A 1980s sporting classic was created in 1984 when BMW hit on the bright idea of fitting the 286bhp M-Power engine from the mid-engined M1 into the 635CSi body. With stiffened suspension, improved damping as well as standard limited slip differential the full-sized coupé became a sublime package, with masses of all-round ability. Values remain significantly higher than non M-Sport coupes. Keep an eye out for fakes, and remember that the M-Sport engine costs significantly more to fix when it goes wrong.
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Information from Octane Magazine.