The Quad 4 (called Twin Cam after 1995) was an overhead camshaft inline 4-cylinder automobile engine produced by General Motors' Oldsmobile division. It was a modern engine for the time, but was criticized for roughness as well as its longevity. Balance shafts were added in 1995, also known as a transitional year from the Quad 4 to the renamed TWIN CAM variant, in 1996 2.4L (LD9) the complete engine makeover was accomplished. The name is derived from the engine's four cylinders and four valve per cylinder layout. There was a single overhead camshaft variant that was produced for a brief time. The Quad 4 used a cast iron block and an aluminum head. The Quad 4 debuted in 1987 and was replaced after 2001 by the Ecotec. Quad 4 engines were produced at GM's Delta Engine Plant (Plant 5) in Delta Township, Michigan. In recent years, it has gained a minor following in hot rodding circles as a period style engine (which looks similar to a 1930's Offenhauser twin cam unit).