Famous Engines

History of Bentley’s 6.75-litre V8

July 14, 2011

Bentley V8 engineBentley production began at Crewe in 1946 in a factory that had been built in 1939 to manufacture the Merlin aero engine. In this post-war period Bentley engines were straight sixes built to power the Bentley Mark VI and the six-cylinder engine was fitted also to the Bentley R-series and S-series until 1959. By 1950 Harry Grylls, then Technical Director, decided that the 4.2-litre straight six – which dated back to 1938 – was reaching the end of its potential and that a new modern unit was called for. A 5.7-litre straight eight already existed, but its weight and dimensions were not really appropriate. A V12 was also considered, but its size and complexity coupled with the growing popularity of large capacity V8s in the USA, convinced Grylls and his team to go ahead with producing a V8.

Jack Phillips, a member of the development team, maintained that it was a completely new design using ideas and cues from the best American V8s of the period. Work began on the 5.2 and 5.4-litre engines in the early ’50s, but once the first prototype engines had been tested in 1953, it became obvious that if the new engine were to have significant power and torque increases over the previous generation, then its capacity would need to be increased to 6.23 litres.

In 1959 a V8 engine was introduced and fitted to the Bentley S2 models. Despite its additional two cylinders and 27.5 per cent increase in swept volume over the straight six, the new 6.23-litre was 30lb lighter thanks to its cast alloy block and cylinder heads. The oversquare – 104.1mm bore x 91.4mm stroke – engine featured a conventional five-bearing crankshaft and a gear-driven single camshaft in the centre of the vee. The overhead valves were operated by self-adjusting hydraulic tappets.

The 6.23-litre V8 stayed in production for the new Bentley S3, which was introduced in 1963. However, the compression ratio was raised to 9.0:1 and two 2.0 inch SU HD8 carburettors replaced the smaller originals. Other modifications included a nitride-hardened crankshaft with larger diameter gudgeon pins to cope with a seven per cent power increase. In 1965, the launch of the Bentley T series saw the V8 receive its first major revision: new cylinder head castings with a different-shape combustion chamber to give yet more power and torque.

In 1970 the cubic capacity of the V8 increased to 6.75 litres. The additional 520cc were obtained by installing a new, long-throw crank coupled with larger con-rods and pistons, increasing the stroke from the original 91.4mm to 99.1mm. By this time the new Director of Engineering was John Hollings, and the Senior Engine Development Engineer was J.Astbury.

Fuel injection made an appearance on the Bentley V8 in 1980, when a Bosch K-Jetronic system was fitted to cars sold in the Californian market, while turbocharging was introduced two years later with the launch of the Mulsanne Turbo. In 1983, Mike Dunn took over as Director of Engineering, and the engine continued to evolve.

In 1987 Bentley ended production of carburetted engines. The fuel-injected 6.75-litre V8 featured many improvements, including a new firing sequence to improve refinement and a reprofiled cylinder head to ease gas flow. At the same time, the crankcase was stiffened by thickening the sump flange and adding another rib, while further development resulted in a stronger block with cross-bolting on the centre and two intermediate bearings. A two-tier manifold was developed to inject fuel at the valve to ensure precise fuel distribution.

In 1994 a new engine management system was fitted with ram-pipe inlet manifold further to increase the torque band. The next year a 406bhp engine was introduced for the Turbo S and Continental S models. In 1997 300bhp LPT version introduced for the Brooklands and Rolls-Royce Spur models, while 400bhp was available for the flagship Continental T. And in the next year a 325bhp version was introduced for the Brooklands R Mulliner model, while the 420bhp derivative was available for the Continental T and Turbo R Mulliner models. In the Arnage T the engine produced 500bhp and 1000 Nm of torque. The 6.75-litre V8 is still used and the latest version of the engine in the 2011 Mulsanne produces mighty 505bhp and 1020Nm of torque.