Wikipedia:
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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.
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