FRANKFURT MOTOR SHOW: Audi S8 gains twin-turbo V8 power

And cylinder deactivation gets the nod for Audi's new high-performance limo
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Better fuel economy, more power, more torque and some innovative technology will head up the headline list for Audi’s all-new 4.0-litre V8 engine.

The 3993cc V8 not only pumps out 382kW of power and 650Nm of torque, but can change character to become a V4, using 12 percent less fuel at 80km/h to bring the heavy-hitting S8 down to just 10 litres per 100km.

It will make its first appearance in the Audi S8 sports limousine in October before finding a home in the Bentley Continental GT next year. A simpler, lower-output version of the engine, with 309kW and 550Nm, will fit into the Audi S6, S6 Avant and S7 before both versions are fiddled with by Quattro for the RS versions.

All-new from the ground up, the S8 engine advances Audi's compact engine design philosophy (with a hat-tip from BMW for that company's defiance of convention). The exhaust valves are sited on the inside of the motor, firing the exhaust gases into the middle of the 90-degree vee, where two twin-scroll turbochargers sit.

Slightly undersquare, the engine has an 84.5mm bore and an 89mm stroke and is made from a new aluminium-silicon alloy.

The S6/7 version of this motor is strong enough for most, with 309kW at 5500rpm and 550Nm of torque between 1400 and 5250rpm, but it lacks the S8/Bentley version’s dual-branch air intake, extra oil cooler, additional turbo boost pressure and its other modifications to the crankshaft, main bearings, compression ratio, valve timing and fuel injectors.

All of that helps the S8’s V8 crunch out 650Nm in a flat line from 1750rpm to 5000rpm, then tops up the performance with 382kW at 6000rpm, yet the story of the performance is that it offers up 400Nm from as little as 1000rpm.

But when you don’t need all that performance, it can swap to a lump-less cam lobe to close both the exhaust and inlet valves on cylinders 2, 3, 5 and 8 to run as a V4, then shuts down the ignition and direct injection as well.

The system only switches across to V4 mode in a tightly controlled set of parameters, including when the engine is producing between 120 and 250Nm of torque, when the coolant has heated to 30 degrees, when the gearbox is in third or higher and when the revs are between 960 and 3500rpm.

There’s an indicator in the dash that changes colour to green when the S8’s running as a four cylinder, but that’s not all. Because V4 engines lack the inherent NVH qualities of a V8, Audi has fitted the S8 with both active engine mounts and Audi Noise Control to the cabin. Both systems work in a similar way, detecting unwanted vibrations and then using noise-cancelling technology to cancel them out.

When the driver needs more power, the sleeping cylinders fire back up within 300 milliseconds.


Engine specifications
S8 spec
3993cc
84.5mm x 89mm
90-degree vee
Direct fuel injection, twin-scroll turbo-chargers (2, one per bank), variable valve timing and lift
387kW @ 6000rpm (S6/7 309kW @ 5500rpm)
650Nm @ 1750-5000rpm (S6/7 550Nm @ 1400-5250rpm)
400Nm @ 1000rpm
12% fuel reduction at 80km/h
7% fuel reduction at 130km/h
10 litres/100km (S6/7 less than 10)

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