The YB series of engines are based on the older inline-4 Pinto engine block, and were introduced in the road-going Ford Sierra RS Cosworth in 1986 with . With 5,000 units built for homologation purposes in Group A, both for rallies and touring cars. Racing versions of the RS Cosworth were developing around , but with the small Garrett T3 turbo on the cars reliability was a problem. A limited edition evolution model was introduced in mid-1987, the Sierra RS500 which included a bigger T4 turbo, with power initially at around the mark in 1987, but in later years climbing to close to some in full racing trim.