After winning the 1973 Race of Champions he announced his retirement , only to be lured back to full-time driving when Penske formed a Formula One team to compete in the final two events of the 1974 F1 World Championship and the entire 1975 season with the new Penske PC1.Donohue was the 1974 IROC champion .
Donohue returned from retirement and became the 1974 IROC champion .