Chevrolet’s ‘W’ series big block ran into a hard power ceiling around 6,500 rpm, prompting a near-total redesign that became the Mark IV, nicknamed the ‘porcupine’ engine for its distinctive splayed valve angles. The new architecture kept key ‘W’ features like side-oiling and Moraine main bearings while dramatically improving high-rpm breathing. Read on to find out exactly what changed under the valve covers.