Audi RS 5 and A6 TDI Concept 2014 Review

The turbocharger is a well-understood technology. Much like a hydroelectric plant, you simply stick a paddle wheel into the middle of the exhaust gas flow and wait for it to spin up.

While a hydro system swings that into alternating current juice, a car attaches a shaft to the paddle wheel and puts another paddle wheel on the other end of it to shove more air back into the cylinders. So, the more gas you have flowing through the exhaust, the more performance you can have.

The trouble has long been what to do when there isn’t enough gas flowing through the first paddle wheel. When you mash the throttle at a turbo engine’s lower revs and not much happens, that’s called turbo lag.

Audi has teased us before with a short drive of an A7 equipped with a prototype electric turbo that’s intended to fix all of that and now as the electric turbo edges towards production, it’s done it again with two cars that are far better engineered than that 2012 A7.

The first is called the A6 TDI concept, based on the single-turbo-diesel version of the 3.0-litre V6 Audi A6. The second is the RS 5 TDI concept, based around the biturbo diesel version of the same motor.

What they both have in common is an electric ‘turbo’ charger, dubbed e-boost, sitting between the intercooler and the induction system. Connected to a 48-volt electrical system and its own lithium-ion battery, the electrical charger (as it’s more accurately known) then simply responds where and when the gas-flow system cannot.

In each of the Audi concepts, the charger has up to 7kW of power available to it to turn the turbo’s compressor and it can respond to the throttle in 250 milliseconds, coming perilously close to the sort of throttle response times you’d expect from a naturally aspirated performance engine.

In the more accessible (well, for a concept) A6 TDI concept, it adds up to 240kW of power and 650Nm of torque from 1500 to 3500rpm, though the electric turbo helps out to fill in the traditional gap below that peak.

For example, it cuts the sixth-gear sprint from 60-120 km/h from 13.7 seconds to just 8.3. And that’s pretty significant.

The immediate impression is one of noise and smoothness, with Audi also taking the opportunity of fitting the concept with a fatter boom box to give it a sound so rich it nearly puts the current RS 5 to shame.

It nearly puts it to shame in a straight sprint, too, with a handful of drag races against the current RS 6 Avant showing the turbo helping it off the line to run with the rocket up to about 40km/h before gallantly falling back.

It’s as smooth as it’s always been, but now its responses are snappy and as crisp as a burnt toast. And there’s no downside, really, except for the additional weight of the DC/DC converter (to make the 48-volt system talk to the car’s standard 12-volt system) and the lithium-ion battery.

Then there’s the RS 5 TDI concept. This is based on the sequential turbo system of the V6 3.0-litre TDI and is a rocket. Audi claims it’s capable of hitting 100km/h in a neat four seconds. Four! And it is capable of reaching up to 280km/h.

And it feels right. Compared to the single-turbo A6 TDI concept, the RS 5 TDI concept feels and sounds outrageously brawny, as if Audi has tried to prove that a diesel could replace the petrol in the RS 5 by noise standards alone.

And it almost could. Even today. The power number might be only 283kW compared to the current car’s 331kW at a lofty 8250rpm, but the TDI Concept’s power peak arrives at a relatively early 4200rpm. But that’s not as early as its torque figure.

Where the V8-powered RS 5 delivers 430Nm of torque at 6000rpm, the RS 5 TDI Concept has 750Nm from only 1250rpm. And beneath that, the electric turbo gets down to work.

It gets to work so well that in a side-by-side sprint, the RS 5 TDI Concept leaves the RS 6 Avant lagging behind up to around 50 km/h before the biturbo V8 wagon begins to haul it in, finally getting on top of the concept at around 90km/h.

But it’s more than that. The entire car is more accurate than any other car fitted with a diesel – or a turbo – engine we’ve ever driven.

Its throttle response is outrageously accurate, letting you tuck the coupe’s nose in to corner apexes from an understeering stance with a little lift off. It allows the car to haul out of corners with instant response and that changes the car’s mood into something far more aggressive.

You can pick up the throttle earlier and it makes the car feel far sharper even than the atmo V8, which can’t match the RS 5 TDI Concept’s torque figure at lower revs and can’t match its throttle response, either. And it’s not as loud or aggressive in the exhaust note, if you can believe that.

It gathers speed at rates that are verging on ferociously sneaky, because the RS 6 Avant in front rarely gets away from it, especially when the two cars are punching out of. It makes effortless things that once took a lot of thought, planning and frustration in normal turbo motors, petrol or diesel.
The beauty of the e-boost is that it only operates up to 3000rpm (because by then there is enough exhaust gas pumping through the turbo wheel to render it unnecessary), when it’s bypassed.

There were people curious about why Audi would call this an RS 5 TDI Concept, because a diesel could obviously never replace high-revving petrol V8s in the RS 5.

But, now that we’ve driven it, the future of petrol engines in high-performance Audis isn’t quite so clear anymore…

2014 RS 5 TDI concept pricing and specifications:
Price: N/A
Engine: 3.0-litre six-cylinder twin-turbo-diesel
Output: 283kW/750Nm
Transmission: Eight-speed automatic
Fuel: N/A