1926 Bentley 6.5 Litre

W.O. Bentley's first vehicles were powered by a four-cylinder engine with a four-valve head and a three-liter displacement. They earned Bentley two Le Mans victories and helped establish the reputation of the company. However, to remain competitive and to be able to carry heavy and luxurious coachwork, the size of the engine and number of cylinders needed to increase. To this end, he began creating a six-cylinder version of the existing engine. The resulting six-cylinder engine was basically the existing four-cylinder unit with two additional cylinders and a displacement of 4.5 litres. While road-testing a prototype in France in 1924, W.O. happened upon a Rolls-Royce prototype that was also undergoing tests. A race quickly ensued and when it was over, W.O. Bentley was convinced that his new flagship needed more performance. The result was the 6 1/2 Litre which may have brought about the demise of the company even further, as it was Rolls-Royce that purchased a bankrupt Bentley early in the Depression, for the purpose of removing it as a competitor.

In period, the 6.5 Litre was known as the Big Six. Its engine was a development of Bentley's three-liter powerplant. First shown at the London Motor Show of 1925, these cars were with customers by March 1926. During its production lifespan over a five year period, a total of 545 examples were built. 362 were the 'Standard' 6½ Litre and 182 were Speed Six specification.

The engine in the 6 1/2 Litre had four tulip-shaped valves per cylinder and was fed by a single Smiths updraught carburetor. The overhead camshaft was driven by two small eccentrics coupled to triple connecting rods, the drive for which came from a helical gear at the rear of the crankshaft. The crankshaft was a multi-disc torsional damper, the compression ratio was 4.4:1, and the ignition was by two magnetos (later coil and single magento) which fed a pair of sparking plugs per cylinder. There was a single-plate clutch and a modified gearbox with notably high ratios. The third gear had a ratio of 1.278:1 which was the highest ever used in a standard W.O. Bentley.

The chassis was given one tubular and four pressed-steel cross members for additional rigidity and strength. The suspension was comprised of semi-elliptic leaf springs at all four corners, for which there were three specifications depending on the chosen wheelbase and type of body. There were three different wheelbase sizes making the 6.5 Litre a very versatile vehicle capable to accommodate a wide range of customer requests and needs. Braking was mechanical to all four wheels. The steering was by worm and wheels and the damping was by Bentley and Draper Duplex or Hartford Duplex.

Top speed was achieved at 3,500 rpm and 84 mph. In 1926, the chassis alone cost circa £1,450 to £1,575, while that of a complete car would have been in excess of £2,000.

The 6½ Litre model spawned the Speed Six, which is said to have been W.O.'s favorite among all the illustrious motorcars he created. Introduced in 1928, it would become the most successful racing Bentley. Modifications included a high-performance camshaft, a compression ratio of 5.3:1, a single-port block, and two SU carburetors. The most popular of the three wheelbase sizes was the smaller 138 inch platform. The other sizes included a 140.5- and 152.5-inch size.

Perhaps the most famous of the Speed Six was the Blue Train Bentley which Woolf Barnato raced in March of 1930 against the Blue Train. The vehicle wore H.J. Mulliner saloon coachwork.

by Daniel Vaughan | Aug 2019