The trouble with Audi’s four-wheel drive A8 4.2 TDI is that it is just too good. Too good for its petrol equivalent, at any rate. Its new 4.2-litre V8 common rail turbodiesel engine gives Ingolstadt’s flagship saloon the sort of performance that makes its popular 4.2-litre V8 petrol sibling look rather ordinary. When fitted with this new engine, the three-year-old A8 delivers effortless ability in all weather conditions. It may just be the most complete luxury car on offer today.The hi-tech V8 can lay claim to being the most powerful diesel passenger car engine on sale. With 326bhp at 3750rpm, the A8 4.2 TDI confidently casts aside the likes of the 300bhp BMW 740d, upcoming 314bhp Mercedes-Benz S420 CDI and the 313bhp Volkswagen Phaeton V10. It’s also a 51bhp increase over the German car maker’s two year-old 4.0-litre V8 diesel engine - proof of the rapid pace of diesel engine development.As staggering as that power figure sounds, it’s the massive low-end torque and resulting flexibility that really dominates the engine’s character, maintaining a solid 479lb ft between 1600 and 3500rpm. Helping Audi’s new V8 scale such heights is a new Bosch developed common rail system featuring the latest piezo injectors.Mated to a beefed-up ZF six-speed automatic gearbox apportioning drive to all four wheels via Audi’s quattro system, the iron-block unit propels the 1995kg A8 4.2TDI from 0-62mph in just 5.9sec – 0.6sec inside the time Audi quotes for the petrol 4.2, and it comfortably reaches its limited 155mph top speed without ever feeling strained. It’s the romping in-gear acceleration, though, that makes the biggest impression. The rapid performance is matched by combined consumption of 31mpg, although if the instant on-board read-out is any indication the real-life figure is more like 21mpg when you’re pushing hard.There’s still some characteristic diesel chatter at idle, but once underway it is thoroughly civilised. The petrol might be more refined overall and possess a more charismatic delivery of power, but the TDI scores a clear victory on pace and, provided you’re easy with the throttle, economy. Not surprisingly, Audi has already confirmed that this new engine will find its way into its new Q7 off-roader later this year.