Cosworth also developed a four-cylinder engine for Ford’s racing and rallying saloons in the late ‘60s and ‘70s. The
Coupled with a five-speed Borg-Warner gearbox the Cosworth YB engine sent torque to a rear axle fitted with a viscous-coupling limited-slip differential. With 204bhp the Sierra could accelerate from 0-62mph in 6.2sec and hit an official top speed of 150mph.The Cosworth’s basic chassis is still much the same as a regular Sierra’s, with MacPherson strut front axle and semi-trailing arm rear set up.
The first Sierra Cosworths used the model’s simple and light three-door shell, with one large rear-quarter window rather than the dual side window shell used by the Sierra XR4i. As well as the rear wing, there are wider arches to house the 7-inch wheels, a front air dam to help reduce lift and deep side sills.
By 1987 Ford wanted to improve the RS Cosworth to keep it competitive, not that it was really struggling. The rules allowed special modifications to be made to the race cars if 500 road-going evolution models were built. Ford identified what was needed to make the Cosworth more powerful, faster and more stable and created the Sierra RS500 Cosworth.