1971 was an important year for American cars, it was the first year that GM and Chrysler published net horsepower ratings in addition to the advertised SAE gross horsepower. The Hemi was rated 425 horsepower gross, but also 350 horsepower net. The 1971 Chevy LS6 454 was rated 425 horsepower gross, but only 325 horsepower net. On the lower end of the power spectrum the Chrysler 340 engine was rated 275 horsepower gross, and 235 horsepower net. The Ford 302 engine was rated 210 horsepower gross, and 143 horsepower net. So the amount that needs to be subtracted from the gross ratings to get the net ratings vary a lot depending on how exaggerated the gross claims were. I go to the automobile catalog to find the 1971 net horsepower ratings, it is not hard to look up. I actually remember the change in SAE ratings very well since my parents were car shopping in 1971 to 1972.
