Since 1974, the leading model of the 911 series has traditionally been called the Turbo. In its standard form, it was presented in Frankfurt/Main in 1999, while in 2004 in Birmingham, the more powerful Turbo S version was presented. The vehicle is conventionally fitted with a permanent four-wheel drive and a viscotic clutch. Thanks to such a powertrain system, it is easier and safer to transmit the tremendous power coming from the 6-cylinder 3.6-liter boxer-type engine, accompanied by two turbo- chargers and developing 420 hp in the standard version, or 450 hp in the Turbo S version. Since the fall of 2003, the 911 Turbo has been available in the Cabriolet body version, with an electrically folded soft top, now also typical of a Turbo S version.