VW-Bus
The longest production flat-four engine is the Volkswagen air-cooled engine, which was produced from 1938 until 2006 and was most famously used in the rear-engined 1938–2006 Volkswagen Beetle and 1950–1990 Volkswagen Transporter. This air-cooled engine was designed by Porsche and was also used in the 1948–1965 Porsche 356, 1953–1956 Porsche 550, 1965–1969 Porsche 912 and 1969–1976 Porsche 914. In 1982, to comply with exhaust emissions regulations a water-cooled version called the Volkswagen Wasserboxer was introduced in the Volkswagen Transporter (T3)