Chevy released the Mark IV big-block or "Rat" in 1965 to replace its "W"-series motors. This new engine featured (among other things) new valve placement and combustion chamber design to increase volumetric efficiency at higher rpm. Starting with 396ci in 1965, Chevy eventually went up to 454 in 1970. In between, you could get a 402 and even an all-aluminum 427 with a single four-barrel that was grossly underrated at 430hp. The kings of the street in the power department were the 1966 427 L72 that was originally rated at 450 hp (it was knocked down to 425 later in the model year) and the LS-6 454 of 1970 that was also rated at 450 hp. Then came the GM Performance Parts crate engines, like the 502/502 and then the monster 572 in 2003 that laid down 720 asphalt-destroying horsepower.