1930 Bentley Speed Six

The original W.O. Bentley automobiles began with the 3-Liter of 1919. Sales success was spurred by the sporting reputation earned on the track, which led to commissions for large and more luxurious coachwork on the chassis. The heavy coachwork and short chassis of the 3-Litre cars prompted a need for more power and a longer chassis. W.O. addressed the performance issue by essentially taking the three-liter four and adding two additional cylinders, displacing 4.25-liters. One of the design changes included the replacement of the vertical shaft camshaft drive with a new system where a crank-driven bevel gear turned a small triple-throw crankshaft, which operated a similar crankshaft on the cam, connected by a set of three connecting rods. This design would continue through the later eight-liter cars.

A prototype car with a Weymann saloon body was tested by W.O Bentley on a country road when he encountered a Rolls-Royce. Both cars were being driven at speed, which led to an impromptu race. W.O. soon recognized the driver of the car as being a Rolls-Royce engineer. In the end, neither was able to gain a clear advantage. This convinced Bentley that more performance was required. This was accomplished by increasing the engine's displacement from 4 1/4 liters to 6 1/2. The new car, introduced in 1925, provided the necessary performance that the luxury car segment demanded.

The 6.5-Litre was not solely reserved for luxury transportation, as competition versions soon followed. The Speed Six was introduced in 1928 and would become the most successful racing Bentley. Captain Woolf Barnato took delivery of the first example in May of 1929. They were similar to the standard models, but with several important performance-oriented upgrades, including a pair of S.U. carburetors on a new square section intake manifold. Horsepower rose to 180, and would later reach 200 hp in LeMans spec racers.

The Speed Six made its race debut at the 1929 Double Twelve at Brooklands, where the Works car lead the race with an average speed of 92 mph before it was forced to retire with a sheared dynamo drive.

At the 1929 24 Hours of LeMans, three 4.5-liter cars were joined by the new Speed Six being driven by Barnato and Sir Henry 'Tim' Birkin. One of the 4.5-liter cars was forced to retire early. The other four Bentleys lead most of the race, with the Birkin/Barnato Speed Six in the lead. By the closing hours of the race, the Speed Six was so far ahead of the rest of the competition, they were instructed by the team to reduce to touring speed to save the cars.

In the end, the Speed Six had an average speed of over 70 mph, and the average would have been higher had W.O. not ordered the cars to reduce the speed after the retirement of the two supercharged Stutz, the Bentley's only real competition.

At the 1930 24 Hours of LeMans, three examples were entered. Barnato and Kidston drove car #4, Clement and Watney drove car #2, and Davis and Dunfee piloted #3. Dunfee crashed #3 fairly early in the race. The two remaining Speed Sixes were left to contest the competitive supercharged Mercedes-Benz driven by Caracciola. Although Caracciola led for a time, the speed of the Bentleys forced Caracciola to use his supercharger extensively, eventually forcing his retirement on the 83rd lap. With the competition out of the race, the Bentleys were once again ordered to slow to touring speed. When the checkered flag fell, Barnato and Kidston were in first place, followed by Clement and Watney in second. For the fourth year in a row, Bentley had won at LeMans.

Between 1929 and 1931 the Speed Six achieved 5 first places and four second-place finishes in major races.

Production
The 6.5-Litre Bentley was produced from 1926 through 1930, with 544 examples produced. One hundred eighty-two of these would be built to Speed Six specification.

Specification
The standard 6½ Litre had a 140 mm (5.5 inch) stroke and a 100 mm (3.9 inch) bore, giving the six-cylinder unit a displacement of 6.6 liters (6,597cc / 402.6 cubic inches). It had four valves per cylinder, an overhead camshaft, and a single-piece engine block and cylinder head cast in iron. With a compression ratio of 4.4:1, a single Smiths 5-jet carburetor, and twin ignition magnetos, the engine delivered 147 horsepower at 3,500 RPM.

The four-speed transmission had a dry-plate clutch and braking was via power-assisted four-wheel finned drums, with the fronts having four leading shoes per drum. This setup allowed the driver to adjust all four brakes to correct for wear by operating a patented compensating device.

Wheelbase sizes varied from 132 to 152.5 inches with most being a size of 150 inches.

The engine of the Speed Six had a single-port block, dual SU carburetors, 5.3:1 compression, and a high-performance camshaft boosting output to 180 horsepower at 3,500 RPM. Wheelbase sizes included 138, 140.5, and 152.5 inch options with the short 138-inch platform being the preferred option.

by Daniel Vaughan | Nov 2019