We've already done a general launch review on the Touareg, but the top turbo diesel version of the new Volkswagen 4x4 is such an extraordinary vehicle that it deserves separate treatment.It's no longer worth agonising over what's the point of luxury off-roaders costing £50,000 or more, which in the hands of many private owners are rarely going to tackle conditions that wouldn't be passable to a cannily-driven Vauxhall Nova. People are lapping up these powerful, sturdy, high-set machines, and, so far from objecting to the colossal over-engineering they offer in comparison with the real-life motoring tasks they'll be asked to perform, want as much of it as the budget will stand.How Volkswagen regards its first-ever full-production 4x4 is summed up in the fact that, when the Phaeton appears, that luxury saloon will be available only through dealerships which also have space set aside for the Touareg.As we've reported elsewhere, the V10 TDI is very well-presented, not just technically but also in terms of its interior design, passenger space, equipment and trim. In fact, the V10 has the top specification in the range, including 18" Nevada alloy wheels, bi-xenon headlamps with cleaning jets, fog lamps, additional chrome here and there, and metallic or pearl effect paintwork as standard.This is the point at which air suspension comes into the Touareg catalogue, with automatic self-levelling and three selectable ground clearance settings. It makes a pretty bulky machine just waft along over bumpy or rough surfaces, even when staying on tarmac.Inside, the V10 has the best of the Touareg seating arrangements. There's 12-way electrical adjustment of the front seats, including variable small-of-the-back support. The car will remember just what setting the driver prefers, and the same kind of memory function applies to the steering column and door mirrors.Like an increasing number of quite bulky vehicles, the Touareg isn't as heavy as a first glance suggests. Some fancy plastics are used for the front wings and the bumpers, while the bonnet is aluminium. You'd never guess, just by looking at the car, or at the finished surfaces of the non-steel parts.But all that stuff is largely peripheral to the appeal of the most powerful passenger-car turbo diesel on the world market. This is a stunningly impressive engine, a 90-degree V10 version of the 2.4-litre five-cylinder TDI offered further down the Touareg price range. That being so, the cylinders in both banks have the same bore and stroke, although doubling the five-cylinder's capacity of 2460cc seems to work out at 4921cc for the V10. Searching the test car for the extra cubic centimetre revealed no clues.In fact, the two banks of the balancer-shaft V10 are constructed almost as separate engines on the same crankshaft. Each has its own turbocharger, intercooler, ECU, air filter, exhaust gas recirculation system and so on. The air intake and exhaust systems are independent, but both banks, of course, are ultimately under joint control, and there's absolutely nothing in the way of driving the car to draw attention to this unusual arrangement in the powertrain.Out on the open road, the V10 TDI is quite amazing, for ultimate power output, colossal torque and remarkably quiet running. As we said in the launch report, the engine is so tucked away below thermal and acoustic covers that you can open the bonnet and hardly notice that it's there. And when you let the acceleration rip, the fruity exhaust note which results certainly doesn't sound like your average diesel.It's not really necessary to describe the performance of the V10. It goes just as impressively as you'd expect for a car with a power output of nearly 310bhp, and a torque peak of more than 550lb/ft (no misprint) at 2000rpm. This is a very quick road machine, with well controlled handling for a 4x4, and a supple ride quality.Volkswagen couldn't source a European gearbox able to cope with the colossal torque. It went to Japan for the transmission, but has made sure it's operated via a six-speed Tiptronic selector, which as usual works very smoothly. Gearing is high enough for the Touareg V10 to motor along in sixth showing nearly 40mph for every 1000rpm; so this is one turbo diesel 4x4 which really is a relaxed motorway cruiser.Especially since Porsche is building its Cayenne version of the same basic design, some observers have been worrying that the Volkswagen badge might not be "strong" enough for a 4x4 range which tops out at over £50,000. For engineering, build quality, equipment and general attention to detail, I'd say the Touareg V10 isn't giving any ground. Engine 4921cc, 10 cylinders Power 308bhp Fuel/CO2 23.2mpg / 329g/km Acceleration 0-62mph: 7.8 seconds Top speed 140mph Price £50,440 Release date 15/05/2010