I try out the launch control in this car, too; you do it in automatic mode, and simply press the brake, floor the throttle, release the brake and feel your entrails wrap themselves around your vertebrae. It's just too easy

From one extreme Porsche to another, and 36 years between them. The differences are immense, the similarities surprising. First, the young one.

It's the new 911 Turbo, the forced-induction version of what you might call the 997.5. Which is to say that it now uses the new A1 engine with direct fuel injection, an oil tank built into the base of the engine (although it's still, technically, a dry-sump unit), a demand-controlled electric oil pump and far fewer separate engine components. And it has the option of the PDK – double-clutch – transmission in place of the unloved, torque-converter-based Tiptronic.

These facts, though, are mere details compared with the numbers involved. Back in 1974, the first 911 Turbo made 260bhp and we were in awe. This latest nearly doubles that output: you can now buy a road-friendly 911 with 500bhp. True, it's just 20bhp up the outgoing model, with a 22lb ft torque hike to match (making 479lb ft), but it's a mighty impressive figure nevertheless.

Drive the new Turbo, as I did in its several guises at Portugal's Estoril circuit and the surrounding roads, and the extra potency feels even greater. The previous model, though monstrously rapid, could feel curiously unengaging unless you lived in its boost zone, upon which it would hurtle to the horizon and your licence cowered in terror. This new engine has a very different character, brought about mainly by the higher compression ratio that the combustion-chamber-cooling effect of direct injection makes possible. Further cooling of the intake air comes with an expansion-type inlet manifold plus, of course, the usual intercooler.

All this means that less boost is needed for the required performance, and maximum boost is 0.2bar less than in the old engine (it's now 0.8bar normally, 1.0bar on temporary overboost). So the difference between off-boost and on-boost feel is less, with turbo response lag less evident and a crisper initial throttle response. That makes for a keener, more responsive 911 Turbo more of the time. It's also usefully less thirsty.

Fine. You've trickled through towns, tickled the accelerator as the road opens out, decided you rather like the new eagerness. Then you see a clear path to the distance, you press the accelerator hard. Time and space now compress with savage insistence.

Long-sighted people have eyes with a shorter distance between lens and retina. Those who indulge in drag races find their eyes do the same thing, thanks to the compressive effects of massive g-forces. It's a good thing that this should improve distance vision rather than compromising it, because full chat in the 911 Turbo brings the distance a whole lot nearer.

It's the sheer persistence of the force that is the most breathtaking part of it. Breath-producing, too; floor the throttle, and although the forward leap begins instantly there's a delay before you hear the rushing blast of exhaust air which all but drowns the combustive part of the flat-six's exhaust note. And then you brake. You're always braking in the 911 Turbo, because bends come up so quickly and they're always sharper than you think.

For the most part, such ferocious acceleration – 0-62mph in 3.4 seconds if you're in a PDK car and you've used the launch control – can be little more than a party trick. The roads just aren't big enough. But, as ever with a truly quick 911, the combination of pace and compactness makes this the wieldiest and nimblest of supercars, which maximises the opportunities for using it.

This being a modern 911 with all the electronic stability stuff, plus four-wheel drive, a limited-slip rear differential and, now, the option of 'torque vectoring' which brakes the inside rear wheel to counter understeer, it lets you use all this power without frightening yourself too much. You can feel the vectoring at work quite clearly on a twisty road driven with verve; turn to quickly into a tight bend and the front is naturally pushed wide, as you'd expect in a tail-heavy car, but suddenly that ceases and you can get the power down early. As ever with a 911, it works best when some drama innate in its singular architecture unsettles it and you then live dynamically through the moment. Nowadays you can take less responsibility for correcting it correctly, but you still know exactly what is going on.

Now, the PDK. Should you or shouldn't you? It is the route to the fastest-accelerating 911 Turbo, but we're talking mere tenths and it's not a racing car. And it is now, at last, available (for an extra £271) with proper upshift-right/downshift-left pull-paddles instead of the cheap, nasty and confusing buttons beloved of Porsche in its sequential-shifters to date. So that's good.

On the road, the paddled PDK certainly shifts quickly and keenly, especially in Sport or Sport Plus modes, and such shifting effortlessness heightens the feeling of being in a high-speed video game as bend after bend is despatched. It all becomes worryingly easy, and you find yourself goaded to go ever faster in search of ever-bigger thrills. I'm not at all sure this is a good thing, and would rather my mental processing power was used instead towards the deeper satisfaction of physical gearchanging and the greater melding between human and machine it engenders. Besides, when the roads are clogged you need something more than a mere switch to keep you amused.

And so to the track. The best lap times will surely be had with the PDK, and I begin with a few laps in the Turbo most appealing to those who don't really get fast cars. It's a PDK-equipped cabriolet, exhaust rush audible at maximum volume, satisfying crackles and spurts with each super-quick upshift. I try out the launch control in this car, too; you do it in automatic mode, and simply press the brake, floor the throttle, release the brake and feel your entrails wrap themselves around your vertebrae. It's just too easy.

Then I score a session in a manual coupé, the 911 Turbo that most of us would actually buy given the required £101,823. And it's a joy, allowing that caressing of brake and throttle as you downshift into a corner and stroke the 911 through it.  These are the moments that make you realise no clever-clogs transmission can ever match the pleasure found in a good manual, and in a car with a 911's dynamic foibles it's more than ever a part of the process.

Yes, even this new Turbo can't hide the fact it's essentially a tail-heavy car with a hefty helping of torque, even if that torque is much more manageable than that of the long-pause-then-blam original Turbo. The tail powers out with great glee, even with the stability control on but set to Sport; with all switched off it's as demanding a machine as a 911 ever was. So I leave it in Sport and have several laps of tyre-squealing delight. Slow in, fast out is still the key, but it doesn’t have to be that slow and you can get on the power sooner than you ever could before.

All this is etched in my mind as I get into the 911 I have just driven: the old one. Which is not a Turbo but the most prized of all roadgoing 911s, a 2.7 RS Carrera lent to me by a touchingly trusting friend. The RS feels like it weighs two-thirds as much (which it does), its gearchange is as loose and demanding of tactile intelligence as the Turbo's is slick and machined, its engine has the razor response that comes from a light flywheel and free breathing, and it sounds revvily fantastic in the way portrayed by every 911 cliché you've ever read.

It's properly quick, too, even if with 210bhp it could get nowhere near the Turbo. But the most heart-warming point of coalescence is in that handling balance. Here in the RS, every characteristic is exaggerated: the pushing-on of the nose on a slippery surface, the squat of the tail as power pours onto the road and edges the tail into oversteer, the tugging of the steering as the forces change up front be it dynamically-changing balance or simply road undulations. This is the 911 as a raw file to the Turbo's Photoshopped JPEG; the Turbo is clearly the more devastating power machine, but to drive the old RS is to understand properly what the Turbo is really doing behind its electronic veneer.

Both are fabulous. That a new Turbo costs half as much as you'd pay for a perfect RS is sobering. Forty-six years on from the 911 breed's birth, it has never been more capable. Nor can it forget its roots.