Explained: Audi RS 6 and RS 7 Performance technical details
Cars these days rely on technology for performance, so here’s how Audi’s quattro all-wheel-drive and associated tech works on the new RS 6 and RS 7 performance.
AUDI IS NEVER going to stop talking about its rally heritage with the quattro, and to be fair why should they, as the way they revolutionsed the rally world with all-wheel-drive technology and turbocharging remains one of the great motorsports game-changers.
The original Quattro saw Michele Mouton very nearly become the first female to win any world motoring championship in 1982, losing only at the last round to Walter Rohrl who became the first double champion. Following Audi’s lead, the rest of the field turned to turbo AWDs and the result was the impossibly fast cars of the Group B category which were so quick over unmade terrain they were banned, and regulations introduced to keep speeds in check.
Those early slab-shaped rallycars didn’t have any computers, but this latest incarnation of the quattro (small q for road cars and capital Q for motorsport applications) all-wheel drive system is of course CPU-controlled.
While torque is nominally set at a sporting 40% to the front, and 60% to the rear, that can be varied as conditions require to maximise traction, or combat understeer (running side) or oversteer (tail goes out, as per photo above!). In the case of understeer more torque may be applied to the rear wheels, and the reverse for oversteer. The centre differential which regulates the front/rear split can entirely lock, which is excellent news for traction, particularly should you find yourself in slippery conditions. The maximum split is 85/15, either front or rear.
One difference with today’s quattro system is torque vectoring. This is a means of steering a vehicle without really turning the wheels, or to be more precise keeping a vehicle on a neutral path around a corner instead of running wide (understeer) or turning in on itself (oversteer).
The vehicles have two types of torque vectoring, one which brakes the inside wheels in, through and out of a corner – that’s really more corner brake control than torque vectoring, but Audi seem to like the name. At the back the trick rear differential does indeed torque vector by being able to send different levels of torque left and right, as distinct from brake individual wheels as shown below (from an A6, but the principle is the same):
What you see above is a standard differential with two clutch packs either side. The red arrows indicate the amount of torque sent to each wheel on the rear axle – the computer-controlled clutches distribute torque (turning force) to assist with under and oversteer corrections. A normal differential would be prone to spin the inside wheel which is unweighted. An easy fix is a limited-slip differential, but that can promote oversteer you don’t want (well, not always) and increases the turning circle of the vehicle as well as tyre wear. Instead, torque vectoring of this nature is the perfect solution.
Power and torque
I suppose I’d better gush about the power and torque figures for the sake of form, so here they are – 445kW and 700Nm good for a 0-100 sprint of 3.7 seconds via an 8-speed automatic gearbox. There’s actually an “overboost” function to 750Nm, but that doesn’t work continuously.
Suitably impressed, let’s move on to fuel efficiency. There’s COD, or Cylinder on Demand – cylinders 2, 3, 5, and 8 shut down when not needed by partially closing the valves and with it disabling the fuel injection. Stricter and stricter emissions standards combined with increasingly efficient turbos and hybrids are fast negating both the need for and feasibility of V8s of any description, so enjoy them while you can. Perhaps this tech will keep the breed alive a little longer, but for us middle-aged types our grandchildren are only going to know the meaning of a V8 from “back in the day” stories. The result, Audi say, is a 10% improvement in fuel efficiency to around 9.5L/100km on the combined cycle, which is not bad for a two-tonne luxury V8.
A mere V8 was not enough for Audi, who have also bolted on a turbo (as a petrolhead, you just have to respect that sort of thinking). Not just any turbo, but a twin-scroll turbocharger. Turbochargers work by spinning a fan (turbine) in the exhaust gas, which drives a compressor that forces more air into the cylinder so you get a bigger bang and more power. But the cylinders in an engine do not push out a steady stream of exhuast gas, it’s more like a pulse as the piston goes up and down, and to make the engine run smoothly each piston in the cylinder is set to run at a different cycle so they produce power and exhaust at different times.
This uneven cycle means that as one cylinder is finishing its power stroke another may be trying to expel exhaust gas, and that can create unwanted interference between cylinders hooked to the same turbo as pressure attempts to equalise where it shouldn’t, sapping power.
A twin-scroll system has two intake and two outputs (“scrolls”), each connected to a different set of cylinders, so this interference is reduced, and exhaust gases can be more efficiently discharged. That means the turbine spins faster, more air is stuffed into the cylinder and the car goes quicker. It also means each scroll can be optimised slightly differently for different rev ranges.
