The 3.0 L (2,996 cc) straight-four engine was designed by ex-Royal Flying Corps engineer Clive Gallop. It was among the first production car engines to feature four valves per cylinder, dry-sump lubrication, and an overhead camshaft. The four-valve SOHC design, with a bevel-geared shaft drive for the camshaft, was based on the pre-World War I 1914 Mercedes Daimler M93654 racing engine. At the suggestion of W. O. Bentley, then serving in the Royal Naval Air Service, one such Mercedes Grand Prix car displayed in the London showroom was confiscated by the British army in 1915 and dismantled at Rolls-Royce for examination. The cast-iron monobloc design and fully aluminium-enclosed camshaft were notable departures from both the Mercedes and contemporary aero engines, contributing to durability. The engine was also among the first with two spark plugs per cylinder, pent-roof combustion chambers, and twin carburettors. It was extremely undersquare, optimised for low-end torque, with a bore of 80 mm and a stroke of 149 mm.
Un-tuned power output was approximately 70 hp, giving the standard 3 Litre a top speed of around 80 mph (129 km/h). The Speed Model could reach 90 mph (145 km/h), and the Super Sports could exceed 100 mph (161 km/h). A four-speed gearbox was fitted. The chassis was derived from a Humber design and developed by Frederick Tasker Burgess (1879–1929), previously chief designer at Humber, who had collaborated with W. O. Bentley on producing the aero engines BR1 and BR2 during the war. Only the rear wheels had brakes until 1924, when four-wheel brakes were introduced.
The 3 Litre was delivered as a running chassis to the coachbuilder of the buyer's choice. Bentley referred many customers to their near neighbour Vanden Plas for bodies. Most bodies took the simplest form — tourers — though all coachwork was bespoke. Customers included Prince George (Duke of Kent), Gertrude Lawrence, and Beatrice Lillie.
Three main variants existed, identified by the colour of the radiator badge.
The Blue Label was the standard model with a 117.5 in (2,984 mm) wheelbase, produced from 1921 to 1929, or a longer 130.0 in (3,302 mm) wheelbase from 1923 to 1929.
The Red Label used a 5.3:1 high-compression engine in the 117.5 in wheelbase chassis and was produced from 1924 to 1929.
The Green Label, produced from 1924 to 1929, was the high-performance model with a 6.3:1 compression ratio and a short 108 in (2,743 mm) wheelbase chassis. 100 mph (160 km/h) performance was guaranteed, though the model carried only a 12-month warranty reflecting its high state of tune, compared to the 5-year warranty on the Blue and Red Label cars.
The 3 Litre chassis was shown at the 1919 London Motor Show, though the engine had not yet been completed. The first customer delivery took place in September 1921. Production ended in 1929, by which time the car had been surpassed by Bentley's own 4½ Litre car. Total production: 3 Experimental, 1,088 standard 3 Litres, 513 Speed Models, and 18 Super Sports.
In the winter of 1926–27, the factory's service department created the first supercharged Bentley when chassis number 220 FR5189 had a Roots-type blower fitted to its 3-litre engine, predating the Birkin supercharged cars by two years. Like the later supercharged 4½ Litre cars, the blower was crankshaft-driven and mounted in front of the radiator.
The oldest surviving production Bentley is 3 Litre chassis number 3, the first Bentley sold, delivered to its original owner in 1921. Bodied by coachbuilder R. Harrison & Son, it carries engine number 4 and UK registration AX 3827. In 2011 it sold at auction for $962,500 including buyer's premium. An original, unrestored 1927 3 Litre Speed Model (Red Label), chassis number 1209 DE, is part of the permanent collection at the Simeone Foundation Automotive Museum in Philadelphia, Pennsylvania. A 1924 3 Litre has been on display at the Shepparton Motor Museum in Shepparton, Victoria, Australia, since July 2019.
This article is based solely on the supplied corpus. No external sources were consulted; claims that could not be substantiated against the corpus were omitted under the drop-the-claim rule.