Audi A6 3.0 BiTDI quattro

What is it?

An Audi A6 that’s been eight years coming. That’s how long Audi brand devotees have been waiting for Ingolstadt to produce a reply to one of the most convincing modern sports saloons of the last decade: the defining hot diesel exec, BMW’s 535d. Which seems prudent in the extreme considering the ascendant car-maker’s phenomenal expansion in other segments, its continued investment and improvement in ‘TDi’ diesel technology across the board, and the obvious marketing opportunities delivered by its five diesel-powered wins at Le Mans.

But finally, that reply has arrived on British shores: it’s the A6 BiTDi. Just like BMW did, Audi has turned to twin-turbocharging to produce a sizeable slug of extra performance from its 3.0-litre six-cylinder commonrail diesel engine. It will be offering it in the A6 saloon, A6 Avant, new A6 Allroad and A7 Sportback this year.

The engine’s basic architecture is the same as it is with the two slighter-tune V6 TDi engines in the A6 range. The 2967cc unit is just 44cm long, weighs less than 200kg, uses a cast iron crankcase and an aluminium alloy head, and has an intelligent dual circuit cooling system which bypasses the crankcase for faster warm-up and greater efficiency on part-load. Its four camshafts are driven by two timing chains, which reduces both friction and weight compared to a four-chain drive system.

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.