Anything made after '72 in the US got hit by the CAFE regulations, and the engineers were throwing everything they could think of on the engines to meet the new limits. We got EGR to lower the temp of the burn in the cylinders to reduce NOX, retarded timing, lower compression, and every kind of vacuum switch they could dream up in '73/74/75 and on. Things didn't get much better until the mid-80's when the OEM's started clawing back ground, ala the Fox Body Mustangs and the Gen 3 Camaros. That's why the traditional "last of the muscle cars" is said to be 1972.