The advertised power drop from 1971 to 1972 ('71 304/210 hp and '72 304/150 hp, for example) is not due to any engine change, but to in the way the automobile industry adopted SAE power rating method. The 1971 and 1972 engines produce the same power. The 1971 and earlier versions were rated using "gross" horsepower, which is with the engines operating with no accessories or drive belts, air filters, nor any exhaust system restrictions. The change to the more realistic "net" horsepower ratings for 1972 and later required the engine to be driving all stock factory accessories and to use the factory air filter system and a simulated factory exhaust. Power is measured at the flywheel for both systems. For example, even with the modifications to more reduce emissions and increase efficiency, AMC's engine was among the strongest factory-spec engines available in an American car in 1971, with more horsepower than Pontiac's standard and Chevrolet's that powered the Corvette.