Built primarily for racing, the aluminium-bodied "100S" (for Sebring) model developed at 4700 rpm. Five works development/special test cars carrying the "SPL" chassis number prefix were developed during 1953 and 1954, with one winning its class at Sebring in 1954 and prompting the "S" designation. Only 50 production 100S cars were made, hand built by the Donald Healey Motor Company at Warwick, delivered between February and November 1955 and carrying the "AHS" chassis number prefix. To minimize weight and improve performance the cast iron cylinder head was replaced by a Weslake designed aluminium one, and the overdrive unit was not installed. Dunlop disc brakes were used all-round, the world's first production car to feature them both front and rear. To further lighten the vehicle, bumpers and hood (convertible top) were eliminated, the grille reduced in size, and the windscreen made of plastic. In all, weight was reduced by approximately . The majority of 100Ss were two-toned white with Lobelia Blue sides. A handful were produced in solid Spruce Green and red, and a single one in black.