Its was running a 320is engine (2.0 m3 engine, was running near on 230bhp), rather than using a true 2.3/5 M3 engine, which meant it could go in a class below, rather than a higher class with Group A rover v8's and 3.0 capri's. It wiped the floor and won the championship with about 3 rounds missed.
The car was used as a test mule for the BMW btcc team, it used parts that we're protyped for the BMW 320 btcc cars. It used hub's, diffs & suspensions, that were way ahead of there time.
The bloke who used to drive still owns the car, albeit as a rolling shell in that liviery. Parts from it were put onto other race cars he's had since.