MINUS TWO, PLUS PLENTY; AUDI'S NEXT-GEN S4 SEDAN ADDS MILEAGE, A BELT FOR ITS SUPERCHARGERBY J. P. VETTRAINO.(NEWS)

Byline: J. P. Vettraino

The all-new Audi S4 dumps one long-standing tradition for an even older one.

When it reaches U.S. showrooms next fall, the 2010 S4 ("Audi S4, AW, Sept. 29, 2008) will trade the previous model's normally aspirated 4.2-liter V8 for a blown V6. Yet its new 3.0 TFSI V6 abandons the exhaust-driven turbochargers that Audi has been developing since the 1970s in favor of a belt-driven superchargersomething the brand hasn't used since the Auto Union Grand Prix cars of the 1930s.

Audi decided early in the S4's development to downsize the V8, but choosing an alternative was problematic. Engineers weighed Audi's recent twin-turbo V6s against the latest supercharging technology using seven key measures, from cost to NVH to efficiency to peak horsepower. They decided that supercharging was superior to turbocharging in six of them.

The 3.0 TFSI features direct fuel injection, which allows a 10.3:1 compression ratio, even with the supercharger tucked into the valley of its 90-degree V. The blower spins roughly three times engine speed, up to 23,000 rpm, and delivers a peak boost of 11.6 psi. This Audi V6 applies the same supercharging technology that is used in the new Corvette ZR1 and the Cadillac CTS-V. The supercharger was developed with Eaton, and each of its twin rotors has four spiral-shaped vanes that intermesh precisely with those on the other rotor as they spin. Air gaps between them measure a few thousandths of a millimeter.

Efficiency gains with the four-vane rotors are substantial, compared with similarly sized conventional superchargers: approximately 20 percent more airflow, with a similar improvement in thermal efficiency. Also significant, crank power required to operate the supercharger drops 35 percent. The intercooler is packaged inside the supercharger case, right next to the intake ports, and it helps dampen noise emanating from the spinning rotors.

The bottom line for the 3.0 TFSI V6: 333 hp at 5,500 rpm, or 11 hp less than the V8. Torque increases 12 lb-ft to 325, starting at 2,900 rpm. With 25 percent fewer cylinders sucking fuel, fuel consumption drops 27 percent, according to Audi. The company projects fuel economy ...