Audi driving modes
Lots of power, but how do you handle it? Like most vehicles, the car’s setup can be changed. In this case the system Audi Drive Select where you can change the car’s mode from amiable family mutt to hungry timber wolf. Actually, that’s not really the terms Audi use, they prefer Dynamic, Comfort and Auto.
Selecting each one changes the gearshift patterns (how long it holds a gear), throttle sharpness (but not overall power), and suspension stiffness. The soundtrack can be changed too. This mode change is independent of the stability control settings which are on, sport and off (more here), although the changing the car’s mode also modifies the stability control algorithms which are looser than those set on the less sporty Audis. There’s also an Individual mode where you get to choose which settings you want from each option.
The stability control, at its default setting on A models will cut in even before the car skids; the computers look at the speed, steering wheel angle and other parameters then decide you’re likely to get into trouble even before you do. With the RS models the algorithim is changed so that you actually have to let the wheels slip a bit before the system cuts in. This is one reason why on our fast laps the system was left enabled – if it cut in you weren’t being smooth enough.
All the RS 6 and 7 range comes with standard adaptive air suspension. Air suspension dispenses with steel coil springs in favour of airbags. Shock absorbers (dampers) are still required though. The advantage of air suspension is the ability to vary the air pressure in the bag, which means stiffness and ride height can be instantly adjusted. Often air suspension is used to lower the ride height of fast vehicles at speed, which provides benefits for stability as well as reducing aerodynamic drag.
There is an option of Dynamic Ride Control (DRC), a suspension interconnect that links diagonal wheels via lines filled with oil controlled by clever valves. The DRC system reverts back to steel coils for spring, and uses the weight shift forces created by braking or cornering to stiffen the suspension so the car stays flatter or reduces nosedive. It is an entirely mechanical system, but its effect is varied by Drive Select. Imagine a car taking a left corner at speed – the vehicle will roll to the right, so the DRC will use that energy to stiffen the right suspension.
The reason for that is stability. Say you hook into a hard left turn – the body will roll relative to the tyres, and the momentum of that roll can be enough to break traction on the tyres. Incidentally, this is why you should always be smooth and progressive with your transitions, for example from braking to cornering, something you learn at Audi driver training.
So why not just stiffen up the suspension? Because then you don’t get the compliance and undulation-absorbing ability when you need it. That’s really the point of all this trickery, stiff suspension when needed, soft when needed as opposed to a literally uncomfortable compromise all the time.
Another option pack includes a steering feature, which changes not only the weight of the electric steering (which is what all steering modes normally do) but also the ratio. This means that at low speeds it takes a small turn of the steering wheel to turn the car, and at higher speeds, that same turn of the steering wheel won’t turn the front wheels as far. This aids straight-line stability at speed, and means less wheel-turning at lower speeds. For track use, Audi recommend disabling this feature so as to have consistency of steering input around the circuit.
Then you can specify ceramic brakes, the new thing in stopping technology. Brakes are made up of heavy discs (rotors) which are clamped by pads to slow you down. A heavy, fast car takes a lot of energy to slow down, and that energy gets converted into heat, a lot of it. The braking system’s job is to do that conversion, and then get rid of the heat so it can accept some more next time you slam on the anchors.
Ceramic brakes use rotors made of carbon ceramic instead of the usual cast iron, for a few reasons. Firstly, it’s better able to dissipate heat, so there’s less chance of brake fade which is that lovely, lovely feeling when you press the brake pedal and the only thing that starts stopping is your heart.
Another advantage is that ceramic brakes are also a lot lighter than standard. The weight saved is in exactly the right place – unsprung weight and rotational mass connected to the wheel, which means weight that is not directly controlled by the suspension which is pretty much everything else bar the wheels and brakes. Reducing unsprung weight is excellent news for handling as the suspension has to do a lot less work.
Ceramics are even good from the daily use perspective. There’s less horrible brake dust to clean, they don’t make as much noise as steel brakes, and they don’t suffer that unsightly reddish corrosion you see accumulate on standard brake rotors after a few days and a bit of moisture. Ceramics also last a lot longer, several times the life of the usual cast-iron equivalents. However, they don’t perform any better when wet.
The end result of all this tech is basically more speed for less effort, but the driver is still pleasantly involved in the process of moving rapidly. They aren’t strictly necessary for road use, but the bigger and faster the car, the better the brakes you need so it is good that they’re an option.