Chevrolet raises the performance bar with big-block Mid & Full-Size models: 1966 CHEVY 396/427 MARK IV: BIGGER IS BETTER. After teasing enthusiasts with a very-late and very-limited-production Z16 Chevelle SS powered by a proprietary 396/375 Mark IV big-block, Chevrolet followed up with a newly-styled ’66 Chevelle SS396 with less horsepower! First introduced with base 396/325 and optional L34 396/360 hydraulic-cam motors, it was a while before they quietly offered the L78 396/375 so...