and block in one-piece made for a complicated and labour-intensive casting and machining. This was a feature shared during that time by the Bugattis which the car was later to compete with. The engine was also among the first with two spark plugs per cylinder, pent-roof combustion chambers, and twin carburettors. It was extremely undersquare, optimized for low-end torque, with a bore of and a stroke of . Un-tuned power output was around , allowing the 3 Litre to reach . The Speed Model could reach ; the Super Sports could exceed . A four-speed gearbox was fitted. The chassis from a Humber was designed by Frederick Tasker Burgess (1879-1929) previously chief designer at Humber who had worked with W.O. during the war producing the aero engines BR1 and BR2. Bentley did not deliver complete vehicles, but—as was customary—provided only a rolling chassis. Only the rear wheels had brakes until 1924, when four-wheel brakes were introduced. Variants There were three main variants of the 3-litre and they became known by the colours commonly used on the radiator badge. There was a definite rule controlling badge colors but astonishingly it has since been established that given "special circumstances" the factory would indeed supply a "wrong" color. Blue label This was the standard model with wheelbase from 1921 to 1929 or long wheelbase from 1923 to 1929. Red label This used a 5.3:1 high compression engine in the wheelbase chassis and was made from 1924 to 1929. Green label Made between 1924 and 1929 this was the high performance model with 6.3:1 compression ratio and short wheelbase chassis. performance was guaranteed. Production The 3 Litre chassis was shown at the 1919 London Motor Show, but the engine had not yet been finished. It took two years to get the engine right, with the first customer delivery in September 1921. Production lasted through 1929, by which time the car had been surpassed by Bentley's own 4½ Litre car. Experimental: 3 3 Litre: 1088 Speed Model: 513 Super Sports: 18 Car later rebuilt and supercharged In the winter of 1926/7 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. This pre-dated the Birkin supercharged Bentleys by two years. Like the later 4½ litre supercharged cars its blower was crankshaft-driven and mounted in front of the radiator between the dumb