The prototype Plus 8 (identifiable by two small bonnet bulges near the centre bonnet hinge) used a Rover V8 engine and the Plus 8 was launched in 1968 using Rover's production engine, itself a re-engineered version of the Buick/Oldsmobile 215 motor (renamed the 3.5 L by Rover) with a compression of 10.5:1 fueled by two SU HS6 carburettors. The high 10.5:1 CR was only usable because 5* (101 octane) petrol was then still available. By 1973, the Rover 3500 saloon was available with a manual 4 speed gearbox and this engine/gearbox configuration was adopted by Morgan although the compression dropped to 9.25:1 with a resulting drop in power. With the adoption of an improved version of the engine developed for the Rover SD1 in 1977, compression increased to 9.35:1 and power increased slightly. After 1981 the engine was fueled by two Stromberg CD175 carburettors, .