The regular Bentley 6½ Litre and the high-performance Bentley Speed Six were Bentley

Bentley

Bentley Motors Limited is a British manufacturer of automobiles founded on 18 January 1919 by Walter Owen Bentley known as W.O. Bentley or just "W O". Bentley had been previously known for his range of rotary aero-engines in World War I, the most famous being the Bentley BR1 as used in later...

 cars in production from 1926 to 1930. They were created out of the desire for more engine power by Walter Owen Bentley by adding two cylinders to the straight-4

Straight-4

The inline-four engine or straight-four engine is an internal combustion engine with all four cylinders mounted in a straight line, or plane along the crankcase. The single bank of cylinders may be oriented in either a vertical or an inclined plane with all the pistons driving a common crankshaft....

 engine used in his Bentley 4½ Litre car. The Speed Six, introduced in 1928, would become the most-successful racing Bentley. Two Bentley Speed Six became known as the Blue Train Bentley

Blue Train Bentley

The term Blue Train Bentley refers to two Bentley automobiles, based on the high-performance 6½ litre Bentley Speed Six model, which became known for their owner Woolf Barnato's involvement in the Blue Train Races of 1930.- Overview :...

s after their owner Woolf Barnato

Woolf Barnato

Joel Woolf Barnato was a British financier and racing driver, one of the "Bentley Boys" of the 1920s. He achieved three consecutive wins out of three entries in the 24 Hours of Le Mans race.-Early life:...

's involvement in the Blue Train Races

Blue Train Races

The Blue Train Races were a series of record-breaking attempts between automobiles and trains in the late 1920s and early 1930s. It saw a number of motorists and their own or sponsored automobiles race against the Le Train Bleu, a train that ran between Calais and the French Riviera...

 of 1930.
The 6½ Litre was inspired by the Rolls-Royce Phantom I

Rolls-Royce Phantom I

The Phantom was Rolls-Royce's replacement for the original Silver Ghost.-Description:The Phantom I is the second model in the Rolls-Royce 40/50 hp line. It was originally and even now correctly referred to as New Phantom...

 as a closed-body car.