The first new Countach was the LP500S; Ferrari had unveiled their Boxer, which was faster than the Countach LP400S, so Lamborghini had to do something. They tried a turbo-charged version of the 4.0-Litre engine, but because of cooling problems, this project as abandoned in an early stage. Another option was tried out, but in the end, the best thing to do was to increase the displacement up to 4754 cc by enlarging both the bore and stroke.
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Information from Octane Magazine.