But a new sort of homogenization is underway. The Audi RS5 TDI prototype that we just flogged at the brand’s handling track at Neuburg an der Donau has managed to reset our notions of diesel engines. This 385-hp 3.0-liter V-6 TDI—still a prototype, but close to series production—has closed gaps with gasoline engines in areas where diesels used to be hopelessly outclassed.
As you might expect with 553 lb-ft arriving at just 1250 rpm, response at tip-in is brisk. From rest, the RS5 TDI charges forward without delay. In a drag race with a gasoline-fueled 553-hp RS6 Avant turbo V-8, the RS5 V-6 TDI held off the more powerful V-8 car for several car lengths. The TDI’s midrange response is also immediate, feeling like a high-performance, naturally aspirated gasoline engine, with none of the laziness and delay typical of turbo-diesels. This RS5 is surprisingly rev-happy, twisting the tach needle all the way to a relatively un-diesel-like 5500 rpm. We estimate the RS5 TDI’s sprint from zero to 60 mph will take about four seconds, with 100 mph arriving in around ten. Top speed is 174 mph, Audi claims. Despite these impressive numbers, the RS5’s 385-hp oil-burner is projected to use less fuel than the 313-hp version of the V-6 TDI it is based on.
The key to the RS5 TDI’s performance and efficiency gains is a Valeo-supplied electric supercharger. It is positioned at the "cold side" of the intercooler and, thanks to a low-inertia rotor, capable of reacting within 100 milliseconds and revving to 70,000 rpm. It can blow at full blast independent of engine load and revs. This allows the engine’s twin turbos to be calibrated for maximum boost—up to 49.3 psi—and power. The leap to 385 horsepower at 4200 rpm is solely achieved by the turbos, not by the supercharger. And the TDI’s ample 553 lb-ft of maximum torque is available from 1250 to 2000 rpm. These figures mirror those of BMW's 381-hp, tri-turbo inline-six diesel, which is an impressive engine, but falls far short of this Audi mill in terms of responsiveness.
The electric supercharger requires a separate 48-volt power supply, and the conversion from the 12-volt system adds cost and complexity. Down the road, cost is expected to diminish as more electrical functions are integrated into a common 48-volt system.
Audi's own, seven-speed dual-clutch transmission would be the perfect companion for this ultrasporty diesel engine, but it lacks sufficient strength to handle the RS5 TDI’s torque. And so the RS5 TDI is fitted with the ZF 8HP eight-speed automatic, a quick-shifting and efficient unit that has become a benchmark among these types of gearboxes.
Lordy that sounds like a fun ride. I'm not thrilled about not having a stick or a dual clutch, but as far as automatics go, that ZF8 is about the best thing since sliced bread!