Sequential turbochargers have been adopted for this high-performance application; a small one (with variable geometry vanes) for increased low-rpm boost and a big one for greater power above 3500rpm, joined by a vacuum valve. The extra pressure put on the engine is handled via modified head cooling, revised intake cam timing and lift and reinforced pistons.
Peak power, at 309bhp between 3900- and 4500rpm, matches that of the BMW 535d exactly, while peak torque (479lb ft at 1450-2800rpm) exceeds that of the BMW twin-turbo. Which is why the performance claims are so serious: 5.1sec to 62mph for the regular A6 saloon. And, as is so appealing with fast diesels, economy is promised to be excellent too, at 44.1mpg.
There is only one transmission on offer with the A6’s new flagship diesel engine, and it’s an eight-speed torque converter automatic partnered with Quattro four-wheel drive. Otherwise, the BiTDi is marketed less like a full-blooded performance model and more like any other engine variant in the range.
You can get one in SE or S-Line trim, and with or without Audi’s Drive Select adaptive dampers, variable ratio sport steering and a Sport rear differential. Our test car was a relatively humble ‘SE’ spec example with none of Ingolstadt’s more trick chassis and driveline gadgets. Oddly, it didn’t even have a ‘BiTDi’ badge to distinguish it.
What’s it like?
Next to a 535d or an XF 3.0D S, the A6 has a noticeable shortage of dynamic poise and sparkle – which we’ll come to address. The BiTDi powertrain’s throttle responsive could be improved, too. When you initially flex the accelerator at everyday cruising pace, it takes a split second for that eight-speed gearbox to kickdown, and another for the turbochargers to come on song. You can summon up too much mid-range torque at times, in a bid to mitigate the delay.
But once the engine’s woken up and hauling, its outright force is formidable. It burns through transmission ratios with disdainful urgency, spinning tunefully between 3000 and 4500rpm, and hurling the big A6 forward with the kind of pace that you could only find the margins of on derestricted German autobahn, above 100mph.
But what about in the outside lane of the M6 – or anywhere else in the UK, for that matter? The BMW 535d has long been revered by Autocar because, hand-in-hand with such fierce speed, it offers the taut, engaging handling of a proper sporting option. The A6 is a different sort of prospect. More comfortable and refined than the BMW, it seems much more geared towards the fleet market than the real-world thrill-seeker. It’s a high-performance business tool, rather than a performance tool well suited to business during the working week – that’s the bottom line.
And the difference is plain. While the A6 BiTDi would soothe away hundreds of miles of motorway in a near-effortless calm, it’s easily stretched through a tightening bend. The car’s steering wheel is large, and its rack slow just off-centre, making the front-end quite tricky to guide on a demanding road. Body roll is no better contained than in a fairly run-of-the-mill diesel family car. And while grip is actually quite equitably split between front and rear axles off the throttle, there’s more power-understeer than a keen driver would like during hard driving; little in the way of natural sporting poise to be enjoyed.
Should I buy one?
Maybe. As we discovered on the car’s launch last year, the A6’s ride and handling is extremely sensitive to optional specification. Spend close to £60k on a fully-loaded BiTDi S-Line, with adaptive damping, sport steering and a sport rear differential, and you could find it a greatly altered machine; a much closer match for a BMW 535d M Sport, even. Such is the richness of Audi’s options list at the moment. Or the paucity of the BiTDi’s standard specification, you could argue.