You notice the muted snarl when you nail the gas. Then you notice the soft rustle of wind around the A-pillars, the distant hum of the tires, and the concert hall clarity of the Bang & Olufsen audio system, even at triple-digit velocities. Yes, the 2018 Audi RS 5 Coupe is all grown up—a smooth, quiet, comfortable, connected grand tourer, albeit one that packs a 450-hp punch and, if you tick the right options box, a top speed of 174 mph.

All taut surfaces and sharp edges, the new RS 5 showcases the crisp design language being developed under the aegis of new Audi design chief Marc Lichte. Only the hood, roof, and trunk are shared with the regular A5/S5 coupes: The grille is shallower and wider, the fenders have been pumped 0.6 inch each side, and the new front and rear bumpers are adorned with the usual performance car perquisites such as vents, spoilers, splitters, and diffusers. All U.S. market RS 5s will come with black trim and 20-inch forged alloy wheels shod with 275/30 tires front and rear.

Although 2.9 inches longer overall, the new RS 5 weighs 132 pounds less than the previous model, and 33 pounds of that weight saving is in the body alone, which makes extensive use of aluminum stampings, extrusions, and castings. A carbon-fiber roof panel is available in Europe, saving an additional 6.6 pounds, but it won't be made available in the U.S.

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Inside is another of Audi's typically classy interiors, rich in stitched leather and soft-sheen aluminum. U.S. spec RS 5s will come standard with carbon-fiber trim—90 percent of buyers of the previous model opted for it—and Audi's impressively configurable 'virtual cockpit' digital instrumentation pack. A new RS-specific head-up display that in shows engine oil temperature, lap time, and shift lights in addition to speed and nav information will make its debut on the RS 5. The 0.6-inch stretch in wheelbase compared with the previous RS5, combined with new slimline sports seats, has given rear-seat passengers almost an inch more kneeroom.

The big news is under the hood, however. At the business end of the MLB Evo platform is the performance version of Audi's new turbocharged V-6, codenamed EA389. In the A5/S5 the engine is a 3.0-liter, but in the RS 5 it displaces 2.9 liters. The reason? A shorter stroke that helps the fuel/air mixture better fill the combustion chamber. The mixture is helped into the cylinders by two turbochargers mounted in the vee that deliver a maximum of 0.95 boost. Peak power is 450 hp, which is line ball with the output of the naturally aspirated 4.2-liter V-8 that powered the previous-generation car. The key difference is the V-8's power peak came just north of 8,000 rpm while the twin-turbo V-6 delivers its maximum power from 5,700 rpm to 6,700 rpm.

The engine drives all four wheels—of course—through a ZF eight-speed automatic transmission rather than the seven-speed dual-clutch auto used in the last car. In the RS 5 the quattro system delivers 60 percent of the torque to the rear wheels under normal conditions, but it can channel up to 85 percent to the fronts if it detects slip. The rear sport differential can also actively vary the torque to each of the rear wheels to ensure maximum traction, and all wheels can be individually braked to improve cornering agility.

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The RS 5 rides 0.3 inch lower than the regular S5. Dynamic Ride Control, which uses steel springs and three stage adjustable shocks connected diagonally via oil lines running through a central valve to reduce diagonal pitching and roll through corners, will be available as an option in the U.S. as part of a Dynamic Pack that includes red brake calipers and the RS sport exhaust with black tips. Those RS 5 owners who want more can upgrade further with the Dynamic Plus Pack, which adds giant 15.7-inch carbon ceramic brakes up front, a tire temperature and pressure display function, a carbon-fiber engine cover, and most importantly ups the car's top speed from 155 mph to 174 mph.

With its bellowing, naturally aspirated 4.2-liter V-8 under the hood, the old RS 5 had a whiff of hooligan about it, a NASCAR good ol' boy in a Hugo Boss suit. This new one is a much more cultured ride, and our experience confirms initial impressions after driving six-cylinder versions of the new Porsche Panamera: This Audi engine is a benchmark V-6. But the laws of physics are immutable, and a 90-degree V-6, no matter how well engineered, is never going to feel as free and fulsome as a 90-degree V-8. Audi's sound engineers have done a spectacular job—the RS 5 growls like a hungry grizzly bear on upshifts and snap-crackle-pops discreetly on downshifts, and it's all done in the exhaust pipes, they say, not through the speakers—but there's no disguising the faint graininess in the delivery.

