Auto Union Type D Grand Prix racing car with dual compressor, 1939
Auto Union developed the 12-cylinder Type D Grand Prix racing car for the 1938 racing season, in which a new international Grand Prix rule limited the size of the engine to 3 litres in capacity. In 1939, a modified version of the Type D was introduced. A characteristic feature of this layout was the Roots dual compressor, the installation of which required an additional air box to be incorporated in the bonnet. The Type D with dual compressor made its first race appearance on 23 June 1939 at the Belgian Grand Prix at Spa-Francorchamps.