Opel used the large American-derived V8 engine for the Diplomat, the imposing sedan that was marketed starting in 1964. In this way, Opel began to occupy a market segment that would then also see the arrival of Mercedes in 1968 with the 300 SEL 6.3. The Diplomat had two engine versions: 4554cc with 190 horsepower at 4600 rpm and, as of September 1966, 5303cc with 230 horsepower at 4700 rpm. Some exemplars came with longer wheelbase (2995mm instead of 2845mm). Production of the Diplomat I series sedan ceased in 1968.