Completely new bodies and chassis were introduced for 1955. A new overhead-valve V8 engine was introduced. (see Engines section below). Sales increased. With the introduction of this V8, the six-cylinder engines were discontinued; a six-cylinder would not return to the full-size Pontiac line until the GM corporate downsizing of 1977. A four-cylinder engine was introduced in the Tempest model line in 1961, followed by an overhead-cam six-cylinder starting in 1966, as well as on the Firebird. It was the first popular-priced, mass-produced engine in America using an overhead-camshaft configuration.