The Porsche 911 model series is a family of two-door, high performance rear-engine sports cars, introduced in September 1964 by Porsche of Stuttgart, Germany, and now in its eighth generation. All 911s have a rear-mounted flat-six engine, and usually 2+2 seating, except for special 2-seater variants. Originally, 911s had air-cooled engines, and torsion bar suspension, but the 911 has been continuously changed and improved across generations. Though the 911 core concept has remained largely unchanged, water-cooled engines were introduced with the 996 series in 1998, and front and rear suspension have been replaced by Porsche-specific MacPherson suspension in the front, and independent multi-link suspension in the rear.