[yogi_123rd]

The big changes back in the 70's was the addition of pollution control. Adding a catalytic converter required the engine to run hotter and to use unleaded gas. Engines back then were not computer driven as today. In order to run the engine hotter, the distributor timing was retarded to top dead center instead of x degrees before tdc and the timing advance mechanism was limited. Running the engine with the timing advanced is great for performance, but the engine runs lean and cool. Emmissions of lean running engines produce pollutants NOX and CO. (I.E. partially burned exhaust gases).
The camshaft was also changed. You will notice all engines in the 70's had drastically lower HP ratings up till about 1980, when the computer started to be used more commonly. The computer allows the timing to be advanced (giving you the power back) and but under control of the oxygen sensors. If a lean burn is detected, the computer retards the timing as needed to run hotter.