The 6.5 litre was developed from a factory test mule and badged as the "Sun" to disguise it's origins and was originally fitted with a 6 cylinder 4.5 litre engine. This car driven by W.O in 4.5 litre format, famously "raced" from the Lyon GP to Dieppe with a similar Rolls Royce driven by a Rolls test driver. The results of this closely matched spirited "race" to the channel port, led W.0 to come to the conclusion he needed to extend the engine to 6.5 litres, with a 20mm bore increase from the 4.5 litre prototype, to produce a performance advantage with a resulting output of 147 bhp. A new dry clutch design which enabled faster gearchanges and power brakes which were adjustable from the cockpit for wear whilst driving, were also refinements added to this model which enhance their usability.

The "Speed Six" model was introduced in 1928 with a major emphasis on performance for more sporting customers. A performance camshaft amongst other modifications resulted in 180 bhp tune for the road with up to 200 bhp available in racing applications. The Speed Six model was victorious at Le Mans in 1929 and 1930 and is inextricably linked with the "Bentley Boys" - Tim Birkin, Glen Kidston and Woolf Barnato amongst others. The pair of "Blue Train" Bentleys which exist - one of which was reputed to have raced the train from the South of France, have fallen into folklore and a number of replicas of varying accuracy (!) of these handsome cars have been made.