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Want to impress your friends? Get gross. Seriously, it’s important to understand how and why engine output ratings dropped so significantly in the early 1970s. First, because of new Federal laws that stipulated low or no lead fuel usage, compression ratios were ratcheted down. Second, Detroit transitioned from rating engines in gross output to net output by using a different SAE (Society of Automotive Engineers) test result. In measuring gross ratings, an engine was dynotested with no encumberments — exhaust, air cleaner, belt-driven accessories, etc. The highest observed number was recorded. In the net horsepower test, the recorded results were obtained with the engine fully encumbered as it would be installed in the car. Hence, the numbers changed dramatically, rather like sipping daintily as opposed to gulping heartily. The following figures graphically display the difference between net and gross horsepower and torque ratings for the 1971 LS5.
