an iroc stands for international race of champions, wich was a contract chevy had with the race name to use as a special trim on the Z28 camaro from 1985-1990. the RS, or rally sport, is a slightly less powerful V8, TBI, rated at 170 hp and produced from 1989-1992. (in the 3rd gen cars) they also offered a fuel injected V6 3.1 litre 140 hp model. if you must choose between the two, get the iroc, and get the TPI, it's much better than the TBI or reg 4 barrel engines.