The Porsche flat-six engine series is a line of mechanically similar, naturally aspirated and turbocharged, flat-six boxer engines, produced by Porsche for almost 60 consecutive years, since 1963. The engine is an evolution of the flat-four boxer used in the original Volkswagen Beetle. The flat-six engine is most often associated with their 911 model, Porsche's flagship rear-engined sports car which has used flat-six engines exclusively since 1963. The engines were air-cooled until 1999, when Porsche started using water-cooled engines.