Porsche returned all-wheel-drive models to the 911 lineup for 2006. The Carrera 4 coupe and convertible, and the Carrera 4S coupe and convertible were equipped with Porsche's Intelligent All-Wheel-Drive system, which transfers power as required to the front axle for maximum grip on all types of road surfaces. A multiple-disc clutch and robust braking system are also part of the package. The 911 Carrera 4, as well as a base 911 Carrera, were powered by a flat, compact, rear-mounted 3.6-liter six-cylinder engine that produced 325 horsepower. Stepping up a notch in power, the 911 Carrera 4S, as well as a standard Carrera S model, received a 3.8-liter six-cylinder engine that pumped out 355 horsepower. A six-speed manual overdrive transmission was standard across the line, while a five-speed automatic was optional. As expected of a Porsche sports car, the 911 came with a long list of features, including air conditioning, power windows, power door locks, leather seats, cruise control, a leather-wrapped steering wheel with a tilt steering column, a trip computer, anti-lock brakes, traction control, stability control, and an AM/FM stereo system with a CD player. Fast and fun were the words most commonly used by drivers to describe the 911. The car's handling, performance, and sporty styling were also cited as positive points.