In pursuit of greater fuel efficiency and more power, the 2011 Mercedes S63 AMG adopts a 5.5-liter twin-turbo V8. So why the hell is it still called the S63?

The new engine, first shown today in Geneva, comes in two flavors: standard and with an optional performance package. In standard form it makes 544 HP and 586 Lb-Ft of torque, but with the performance package boosted, literally, to 571 HP and 660 Lb-Ft. These figures easily eclipse the old S63, which also insisted on using an engine that didn't match the badge, in this case a 6.2-liter naturally aspirated V8 that's in virtually every other AMG product. That car made a paltry 518 HP and only 456 Lb-Ft of torque.

The standard 2011 S63 is limited to 155 MPH, while the performance package-equipped car will hit 186 MPH.

Amazingly, all that extra power comes with a 25 percent increase in fuel economy. That comes courtesy of the 2010 E63's 7-speed SpeedShift MCT gearbox and it's selectable drive modes, as well as direct fuel injection, start/stop operation and regenerative braking. Mercedes says the new S63 will manage 23.5 MPG (US), albeit tested in the less difficult European standards. That may not sound like a huge deal, afterall AMG customers are buying for performance, not economy, but it should help squeak the S63 through without a gas guzzler tax, saving its customers money.

While the new engine will be fitted to the existing S-class, it'll premier in a concept car that is said to be inspired by the 1971 300 SEL AMG endurance racer.

Of course the car's been misnamed for as long as the 6.2-liter's been around, but now, even more-so. No explanation from Mercedes on the name is forthcoming, but we'll let you know if we get an official answer.