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It was difficult to resist the temptation to take the old RS 5's free-revving V-8 to its 8,300-rpm redline, not just for the hell of it, but also because that's where the power was. It's difficult to justify taking the new RS 5's V-6 much beyond 5,700 rpm, even in Dynamic mode, and not just because that's where the power curve peaks before running flat through the next 1,000 rpm. The old V-8 developed 317 lb-ft at 4,000 rpm. The new V-6 has 442 lb-ft available from 1,900 rpm through 5,000 rpm. It's a torque monster, and rapid progress is most effortlessly achieved by short shifting before the digital tach starts flashing red to warn of the approaching rev limit.

As a result the new RS 5 doesn't sound as hellfire fast as the old one and doesn't feel it, either, thanks to better damping and more consistent steering that help deliver a calmer dynamic demeanor. Yet the stylish Audi coupe proved impressively fast over the switchback mountain passes on our test route through the tiny principality of Andorra, high in the Pyrenees Mountains between France and Spain.

As with all MLB-based performance Audis, the new RS 5 rewards precise corner entry when driven briskly. Although the 2.9-liter V-6 is 68 pounds lighter than the 4.2-liter V-8, its mass is still all right over the front axle; this is not a chassis that naturally wants to rotate in a hurry. But those big front tires deliver a ton of grip, and the RS 5 will go exactly where you point it, especially with the optional Dynamic Ride Control system offering additional support to the outside front corner of the car. Then it's a matter of a little patience before getting hard on the gas as the corner opens up. The Audi digs deep into that rich vein of torque and, with the rear diff carefully monitoring the traction algorithms, slingshots out of corners.

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At low speeds in Dynamic mode, the suspension still returns sharp vertical motions on a rough road, but there's more finesse to it all than in the old car, and as speeds increase the ride calms down nicely. Again, the Dynamic Ride Control system seems a worthwhile option, deftly quelling excessive diagonal pitch and roll motions. For daily driving, switch the Audi Drive Select to Auto mode, let the eight-speed automatic do the thinking, and sit back and enjoy the RS 5's quiet and comfortable cruising capability. There's plenty of power on tap when you need to pass or plug a gap in fast-moving freeway traffic. There's no need to fuss over whether you're in the right gear—torque is your friend.

The 2018 Audi RS 5 is more restrained car than either the old RS 5 or AMG's loud and urgent C63 S Coupe, and that's just the way the folks at Audi Sport say they want it to be. The new RS 5, they insist, is the grand turismo of the Audi Sport RS lineup, a car designed to be driven long distances at high speed without raising a sweat. It's still quick—the claimed 0-60-mph time of less than 3.9 seconds is right in C63 S territory—but it's more relaxing. Understand that, and you won't be disappointed. First cars arrive in the U.S. in the first quarter of next year, with a price tag expected to be around $70,000.

Audi chases economy as well as performance

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It might be the performance engine of the new Audi V-6 lineup, but the twin-turbo EA389 engine uses an innovative combustion process developed by Volkswagen Group engineers to save fuel. It's called the B-cycle, and it is essentially an evolution of the Miller cycle process invented by American engineer Ralph Miller in the 1950s and used by Mazda in the supercharged 2.3-liter V-6 that powered the Millenia sedan more than a decade ago.

The basic idea behind a Miller-cycle engine is to leave the intake valve open on the compression phase longer than normal and using a turbocharger or supercharger to compensate for the fuel-air mix forced out of the cylinder and through the open valve by the rising piston. The advantage is the mechanical compression ratio can be kept high—it's 10.0:1 in the EA389—and the compression phase short. Combustion takes place in a relatively small volume, and the expansion phase is longer than normal, enhancing efficiency.

Audi compensates for the Miller cycle's inherently smaller cylinder charge with twin turbochargers and the two-stage Audi Valvelift System (AVS). At higher load and engine speed ranges, AVS closes the inlet valve later. The opening time increases from 130 to 200 degrees crankshaft angle, while at the same time the valve lift increases from 0.2 inch to 0.4 inch.

Another key element of the Audi design is the central position of the injectors in the combustion chamber. The common-rail system directly injects the fuel into the combustion chamber at 3,625 psi to ensure a homogeneous spray pattern and a uniform propagation of the flame front.

Audi claims the B-cycle system, which has been specifically designed to be most effective under partial throttle loads, helps the EA389 deliver 17 percent better fuel efficiency (as measured on the European cycle) than the 4.2-liter V-8 it replaces, despite producing the same power and 39 percent more torque.