1928 Bentley 4.5 Litre

Much of the legacy and cachet of Bentley Motors is attributed to its accomplishment during its first dozen years, from its establishment in 1919 by W.O. Bentley until it ceded control of the company to Rolls-Royce in 1931. The accomplishments on and off the race track, with many of those by the so-called 'Bentley Boys,' earned the company many headlines and sales. Early success in the 1922 Isle of Man Tourist Trophy, when Bentleys finished second, fourth and fifth to take the Team Prize, led to the introduction of the TT Replica, later known as the Speed Model. By the middle of the decade, however, the 3-Liter's competitiveness was on the wane, coupled with the fact that too many customers had been tempted to install unstable heavy coachwork to the 3-Liter chassis rather than endure the expense and complexity of Bentley's 6½-liter.

After victory in 1924 and subsequent failings in 1925 and 1926 with the 3 Litre, Bentley sought to increase displacement in their four-cylinder car for the following year.

The first team car equipped with the 4.5-liter engine was 'Old Mother Gun,' which was essentially a three-liter team car fitted with a new cylinder block. Driven as a prototype before production, it proved incredibly fast during the 1927 Le Man's race until the historic Maison Blanche crash involving eight cars including all three Bentleys. With two cars out and the third badly damaged, Bentley still managed to win the race with drivers Dudley Benjafield and Sammy Davis in a 3 Litre Speed model. This was Bentley's second victory in the endurance classic and they had done it with a considerable 20-lap margin, a damaged car, and after undergoing repairs in the pits that cost them a half hour. A year later, Woolf Barnato and Bernard Rubin drove a 4 1/2 Litre Bentley to victory after a race-long duel with the Stutz of Edouard Brisson and Robert Bloch. The 4 1/2 Litre Bentley would repeat its victory at LeMans in 1929. Among the other notable accolades include a victory at the French Grand Prix and 500 miles of Brooklands in 1930.

The 4 1/2 Litre Bentley was introduced in chassis form in October 1927 at the Olympia Motor Show and it successfully combined elements of both the 3 Litre and the 6 1/2 Litre models. It had a wheelbase size of 130 inches, an overall length of 172 inches, and was both spacious and heavy, yet well-balanced and nimble. The suspension used semi-elliptic leaf springs at all four corners and stopping power was provided by conventional, 17-inch drum brakes that were finned for improved cooling.

The 4,398cc four-cylinder engine had a 3.9-inch bore and a 5.5-inch stroke. It used the same bore and stroke as the six-cylinder 6½-Litre but retained the 3-Litre's shaft-and-helical camshaft drive. There were two SU carburetors and dual ignition with Bosch magnetos. A single, crossflow, overhead camshaft actuated four valves per cylinder - a rather advanced design considering most cars used two valves per cylinder in a side-valve cylinder head. Bevel gears drove the camshaft on a vertical shaft at the front of the engine, similar to the 3 Liter engine. In the road-going guise, the engine produced 110 horsepower; it produced 130 horsepower in racing tune. The engine was backed by a four-speed manual unsynchronised transmission.

Most of the customer chassis was clothed as saloons or tourers. Between 1927 and 1931, the company built 720 examples of the 4½ Litre cars, including 55 with a supercharged engine popularly known as the Blower Bentley. All but 11 of the naturally aspirated 4½ Litre cars rested on the 10-foot, 10-inch wheelbase chassis, with the balance resting on the 9-foot 9.5-inch platform. Vanden Plas provided touring bodies for 669 Bentley vehicles between 1922 to 1931.

W. O. Bentley was the chief engineer of the company he had founded and was reluctant to allow major modifications to his engine, and this included the addition of a supercharger. So when 'Bentley Boy' Sir Henry 'Tim' Birkin was searching for new ways to extract more performance from the proven 4½-Liter model, which was becoming outclassed by its rivals on the racetracks of Europe, he embarked on a private venture. With the help of engineer Amherst Villiers, a supercharger was placed at the end of the crankshaft, in front of the radiator. Although it increased the performance of the engine, it did cause increased understeer due to the additional weight at the front. A special crankshaft, pistons, and lubrication system were used, and a guard protected the two carburetors located at the compressor intake. Horsepower rose to 175 hp at 3,500 RPM with 9½ lbs boost for the touring model and 240 horsepower at 4,200 for the racing version, which meant it was more powerful than the 6½ Litre Bentley. W.O. Bentley never supported the development of the supercharged car and was quoted as saying how much he 'disliked the easy shortcut provided by the supercharger.' His preference for increasing the engine's output was to increase its displacement capacity, and he preferred reducing the weight at the front by using Elektron castings. However, since Sir Henry Birkin was an influential customer, backed by the wealthy Honorable Dorothy Paget and Woolf Barnato, the Supercharged 4½-Litre Bentley come to fruition.

The 'Blower' Bentleys used a Villiers Supercharger Mark IV, of Roots-type with twin paddle rotors, which drew a mixture from twin SU carburetors and was driven off the front of the crankshaft.

by Daniel Vaughan | Oct 2021