1930 Bentley 4.5 Litre

The Bentley 3 Litre model won the 24 Hours of Le Mans in 1924 and the 3 Litre Speed model in 1927. It had been proudly introduced by founder W.O. Bentley in 1919 on Stand 126 at the Olympia Motor Exhibition. This model, in mildly developed form with its classic radiator design, British Racing Green livery, and leather-strapped bonnet would become legendary in motor racing history. Among the accolades achieved was a second, fourth and fifth place finish in the 1922 Isle of Man Tourist Trophy earning the Team Prize.

When it came time to replace the aging 3 Litre model, Bentley did so with the 4½ Litre. The company had introduced a 6½ Litre model in 1926 which used a larger chassis than the 3 Liter paired with an inline-6 cylinder engine with a 6,597cc displacement (6.5-liter). The 4½ Litre model that followed in 1927 essentially combined the chassis of the 3 Litre and a four-cylinder version of the 6.5-liter engine.

It is believed that the first prototype engine was installed into a 3-liter chassis (the 1927 Le Man's practice car) and was later fitted to the first production 4½-liter chassis for that year's Grand Prix d'Endurance at the Circuit de la Sarthe. Nicknamed by the team 'Old Mother Gun,' it was driven by Frank Clement and Leslie Callingham who set the fastest lap of 73.41 mph before retiring early due to the infamous 'White House Crash' pile-up. Woolf Barnato and Bernard Rubin drove a 4½-Litre to victory at LeMans in 1928, the first of three wins by Barnato in the 4½-Litre.

Production continued through 1931 with a total of 720 examples (possibly 733) constructed including 55 cars with a supercharged engine, commonly known as the Blower Bentley.

The engine used features of the original four-cylinder engine along with some from the later 6-cylinder on which development had begun in 1926. Its bore measured 100mm and stroke at 140mm resulting in a displacement size of 4,398cc (268.4 CID / 4.5-liters). The water-cooled, single-overhead-cam, cast iron four-cylinder unit with four valves per cylinder, two SU carburetors, five main bearing crankshaft, and dual ignition with Bosch magnetos, it developed 110 horsepower at 4,000 RPM. Like the 3-Litre, the camshaft was operated by bevel gears on a vertical shaft at the front of the engine. During its production lifespan through the takeover by Rolls-Royce in 1931, few modifications to the engine were deemed necessary attesting to its overall excellent design. The engine was paired with a four-speed manual transmission and the chassis consists of channel-steel, front and rear suspension by semi-elliptic leaf springs, and 4-wheel 17-inch drum brakes finned for improved cooling. The standard chassis had a 130-inch wheelbase size and eight examples were given a short, 1175-inch platform.

Over twenty percent of the 4½ Litre received coachwork by Vanden Plas. Additional coachbuilders included Gurney Nutting, Harrison, Freestone & Webb, Thrupp and Maberly, Salmon & Sons, and Weymann.

The 4½ Litre Blower
W.O. Bentley was a purist and refused to attach a supercharger to his 4½ Litre model. At Sir Henry 'Tim' Birkin's insistence, engineer Amherst Villiers applied a Roots-Type supercharger to the engine, extracting even more performance from the 4.5-liter engine. It was placed in front of the radiator and at the end of the crankshaft which added weight to the front and increased understeer. More than just the adoption of a supercharger, the engine received special pistons, crankshaft, and lubrication system. The road-going touring versions delivered 175 horsepower at 3,500 RPM and in racing guise, output was measured at 240 bhp at 4,200 RPM. This meant it was more potent that its 6½ Litre sibling.

During the first year the company contested Le Mans, the 3 Litre of Frank Clement and John Duff was delayed by stones that damaged a headlight and punctured the fuel tank. With the Bentley damaged, the Chenard-Walckers of René Léonard / André Lagache and Christian Dauvergne / Raoul Bachmann finished the 1923 Le Man's race in 1st and 2nd. Applying this knowledge forward, the 4½ Litre and the Blower models were given a guard to protect the pair of carburetors located at the compressor intake, and similar protection for the fuel tank.

The first twenty-five Blower cars were given smooth-cased 'blowers' and chassis numbers beginning with 'SM.' The subsequent 25 chassis numbers began with 'MS' and had ribbed blowers to aid cooling.

by Daniel Vaughan | Dec 2022