The Lancia Delta S4(Production: 5423) is a Group B rally car that competed in the World Rally Championship in 1985 and 1986, until Group B cars were banned from competition by the FIA. The car replaced and was an evolution of the Lancia 037 Monte Carlo. The S4 took full advantage of the Group B regulations. It featured a mid-mounted engine and all wheel drive for superior traction and handling. The car's 1759 cc four cylinder engine combined supercharging and turbocharging to reduce turbo lag at low RPM. Officially the car produced 550 horsepower (410 kW). Unofficial figures claimed the S4 could accelerate from 0 to 100 km/h on gravel in just 2.3 seconds. A engine capacity multiple of 1.4 was applied to forced induction engines by the FIA and the choice of 1759 cc put the S4 in the under 2500 cc class which allowed a min weight of 890 kg. The combined super/turbo system was a development of the 037 engine that produced with a supercharger only. This was competitive with the Audi Quattro in 1984 which also had but by the 1985 season Audi's S1 was homologated with plus meaning a turbo had to be added to the S4 engine design to remain competitive.

Like Peugeot's earlier 205 T16, the mid-engine Lancia Delta S4 was a Delta in name and body styling only (for marketing purposes), and shared virtually nothing in terms of construction with the production front-engine Delta. The chassis was tubular space frame construction much like the 037. It featured long travel double wishbone suspension front and rear with a single large coil over at the front and separate spring and twin shock absorber at the rear. The bodywork was made of a carbon fibre composite with front and rear bodywork fully detachable for fast replacement due to accident damage and ease of access during on-event servicing.The bodywork featured several aerodynamic aids including bonnet opening behind the front mounted water radiator with Gurney flap, front splitter and winglets molded into the front bumper panel, flexible front skirt, and rear deck lid wing that featured both a full aerofoil wind section twinned with a deflection Spoiler. The doors construction style was brought from the 037 with a hollow shell all Kevlar construction that had no inner door skin, no door handle or window winder. The door was opened with a small loop and the windows were fixed perspex with small sliding panels to allow some ventilation and passing of time cards etc. For homologation purpose was built 200 S4 Stradales, it was middle engined (1759 cc) street car.

The Group S Lancia ECV was to replace the Delta S4 in the 1987 season but Group S was scrapped along with Group B and Lancia used the production-derived Delta in 1987.

In competition the car won its first event, the 1985 RAC Rally in the hands of Henri Toivonen and carried Markku Alén to second in the drivers' championship the following year. For two weeks after the end of the 1986 season Alen was champion until the FIA annulled the results of the San Remo Rally due to irregular technical scrutineering. Alén had won that event and the loss of points handed the title to Peugeot's Juha Kankkunen. All told, in 1986 there were 3 wins for the Delta S4 (San Remo not included). The Monte Carlo Rally by Toivonen, Rally Argentina by Massimo Biasion and the Olympus Rally by Alén. The car also won the 1986 European Rally Championship with Italian driver Fabrizio Tabaton, whose car was run by Italian team HF GRIFONE in ESSO livery. The factory supported Jolly Club team also ran cars in TOTIP livery, one of which was for Dario Cerrato.

Unfortunately the car's legacy was tainted by Toivonen's fatal crash on the 1986 Tour de Corse, where the Finnish driver inexplicably missed a tight left-hand hairpin bend and plunged into a ravine, where his S4's fuel tanks exploded.

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