When cars reach a certain age, manufacturers do a predictable thing. They’re not old enough to justify being replaced but sales are sliding as newer rivals come along; if you do nothing, soon they will be on the skids. The trick is to appear to produce a new car while in reality changing hardly anything at all. You design some new lights and bumpers because that way you can change its looks dramatically for little money. Then you reskin the interior and, voilà, maximum perceived change for minimum incurred cost. That is the game.

Unless, of course, you are Porsche and the car in question is the 911. Then what you do is change the way it looks not at all and instead spend four years designing two brand-new and no doubt hideously expensive engines to replace two already brilliant ones that appeared in no need of replacing. And, just to make sure no one notices, you decide to give these new engines the same 3.6 and 3.8 litre capacities as the old ones. Then you spend even more time fitting an all-new braking system no one’s going to see either and quietly fine-tuning the suspension.

As an approach, it is subtle to the point of invisibility and admirable in its engineering purity. Whether it will prove commercially sensible – when Porsche’s UK sales have tumbled by more than 20% so far this year – remains to be seen.

As you’d expect, both engines are more powerful than before and, thanks in no small part to direct fuel injection, use less fuel and emit less CO2 as well. But if you really want to extract maximum efficiency from the new power plants, you need to spend a further £2,338 on the other little toy Porsche has been working on – a seven-speed semi-automatic transmission. This works in the same way as the direct-shift gearbox (DSG) you can fit to something as humble as a Volkswagen Golf – it uses two clutches: one to engage the gear you’re in; the other to preengage the one you’re going to need next. So when you request the next gear with the press of a paddle (or the push of a gearlever), it arrives pretty much immediately.

What’s clever about Porsche’s application of the technology, called PDK, is that the seventh gear acts like an old-fashioned overdrive, so that at a 75mph cruise it shows just 2000rpm on the clock – with Porsche’s old automatic gearbox, it would do just 50mph at those revs. The result is a 13% improvement in fuel consumption and a 15% reduction in CO2 emissions, meaning that if you buy your 911 with the smaller engine and the trick gearbox, it avoids band G vehicle excise duty.

So much for the theory. On the road the new engines lose no time in pressing their credentials on you. In fact the smaller, cheaper motor is now so strong that there seems little point in spending the extra on the 3.8 litre S version, which is only a little faster (0.2sec to 60mph) but a lot more expensive (£70,360 versus £63,070). Instead buy the S for its standard active suspension system, which in its soft setting makes the 911 a sensationally comfortable place to pass many high-speed hours.

The advantages of the new PDK gearbox are clear to see too: leave it in automatic mode and it’s as smooth as a Mercedes limousine, but knock the lever into its manual plane and shift times are quicker than you’d ever manage by hand (though nothing like as fast as the shifts executed by the paddle-actuated manual gearboxes used by Ferrari).

And yet something less tangible but no less important has been lost with this new generation of 911. Those engines, for all their power, have less character than before. They are smoother than ever, but quieter and less interesting to listen to. Porsche has been making flat-six engines with an inimitable sound for 45 years, but these could have been engineered by BMW. And that’s not bad; just sad. Likewise the PDK may make the 911 quicker from point to point, but the paddles on and behind the steering wheel are dreadfully located and substantially remove the fun of changing gear. At least that is something you can opt out of.

Likewise, while those suspension modifications have made the Carrera quicker point to point, it now feels almost too easy. A 911 should always challenge its driver, if not to a duel to the death then at least to put in what he or she intends to get out of it. And here another essential part of the 911 spirit and its unique interaction with the driver has been diluted.

Make no mistake: I was much taken with this car, as I have been with most 911 coupés that have come my way over the years. I’d love to live with a basic, manual Carrera, particularly now that the cabin’s unfathomable switchgear has been fixed. Also Porsche is to be applauded for not taking the easy option of a cosmetic 911 update but instead trying to bring in changes that make a proper difference.

To many people these changes will be welcome: after all, a 911 that’s quicker, more comfortable, more frugal and less polluting doesn’t sound a bad thing, does it? Fact is, the 911 has grown up and I must accept I am in a minority who’d rather it hadn’t.

Prizewinners

LAST week we offered three pairs of tickets for the British Grand Prix race next Sunday to readers who could name the pit-stop error that cost Lewis Hamilton vital seconds in last year’s British Grand Prix. The answer was that he drove off with the fuel pump attached. Congratulations to Mike McWhan of Derbyshire, Ann Muller of Staffordshire and Clive Davies of Hertfordshire

Vital statistics

Model Porsche 911 Carrera

Engine type 3614cc, six cylinders

Power / torque 345bhp @ 6500rpm / 288lb ft @ 4400rpm

Transmission Seven-speed semi-automatic

Fuel / CO2 29.4mpg (combined) / 225g/km

Performance 0-62mph: 4.9sec / Top speed: 180mph

Price £63,070

Road tax band F (£210 for 12 months)

Verdict More capable but less characterful than before

Release date July 5

The opposition

Model Jaguar XK £60,995

For Looks good; superb blend of ride and handling; lively engine

Against Performance only adequate; small cabin

Model Mercedes-Benz CLK 63 AMG £66,805

For Knockout power of huge V8 motor; decent room in the rear

Againt Ageing design; awkward looks; expensive