Audi has targeted their new Diesel bi-turbo engine (called BiTdi), which is available for now only on the A6 models, against the 5 series F10 535d, which produces the same power output of 313 HP. As the following videos will show, the 2012 BiTdi is almost as fast as the top of the Diesel range offered by BMW, the M550d.
Both have under the bonnet a 3.0 liters Diesel engine, but the BMW M550d has an output of 381 hp and 780 NM, and the A6 only 313 hp and 650NM. Also both put the power down through All-wheel-drive transmissions: XDrive and Quattro, thus making them very fast off the line.
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.
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.
A nice video showing how much Diesel bi-turbo technology has been evolved in the latest years.
My E60 535d with remapped ECU (350hp/700nm/1730kg) is almost 1 sec faster than the above on the 100-200 km/h acceleration range, 12.5 seconds in comparison with 13/13.5s (M550d/A6), mainly due to the fact that the weight is almost 200kg less.