Does Porsche's hulking Cayenne Turbo truly represent a much-hyped meeting of size and speed? Sean O'Grady puts the monster through its paces at Brands Hatch

The first revelation was that the Cayenne is an even faster and a much more stable beast than I ever thought it was going to be. Now I should know all that, having read about it so many times and with the thing being the product of a great engineering enterprise like Porsche. But still I didn't really believe the hype. Now I know better. That leads on to my second surprise, which is that I managed to get round the circuit a few times without sacrificing life and limb.

The basics of driving fast round a circuit are fairly straightforward. In fact I'd learned them before on previous track days, notably at Jonathan Palmer's Bedford Autodrome in a dual-control Vauxhall VX220. Nothing really prepares you for going round a U-turn sideways at 100mph. That I managed to control the Vauxhall at all was down to an excellent instructor. On this occasion, too, I was to be blessed by a guardian angel, in the shape of Anna Imrie, a former rally-driver who helps Porsche-owners find their feet. Her opening words were balm: "It's OK. We're not going to go ten-tenths today." Then she reminded me about the importance of skill - that success on a track is all about making the car spend the least possible time steering and the maximum amount of time with its wheels going straight on, which means more power delivered more stably. That, in turn means making sure you take the right line around corners, cutting the apex, and running nice and wide coming out of it.

Before we set off, she made sure that my safety helmet was on straight, that the tyres were set up properly, that the standard-production Cayenne Turbo was on the right settings (sports shocks and lower ride height) and that the controls were well placed for my little legs. Then she demonstrated how it was done, picking up speed on progressive laps of the Indy Circuit, taking every corner smoothly.

Then it was my turn, and after a few times round the circuit I just about managed to get the hang of some of the principles and part of the practice. However, I was always too timid to slow down, for fear of losing speed and because I expected the car to take it. It didn't. When I failed to brake hard going into the sharp corners, the Porsche would squeal like a real porker. I don't really know why I kept on forgetting to brake hard enough, but I was a slow learner. However, despite some kerfuffle from the standard road tyres, the Porsche didn't slip-slide away, and it stayed in control of me. (Have I got that the right way round?) Mercifully my track times weren't recorded.

The main pleasure - as opposed to thrill - in taking a Porsche Cayenne at 100mph-plus around the Indy Circuit is that it takes away the temptation to go daft on the M40. Indeed, that goes for any car. The Government ought to consider offering tax breaks for really bad drivers to take a few days out every year and thrash their Golf GTI/Mini Cooper/BMW/whatever round a bit of tarmac designed for that purpose. I think that the reason why some drivers go so fast on public roads is simplify to satisfy their own curiosity. They know, as with me and the Cayenne, what the car's supposed to do - they've got the top speed and acceleration stats in front of them, and they've seen it on Top Gear or read about in the press - but they want to test the limits for themselves. Nothing wrong with that, except that so many even quite ordinary modern cars are so thoroughly sorted that most drivers have no hope of reaching their car's peak performance on the road. Or if they do, they end up doing something irresponsible, like that policeman who took his new Vectra up to 159mph on the M54 just so he could see what it felt like at that speed. A day on the track would answer most of those questions, and might just get the addiction to speed out of the driver's system for a week or two. By the way, I think that it'd be fun to have a Porsche Cayenne race at Brands Hatch, but maybe I'm getting carried away here.

Which brings me back to my new friend, the Cayenne. I was intrigued to see whether such a big beast would live up to the claims made for it and, as I found, there is only one sort of place where you can do that safely (and enjoyably). The skill of the engineers at Porsche (and Volkswagen Group, as the Cayenne and VW Touareg are related) is in making such a hefty vehicle go so fast. And the answer to that is power; lots of it from that turbocharged 4.5-litre V8.

The crucial thing here is the power-to-weight ratio. Here we must metaphorically remove our crash helmets and don anoraks. You see, the Porsche Cayenne Turbo boasts 191bhp per tonne. For comparison, a standard Porsche Boxster runs to 185bhp/tonne, a Golf GTI to 147bhp/tonne and the best Range Rover to 150bhp/tonne. To beat the Cayenne on that measure you have to find something like a 911, a Caterham (you can get one of those with 500bhp/tonne) or an M5 BMW, and you can't get a calf in the back of those.

The Cayenne's impressive power-to-weight ratio (and the torque it implies) is equally useful when it comes to going for a little bit of light off-roading, with the Porsche's ability once again countering its image. A brief trip on the "nursery slopes" at Brands Hatch proved the point. The Cayenne may not have the Kilimanjaro-ready ability of a Land Rover Defender or Discovery, or a Range Rover, but it acquits itself perfectly well. A rare mix of abilities, then, shared with such as the Range Rover and Jeep Grand Cherokee, although I do prefer the Rangie's superbly fresh Scandinavian-inspired interior. Indoors, the Cayenne is tasteful but rather sombre; the smell of black leather is also close to overpowering.

However, I must return to an older theme of mine: the scourge of the 4x4. Is the Cayenne a superbly engineered car? Yes. Does its handling belie its vast dimensions and huge weight? Indeed so. (It even took me round a world-famous racetrack without embarrassing either of us.) Is it an ideal city or suburban car? Never in a thousand years - which is about as long as I'd need to pay off the loan on this £70,000 special. It is wieldy - you do soon get used to its outsize dimensions and it renders molehills out of road humps - but it is just too big, too profligate and too plain naughty for it to convince the environmentalist/puritan/miserablist in me. It's OK for fast farmers, though, and following my own train of thought, the Cayenne is almost good enough to make me buy a nice bit of land (somewhere in Kent, near Brands Hatch perhaps).

The Porsche Club Festival, the largest-ever gathering of Porsche cars, takes place at Brands Hatchon 28-29 August (0870 950 9000;

Search for used cars