In 1964, the base GTO engine was a 389 cubic inch topped with a Carter four bbl. carburetor, high compression heads and a moderate lift camshaft. So equipped, output was rated at 325 horsepower. As potent as it was, the hot set-up consisted of a set of three vacuum controlled Rochester two bbl carburetors, known as the tri-power option. Hydraulic lifters and a 10.4:1 compression ratio gave the tri-power its 348 horsepower.

For 1965, a more radical camshaft and improved intake manifolds upped the output of the single carburetor engine to 335 horsepower and the tri-power engine to 360 horsepower.

1966 saw the same line-up available with the XS option; it featured a more radical camshaft and stiffer valve springs. With a functional air intake scoop, it was the first Ram-Air engine. 190 XS engines were built, making it one of the rarer Pontiac GTO options.

Big changes came in 1967; a GM corporate ban on multi-carburetor engines meant that the legendary tri-power engines were no longer part of the lineup and all GTOs featured the Rochester Quadra-Jet. Anti pollution equipment made its appearance on cars delivered in California. A 400 cubic inch motor, rated at 335 hp, was introduced. Also available was a two bbl carburetor with a tame 8.6:1 compression ratio; output was 255 hp. Two other 400 cubic inch engines, rated at 360 hp, were part of the lineup.

In 1968 a Ram Air II was added with beefed up internal parts and rated at 366 hp. In 1969 the Ram Air IV featured 370 hp via improved heads, a more radical cam, and aluminum intake manifold. This was a serious performance engine and air conditioning was not available.

The choices really slimmed in 1972 with only the 400 cubic inch (250 hp) and 455 cubic inch motors (300 hp) available. Changes in 1973 included reduced compression ratios (8.0:1, both engines) and less power, (230 hp and 250 hp respectively). Pollution control regulations became aggressive in 1974, the last year of the GTO. Only one engine was available, a 350 cubic incher rated at an anemic 200 hp.