At the time GM claimed "virtually no difference in driveability!" The Exhaust Gas Recirculation valve lets exhaust gas into the intake manifold, after the engine is up to operating temperature and when the engine is under heavy load. It serves to reduct NOx emissions by reducing peak combustion temperatures. You are in effect diluting incoming mixture with spent exhaust gas which is inert. The carb needs to be leaned out a bit at full throttle to compensate for the "air" that has less than the normal 20.9% oxygen.
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