The Lancia Delta S4, known internally by its Abarth project code SE038, stands as a significant milestone in automotive engineering, particularly for its pioneering use of twincharging technology in the demanding world of Group B rally racing. Manufactured by Lancia, this car competed in the World Rally Championship during 1985 and 1986, before the Group B class was ultimately disbanded. The Delta S4 was an evolution of its predecessor, the 037, and was engineered
to fully exploit the Group B regulations of the time. Its design featured a midship-mounted engine and all-wheel drive, which provided superior traction on various loose surfaces, a critical advantage in rally competitions.
Engine Design and Twincharging Innovation
At the heart of the Lancia Delta S4 was a 1,759 cc (1.8 L) inline-four engine, which ingeniously combined both supercharging and turbocharging. This twincharged system was a direct development from the 037 engine, which had previously generated 325 hp (242 kW) using only a supercharger. The primary motivation for integrating both technologies was to significantly reduce turbo lag at low engine speeds. Turbo lag, a delay in power delivery as the turbocharger spools up, negatively impacts driveability, especially in rally where immediate response is crucial. By employing a supercharger for low-end boost and a turbocharger for higher engine speeds, the Delta S4 achieved a more linear and responsive power delivery.
The FIA's regulations applied a 1.4 multiplier to forced induction engines for classification purposes. The Delta S4's 1,759 cc engine, when multiplied by 1.4, placed it in the under 2,500 cc class, which allowed for a minimum vehicle weight of 890 kg (1,962 lb). This strategic engine sizing, combined with the twincharging system, enabled the car to produce an impressive maximum output of 490 PS (360 kW; 483 hp), with some sources even claiming up to 500 PS (368 kW; 493 hp).
Overcoming Boost Threshold and Parasitic Load
The Delta S4's twincharging system was specifically designed to address the limitations of turbocharger technology prevalent at the time. It featured a relatively large Kühnle, Kopp & Kausch 27 turbocharger, which had a boost threshold of 4500 rpm. Below this threshold, turbochargers of that era offered little to no response, a phenomenon sometimes mistaken for turbo lag but more accurately described as boost threshold. This characteristic severely affected driveability. The Abarth Volumex R18 supercharger worked in tandem with the turbo, providing immediate low to mid-range boost. Superchargers, being mechanically driven directly from the engine's crankshaft, do not suffer from lag, thus improving engine response and overall driveability of the S4.
However, this direct mechanical connection means that superchargers impose a significant parasitic load on the engine, particularly at higher engine speeds. Lancia's engineers designed their twincharger system to manage this. The supercharger was tasked with providing instantaneous boost in the lower engine speed range, while the system transitioned to the turbocharger for more efficient operation at higher engine speeds. This intelligent switching mechanism allowed the Delta S4 to harness the benefits of both forced induction methods, ensuring powerful acceleration and sustained performance across the entire rev range, a critical advantage in the competitive world of rally racing.
Chassis and Aerodynamic Innovations
Beyond its revolutionary engine, the Lancia Delta S4 was a marvel of rally car design. Similar to Peugeot's 205 T16, it was a silhouette race car, meaning it shared virtually no construction elements with the production front-engine Delta, despite its name being used for marketing. The chassis was a tubular space frame, much like the 037, providing a rigid and lightweight structure. It featured long-travel double wishbone suspension at both the front and rear, with a single large coil-over at the front and a separate spring with twin shock absorbers at the rear, designed to handle the extreme conditions of rally stages.
The bodywork was constructed from a carbon fiber composite, with front and rear sections fully detachable for rapid replacement in case of accident damage, facilitating quick servicing during events. Aerodynamic aids were extensively integrated, including a bonnet opening behind the front-mounted water radiator with a Gurney flap, a front splitter, winglets molded into the front bumper panel, a flexible front skirt, and a rear deck lid wing that combined a full aerofoil wind section with a deflection spoiler. The doors, inspired by the 037, were hollow shell all-Kevlar constructions without inner door skins, handles, or window winders, opened by a small loop, and featured fixed perspex windows with small sliding panels for ventilation and passing time cards. The all-wheel-drive system, developed with Hewland, allowed 60-75% of torque to go to the rear wheels, paired with a 5-speed Hewland transmission.
