Professional navigation system (a simpler system is standard) that updates data free.

Not that getting lost behind the lovely six-cylinder engine would always be a bad thing. Keep the hood down and take the long way home.