The Bentley 6½ Litre and the high-performance Bentley Speed Six were rolling chassis in production from 1926 to 1930. The Speed Six, introduced in 1928, would become the most successful racing Bentley. Two Bentley Speed Sixes became known as the Blue Train Bentleys after their owner Woolf Barnato's involvement in the Blue Train Races of 1930. Background By 1924 Bentley had been in business for five years. He decided to build a larger chassis than the 3 Litre, with a smoother, more powerful engine. The new chassis would be more suitable for the large, heavy limousine bodies that many of his customers were then putting on his sports car chassis. The resulting car would be more refined and better suited for comfortable general motoring. Prototype race Bentley built a development mule with a 4¼ L straight-six engine derived from the 3 Litre's four cylinder engine. To disguise the car's origin, it had a large, wedge-shaped radiator and was registered as a "Sun". The chassis was given a large very light weight Weymann-type tourer body built by Freestone and Webb. W. O. Bentley combined one of his road tests of the Sun with a trip to see the 1924 French Grand Prix in Lyon. On his return trip to the ferry at Dieppe, W. O. encountered another disguised car at a three-way junction. W. O. and the Rolls-Royce test driver recognized each other and began racing each other along the routes nationales. This street race continued until the Rolls-Royce driver's hat blew off and he had to stop to retrieve it. The Sun's tyres were heavily worn when W.O. got to the ferry at Dieppe. 6½ Litre Realizing from the impromptu race that the Sun had no performance advantage over Rolls-Royce's latest development, W. O. increased the bore of his six-cylinder engine from to . With a stroke, the engine had a displacement of 6.6 L () Like the four-cylinder engine, Bentley's straight-6 included overhead camshaft, 4 valves per cylinder, and a single-piece engine block and cylinder head cast in iron, which eliminated the need for a head gasket. In base form, with a single Smiths 5-jet carburettor, twin ignition magnetos, and a compression ratio of 4.4:1, the Bentley 6½ Litre delivered at 3500 RPM. Although based on the 3 Litre's engine, the 6½ engine incorporated many improvements. The 3 Litre's cone-type clutch was replaced by a dry-plate design that incorporated