Despite storm clouds on the muscle car horizon, the 4-4-2 returned in 1971 with only minor modifications from the previous year. Engine output was down for 1971 due to a lower compression ratio (8.5:1), which affected all of GM's engines as the result of a corporate policy requiring engines to run on lower-octane regular leaded, low lead, or unleaded gasoline, in preparation for the introduction of the catalytic converter on 1975-model cars. The base 455 was rated at , with the W-30 achieving a rating of . The W-27 option was downgraded to an aluminum cover for the cast iron differential housing.