"flexibility down to 6 mph (10 km/h)" and the ability to "climb a hill of 1 in 9 maximum gradient, complicated by bends", all of which supported the manufacturer's contention that power, along with low speed torque, were adequate. In 1951, a -litre, 4.6 L (4,566 cc/278 cu in) version of the engine was introduced. The increase in displacement was accomplished by increasing the bore from -inch to 3 5/8 inch. The later version is sometimes casually referred to as the "big bore" engine, the earlier version as the "small bore" version. The 4 1/2 L version of the engine is as well equipped with a Vokes 30 (later: Vokes 62) full flow oil filter (some earlier 4 1/4 L cars have since been modified to a full flow filtration system). Carburation in RHD (right hand drive) cars were two horizontal constant-vacuum SU carburetors (type H4 up to B81HP, type H6 from B83 HP on). LHD (left hand drive) cars had a single dual downdraught Stromberg carburetor type AAV26M and a different inlet manifold as fitted in the Rolls-Royce Silver Dawn and Silver Wraith. A four-speed syncromesh manual transmission was fitted in all Bentley MK VI with the change lever to the right of the driver on RHD cars and on the column on LHD versions. -litre cars had chassis numbers from B 1 AJ through B 400 LJ, with the final two letters indicating the series in which it was built. The "big bore" cars serial numbers begin with B 1 MB (although B 2 MD was the first big bore Mark VI built) and ended with B 300 PV (although B 301 PU was the last one built). Each alphabetic series only contained either even or odd numbers, and 13 was always skipped for the odd-numbered sequences. The was referred to as the L and can be quickly identified from its single exhaust in RHD cars. The is known as the L and features a twin exhaust in RHD cars. In LHD cars the (much less restrictive) twin exhaust system was only fitted with the introduction of the R-type. In addition for "standard steel" Mark VI saloons the single hinged ventilation flap centrally mounted on the top of the bonnet, directly ahead of the windscreen was replaced, on later cars, with two hinged ventilation flaps, mounted at or slightly below knee height, one on each side of the