Forty years after the end of the “classic” muscle car era, there is still some confusion over horsepower ratings, especially how they relate to today’s cars. Let’s try to clear it up.

Prior to 1972, American carmakers used the SAE (Society of Automotive Engineers) “gross” measurement of horsepower. Gross meant the figure was taken from an engine running on a test stand, with no air cleaner assembly, accessories or exhaust system connected.

By 1971, carmakers began reducing compression in many engines in order to meet upcoming emissions requirements and to use unleaded fuel. General Motors and Chrysler began advertising both gross and SAE net figures in 1971, derived from an engine tested with air cleaner assembly, accessories or exhaust system connected.

The net ratings, which were applied across the board for 1972, must have been a shock to some customers. Suddenly, muscle cars appeared to lose 100 hp or more.

For example, the Corvette’s optional LT-1 350 cu. in. small block V-8 had 370 gross hp in 1970 (with 11:1 compression), then a 330 hp gross rating (with 9:1 compression) for 1971-1972 with a 255 hp net rating. The mighty Chrysler 426 cu. in. Hemi kept its high compression and 425 hp gross rating for 1971 and showed 350 net hp.

Jim Campisano, editorial director of Muscle Mustangs & Fast Fords and Super Chevymagazines, has revisited the classic muscle car horsepower topic numerous times over the years. The magazines have compared old and new muscle cars and have also put classic models on a chassis dynamometer to record rear-wheel horsepower.

“Rear wheel horsepower was at least 30 percent lower than the reported gross figure, in some cases even more,” Campisano said.

​​​​​​…..