Back in February 2011, in South Africa, Top Gear climbed into a car that looked very much like a Porsche 911, but such was the secrecy we can only talk about it now.

A 911, but not as we know it. Under the tape and mesh lights covers and false turbo-look side vents, it's running all-new body and chassis. But then when did a new 911 ever look much different from the old? Not since ever.

It even sounded the same: again, no surprise given its flat-six isn’t vastly modified from what today’s retiring 911s have.

We’ll have the full story in next month’s Top Gear magazine, but for now, here are some of our impressions of our time in Porsche’s prototype 991, only the third new 911 design in the car’s 48 year history. It's going to be formally unveiled at the Frankfurt show in September.

First, the big surprise: it felt less rear-engined. Less 911-ish, more Cayman-like. Because, in a way, it is. Of course, the engine’s where it always was, hanging out the back. But the rear wheels have moved 70mm back to meet it, reducing its leverage over the rest of the car. So the mass of that engine has less see-sawing effect when you go over crests or dips. If those events coincide with a corner, there's less corkscrewing body movement.

The new shell also has its front wheels further forward, and a more sloping windscreen centreline. So it's roomier and more aerodynamic. But the screen pillars are still fairly upright to make the car as easy to see out of as all 911s are, and most other supercars defiantly aren’t.

Another fundamental change: for the sake of fuel economy, the steering is now electrically powered. Potentially dismaying news, given the dishearteningly remote systems used elsewhere, but August Achleitner, boss of the 911 line and our driver for the day, is insistent it will offer less interference on straights but the same feel as the old car in bends. We shall see.

Though the longer wheelbase gives a little more cabin room, the overhangs are shorter, so overall length doesn't rise much. And it's no wider. Still a compact, useable, glassy car. Good.

They moved the wheels by changing the gearbox – the new manual is a version of the current three-shaft PDK box, and this has its driveshaft flanges further back than the old manual. Obviously in converting the PDK to a manual, they won’t have blocked off any of its seven speeds. Prizes for naming any other seven-speed manual? (Trucks and tractors not allowed.)

But the new platform isn’t just about moving the wheelbase relative to the weight, it’s also about actually cutting weight. Aluminium is used for the floor, the main structure front and rear and most of the external panels. The rear wing and inner and outer bodysides are steel and so is the front crash structure. The new shell and other weight improvements shave somewhere between 30-40kg out of a base 911 Carrera.

If they’d left it as all-steel, it would have put on weight, not lost it. Maybe by as much as 60kg heavier than before, because it’s longer and has more kit and its standard wheels have gone up to 19-inch.

Performance numbers aren’t final yet, but in the Porsche way they’ll all be a sliver better than the cars they sell now. For economy the base car drops to a 3.4-litre from a 3.6, but it’s still very slightly more powerful than it was at 350bhp – 20bhp more than a Cayman R’s 3.4 because it has its own cylinder heads and revs higher.

The Carrera S still has a 3.8-litre, but now revving up to 7800 because of new injectors, camshafts and their drive. That means 400bhp.

And economy rises on all. Over recent history Porsche has always managed at least a 10-percent improvement for each new model. The engineers don't shake their heads when you suggest they've done the same this time. That would put the 3.4 Carrera PDK at about 33mpg and roughly 200g/km. That’s ruddy unbelievable for a GT that’ll do 0-60 in the mid-fours.

Style-wise, the full drum-roll debut is at the Frankfurt show in September – to make our prototype resemble 997s the car was well-padded with matte-black plastic wraps to dull the curves, plus bra and pants front and rear. But you can see the curvier line to the top of the rear wings, and the way the rear window sits slightly recessed in the bodywork. Inside the cabin uses lots of Panamera parts, but it doesn’t feel like a saloon in here. Just a sports car with shedloads of buttons for all the optional equipment.

For more, you’ll have to wait for the full story in the next Top Gear magazine (out Wednesday July 13). And then once again, when we actually drive it. But do let us know your initial thoughts in the meantime…