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>> “In the past, ratings were made up, at best.” <<
In the ‘60s horsepower was deliberately under rated to keep insurance costs down.
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Only a few were under rated. Most were NOT delivering the advertised hp to the flywheel.
Most ratings were bare motor without accessories or mufflers running with modified carburetors and undriviable tunes.
By the mid-sixties, the advertised outputs of many bread-and-butter engines were still unrealistically high,
http://ateupwithmotor.com/terms-technology-definitions/gross-versus-net-horsepower/