When is a minor facelift a disguise for heroic technical innovation? Easy - when it's a second-generation Porsche Carrera 997. Under the blink-and-you-miss-it camouflage of new lights and exterior mirrors, different bumpers and larger air intakes, hides a compelling all-new direct-injection engine and clever seven-speed dual-clutch gearbox that ensure the new 911 is now the world's cleanest, greenest supercar.

You won't find a superior expression of miserly muscle than Porsche's 3.8-litre Carrera S. With more efficient combustion and a higher 12.5:1 compression ratio, the mighty big-bore S now permits 7500rpm (up 200rpm) to push outputs to 283kW at 6500rpm and 420Nm at 4400rpm. Impressive increases of 22kW and 20Nm, especially when mated to the optional (an eye-watering $7000) PDK gearbox for a combined 10.2 litres per 100km fuel figure and just 250g/km CO2 emissions. Among serious performance cars, the S is only bettered by the even greener, marginally slower, 254kW, 3.6-litre Carrera's 9.8 litres and 225g/km.

And the latest S is incredibly quick, the performance instantly accessible and, in PDK guise, far more consistent than the balancing dance of clutch pedal, throttle and gearlever. Engaging launch control (part of the pricey $2980 Sports Chrono option pack) demands no more than selecting 'Sport +' on the console, left foot on the brake and mashing the throttle to the floor to send the engine revs to 6500rpm. Lift off the brake, a momentary pause before the rear tyres light up, and the Carrera S erupts. With the traction control and stability control minutely balancing wheelspin and adhesion, the Porsche hooks up flawlessly. We ran 0-100km/h in 4.5 seconds, 0-400m in just 12.5sec. That's a massive 0.5sec quicker to 100km/h than the previous 261kW, manual-tranny S tested (Wheels, August 2007), and 0.3sec faster than the 309kW, all-paw Audi R8 to 400m. Furthermore, the times are repeatable: in four runs the variation over the 400m amounted to only 0.03sec, an impossible-to-duplicate level of uniformity compared to the standard three-pedal operation. Still ...before automatically ticking the PDK and Sports Chrono boxes, there are a couple of negatives that diminish the appeal of the long-awaited dual-clutch 911. Porsche's decision to go with an evolution of the old five-speed Tiptronic auto rocker-switch gear selectors on the steering wheel spokes makes no sense. Instead of the left-downchange, right-up paddle arrangement adopted by most manufacturers, you push either switch away to climb gears, or pull backwards to drop down a ratio. The cheap alloy-look plastic switches are so poorly positioned we preferred to use the edge of the right thumb knuckle to change up. In a week's driving, shifting gears manually never became intuitive.

Nor does the selector lever, symbolically sprouting from the console, offer an appealing alternative. Carrera Cup racing cars are set up correctly: you push forward to go down a ratio/pull backwards to go up, the opposite of the stupid set-up in the road-going S. All of which is a shame because otherwise this is a brilliant gearbox; quickly adapting to the driver's style when you're in assault mode, yet near flawless as an auto in Drive. Yes, it's better, even, than the dual-clutch BMW and Ferrari systems, while matching the smoothness of the old auto.

The ratios are essentially the same as the manual: fifth and sixth are identical, except the PDK gearbox adds a tall overdrive seventh (57.2km/h per 1000rpm versus sixth's 40.3) to help achieve those eco numbers. When asked to lope along, PDK reaches the taller gears in less time and culminates in seventh. Go for the whip in the sport modes and downshifts are accompanied by a rev-matching blip of the throttle, while kick-down will drop top gear to second without visiting any of the intervening ratios and without any perceptible delay. Thankfully, the transmission won't upshift at the 7500rpm rev limiter nor automatically downshift while in manual mode, unless you tap into the kick-down switch at the bottom of the throttle's travel. Porsche expects PDK to be mandatory in five years, but we reckon the manual gearbox will remain, at least until the paddle complication is resolved.

Beyond the engines, the supporting cast is brilliant. Porsche played with the 911's dampers, springs, and anti-roll bars to deal with the extra power. All steel-rotor 911s now have 330mm brakes straddled by four-piston calipers all round. The $19,900 carbon-ceramics option, fitted to the test car, bumps the rotor size to 350mm and increases the front piston count to six per wheel. Given the price, I'm pleased to report these are probably the best brakes I've tested: quiet, progressive and linear, fade-free and exceptionally powerful.

The driving experience is, predictably, pure 911: the car communicates constantly with the driver. The thin-rimmed steering wheel performs an interpretive dance in your hands, sharing with you its fascination for changing road surfaces, cambers, and grip levels, although, sadly, the variable-ratio rack introduced on the first-gen 997 lacks the on-centre immediacy of the 996's steering. The PASM adaptive suspension, standard on the S, delivers a ride that is all-day comfortable without ever allowing body movement to intrude. However, tyre roar is excessive to the point of being tiring, the ever-bigger rubber (now 295/30ZR19 rears) constantly feeding a drone into the cabin. The 911 still doesn't have the handling consistency or balance of the mid-engined Cayman, but the levels of grip and adhesion mean that any sane driver rarely employs all that power, at least on the road.

Of course, it's all still recognisably 911 inside that snug cockpit, though the centre console has been tidied, many of the fiddly little switches have migrated to the new touch-screen communications interface. Focused and familiar, it's essentially business as usual.

Sales of the 911 are down 56 percent so far this year, more a measure of the lousy economy than the quality of the car. Porsche has done it again and built a better 911.