Homologation is a strange word, but one that's associated with some of the rarest and most impressive road cars ever built. In simplest terms, homologation cars are vehicles that were built to comply with specific rules for racing — most notably the rule that a certain number of "race-spec" cars must be built and sold to the general public before that car can be used for racing. In the late '60s, Ford wanted to put its new 429ci V8 engine into racecars, but NASCAR required them to put it into at least 500 road-going cars first. Thus the decision was made to shoehorn it into a Ford Mustang, and the Boss 429 was born.