A general motors engine made from 1987 until 2002, which came in several different varietes and underwent several redesigns until it was replaced with the ecotec in 2003. It is a dual overhead cam, 16 valve, inline 4 cylinder gasoline engine. Various versions produced made from 150 up to 190 horsepower, depending on year model and vin number. Displacements ranged from 2.3L to 2.4L. A single-overhead-cam, 8 valve version was also produced as an economy version. It had the same displacement but only produced 110-115 horsepower.
The Quad 4 was once hailed as the powerplant of the future, having the power of a V-8 while the effeciancy of a 4 cylinder. It turned out to be a problematic engine. Before GM worked out the issues with head-gaskets, the ignition system, and other misc. problems the engine had already earned a bad reputation. This is especially because of the single-overhead-cam version, which used a horrible amalgamation of parts and was the worst version of this engine ever made.
The shining redeption of the Quad 4 was its potential as a FWD performance powerplant. It helped Oldsmobile win many SCCA SOLO2 and ISMA competitions, and set a land speed record in the early 90's with the Oldsmobile Areotech. Many hot rodders and tuners thus choose to use the quad 4 because of its potential.
I'm glad I have the 190 horsepower, W41 Quad 4 instead of the crappy single overhead cam version that everyone complains about.