We'll come to other features shortly, but the engine is the most important thing. The 3.6-litre six-cylinder unit (which, in these days of legislated noise suppression, produces a hum rather than a roar) is fitted with two turbochargers and Porsche's variable valve timing system, produces 414bhp, and is exceptionally flexible, giving 415lb/ft of torque across a wide rev range.It's easy to think of figures like 414bhp without fully appreciating just what kind of effect such outputs have on a car. In a straight line, the 911 Turbo is incredibly quick. A 0-62mph time of just over four seconds (in the six-speed manual version tested here - the Tiptronic version is a little slower) is remarkable, but even more so is the way the 911 will shoot up to 100mph with only a brief prod on the throttle in third or fourth gears. Overtaking, needless to say, is childishly simple, holding on to your licence perhaps less so.Of course, this kind of performance is all but unusable on the public road, so it's just as well that the Turbo is very easy to drive at more moderate speeds. You can make pretty decent progress without allowing the digital boost gauge to register anything above 0.0 bar. And although all the major controls are firm, they are not heavy. For a car of this type, the gear linkage is astonishingly good (with a slight tendency to rubberiness), and the clutch is gentle enough to allow seamless changes without major effort.The ride quality is also very high. We've tested many cars which suffer in this respect from having low-profile tyres. The rubber on the Turbo is probably the lowest-profile (30) we've come across at CARkeys, but this does not lead to the usual jarring over bumps. Fairly soft suspension also contributes to a very smooth run over nearly all road surfaces.This softness, plus the fact that the tyres are very wide, plus the four-wheel drive system, creates a huge level of grip. And in case ambition threatens to triumph over adhesion, there's also a traction control system (which can be switched off, though I wouldn't even consider doing so for road use).For all that, though, the 911 is not a car I fancy hustling through bends. This is because of the one thing that's wrong with the engine - not its behaviour or its massive power output, but its location. We've discussed this already (in a track test of another 911 versus the Boxster S) so I won't go into it again in detail here, but the basic story is that putting the engine behind the rear wheels rather than ahead of them creates an imbalance which no amount of fancy chassis work can ever correct.Even at quite moderate speeds it's very obvious that the engine is squeezing the rear wheels on to the road surface and attempting to lift the front wheels off it. As you turn in to a corner there is always a feeling that the car is wondering whether it should understeer or oversteer, depending on whether the mass of the engine decides either not to follow the rest of the car at all, or to start moving and force the rest of the car to follow it.I should say at this point that neither of these things actually happened at any point in the test. But the suspicion was that if I pushed any harder, one of them would. I'd be quite happy to spend an afternoon balancing the two on a race track, but on a public road I'm just not prepared to take the risk.So, while the Turbo can make its way along favourite quiet roads faster than almost anything else I've driven, the speed was entirely down to how quickly the car accelerated along the straights. Apex speeds were anything up to 10mph down on what I would consider safe and easy in many family hatchbacks.If the engine were in the middle, the Turbo would be twice as good a sports car as it is now. There would be no room for the back seats, of course, but these are far too small for adults in any case, and I suspect that most owners use them simply to carry things that will not fit in the tiny luggage compartment under the bonnet.Criticism of the engine location, however justified, may be irrelevant. For a start, Porsche is one of the most profitable manufacturers on the planet, so it presumably knows what it is doing. And I doubt that anyone who buys a Turbo seriously thinks they will ever get the chance to use it to its full potential. If you're attracting a lot of attention by driving a beautiful car through the middle of a city, it doesn't really matter where the engine is. The point is that you possess something which has all that potential, and can be seen to possess it.All this comes at a price. The Turbo costs nearly £90,000, and running one must be a severe financial drain. The combined fuel consumption figure is over 20mpg, but that test won't involve using anything like the full performance. In real life the fuel gauge needle plummets towards zero at an alarming rate, and it's difficult to get much more than 200 miles from a full tank. Those tyres (and indeed the pads in the superbly effective brake system) will not be cheap to replace, and even normal servicing has to be a costly affair - as I peered through the tiny bootlid I realised that you would have to take the engine out just so that you could reach the spark plugs.In other words, owning a 911 Turbo means spending a great deal of money on a car which has abilities that are almost never going to be explored. To me, that is not satisfactory motoring - yet I was sorry when I had to hand the car on, and wished I could have spent more time with it. Clearly the Turbo has a lot of appeal, even to people who don't necessarily want to acknowledge this.Second opinion: Philosophising apart, this is a genuine supercar There are new ones supposed to be coming on the market which won't get off the line any quicker. 0-62mph in 4.2 seconds is fantastic production-car acceleration, and it's almost impossible to describe the light-the-blue-touch-paper effect to somebody who hasn't experienced it. Yes, Porsche has had to do a lot of fancy engineering to counteract the daft engine position 911 fanciers insist on. But it's done just as much to make this an entirely biddable car at round-town speeds. And a supercar with little details like door armrests which double as lidded stowage boxes - very civilised. Ross Finlay. Engine 3600cc, 6 cylinders Power 414bhp Transmission 6-speed manual Fuel/CO2 21.8mpg / 309g/km Acceleration 0-62mph: 4.2 seconds Top speed 191mph Price £88,060 Details correct at publication date