The way it finally changes into fifth gear at about 150mph is particularly satisfying, I thought. Trouble is, the limiter means there's only another 5mph to go. Freed of that constraint, the Audi RS6 - the company's most powerful production car ever at 444bhp - would reach, according to its prime creator Stephan Reil of quattro GmbH, '303 to 305km/h'. Approximately. Which, if we divide by the required 1.609, gives us 189mph. Approximately. Here's another statistic. Between 1950rpm and 5600rpm, the torque of the V8 stays at a constant 413lb ft. What if the torque wasn't electronically limited to this rarefied plateau, if the twin turbochargers could deliver their maximum 0.8bar boost whenever they liked? 'Well, of course we could have more torque, maybe over 600Nm (443lb ft). But there's durability to consider.' This is truly a mighty engine, mightier even than those of the Jaguar S-type R and the BMW M5 which can muster just 400bhp apiece. Blip the throttle under load and, on its way to the virtually lag-free release of revs and thrust that is the kickdown, it roars like Tony the Tiger in need of another Frosties fix. As you accelerate to, then through, the 3000rpm mark it takes on a deep beat an octave below the usual V8 throb, and sounds like a close-quarters encounter with some helicopter blades. The RS6 is, as you would expect, based on the V8 S6 and shares that car's widened wheelarches and aluminium bonnet. The engine, wheel and cosmetic changes are obvious, but the RS6 also has a suspension tweak of heart-warming simplicity. Dr Alex Moulton, inventor of British Leyland's Hydrolastic and Hydragas systems as seen on the original MGF, would surely appreciate this entirely electronics-free approach to adaptive damping. That's right; the dampers displace fluid from one wheel to another via interconnected pipes. In this case, the connection is diagonal: front left to rear right, and vice-versa. The pipe runs into, and out of, a cylinder on the other side of whose piston is a pressurised, gas-filled chamber. There's also a by-pass valve, through which fluid passes during body roll because the extension of one damper can accommodate the compression of the other without moving the piston against gas pressure. Here, the dampers are at maximum stiffness for crisp cornering. However, if one wheel hits a bump, the fluid is forced into the cylinder and moves the piston against gas pressure, resulting in less of a build-up of damping forces and a more supple ride. Or if both wheels' suspensions compress simultaneously, as on an undulating road, fluid from both dampers in each pair pushes the gas-piston up further, depriving the dampers of the initial extra stiffness they normally have and giving a linear damping response for smooth body control without choppiness. Audi calls it, simply enough, Dynamic Ride Control (DRC). Invented originally by Yamaha as an anti-dive system for motorbikes, it's a joint venture between a Spanish damper company (APA) and a Japanese one (Kayaba). So we're on a clear autobahn in Audi's first twin-turbo V8. It's almost a twin-four engine, because the airflow meters, the KKK turbos with their ultra-light turbine wheels, the intercoolers, the plenum chambers and the exhaust systems are separate for each bank, a mixing of air occurring only at the hefty, 82mm, drive-by-wire, driving-style-adaptive throttle body. As an overtaker, it's extraordinary. There's no manual alternative, but the five-speed autobox (borrowed from the mighty A8 W12, and reprogrammed for much faster shifts) kicks down instantly when asked. In fact it's a little too eager here when in non-sport mode; an engine this torquey can happily hold a high gear, although kickdown is good if you want to emulate the forces that can bring on a 4.7-second 0-62 time. Surely no saloon can beat that. Switch to manual Tiptronic mode, and you can use Alfa-like paddles on the steering wheel. And here's why this is the best auto-turned-manual I've yet tried. The upshifts are instant yet smooth, the downshifts positive and rev-matched with a transmission lock-up which makes it feel like a real manual-based 'box. Think Ferrari F1 without the surges and clonks. So we're into proper driver involvement. Chassis-wise the old RS4 was sadly lacking here, but the RS6 is the most talkative Audi since the original Quattro. There's still some of the usual Audi rubberiness in the initial steering motion, depriving it of the mechanical precision of its Jaguar arch-rival, but once loaded-up in a corner the Audi proves agile, spectacularly grippy on its 19in wheels, and unexpectedly throttle-adjustable given its four-wheel-drive layout. Back off in a fast bend, and you can cue a drift like an Impreza Turbo's. And yes, it rides with decent suppleness despite the firm body control, so DRC is clearly working. It stops very effectively, too, as you would expect with eight-pot front callipers and decoupled, directional discs pinned to separate hubs for stress-free expansion. So, does RS6 trump M5 and S-type R? For pace, yes. For ultimate driver involvement, not quite. But it's a close thing. Hail the ultimate quattro.