Although Porsche began production of the 911 in late 1964 (with the 901), 1965 is considered the first full year of production and the formative year of the 911, with lots of changes occurring in production. The flat-six 2.0 liter engine producing 130 hp remained a constant, but small changes were made to interior and exterior components. These early 911s are considered by many to be the purest and finest automobiles ever built and are consistently amongst the most sought-after models by collectors. It was also produced as a 912, which was essentially a 911 coupled to a 1.6 liter four-cylinder engine derived from the 356.