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.