In the 1971 ZR2, the engine was retuned ... The ZR2 package for the Corvette was a replacement for the LT1-powered ZR1 model.
From 1971 onwards ... the Corvette wasn’t out of the game—its 454-cube LS6 was rated at 425 horsepower. It was a downgrade from the 450-hp output of the 1970 version of the engine.