The 1964 GTO started it all. By stuffing a big-car engine into their mid-sized Tempest, Pontiac created the muscle car. Although this was against official GM policy at the time, by offering the GTO as an option package on the Tempest, rather than as a separate model, they were able to get away with putting the 389 cubic inch engine into an intermediate. The initial success of the GTO ensured its continuation, and by 1966 it became a stand alone model. By 1970, you could order a GTO with a 455 cid engine producing 360 horsepower.