1971 was the year the car insurance industry brought an end to the original ‘muscle car’ era. In an effort to comply with federal regulations requiring leaded fuel to be phased out, General Motors lowered compression ratios of all engines to a maximum of 9.0:1.

Not only did this actually cut horsepower (compared to an 11.0:1 engine), but General Motors also began rating their engines in SAE NET horsepower. In 1971, engines were rated in both SAE NET & Gross HP.