Design and development
Two veteran AAR designers were tasked with leading the design of the MkIII chassis. John Ward focused on the tub and mechanical aspects of the car, while Hiro Fujimori was responsible for aerodynamics and styling —[1] hence, the car was internally codenamed WFO 91 for Ward, Fujimori and Others, 1991. Team members nicknamed the project "Wide F'n Open".[2] The design parameters given to the team were not groundbreaking and the designers looked to avoid risky, cutting-edge solutions wherever possible; rather, for reliability's sake, the mantra was to make the chassis simple while taking full advantage of proven technologies. However, there was one significant technological advance planned: the MkIII would be the first carbon-fiber monocoque manufactured in-house by AAR.[4]
"What we did with the MkIII was to address just about every weakness of the MkII," Gurney said. "It was not a styling exercise; everything was driven by functional considerations, and it was really a clean sheet of paper from virtually every standpoint."[5]
One of the primary flaws with the HF89/90 had been a persistent lack of front grip; combined with overwhelming amounts of rear downforce, this created an intractably imbalanced car with a significant tendency to understeer. On the slower, more technical circuits on the IMSA schedule, this was a major competitive disadvantage.[4] Thus, on the MkIII, the team first focused on creating more front downforce. This led to the inclusion of an integral front diffuser in the nose to stabilize underbody airflow and create a low-pressure zone directly under the front wheels. This was a break from the tradition of GTP and Group C cars, which had almost exclusively adhered to a single underbody with one set of tunnels running from front to rear.[5] Working in tandem with the standard ground effect tunnels under the rear of the car, the nose diffuser would balance front and rear grip while minimizing drag.[2] To test and refine the design, a 20%-scale wind tunnel model was built, accurately replicating not just the car's external features but internal components as well — detailed down to the car's wiring, air ducting and engine bay plumbing. Several different nose concepts were modeled and discarded before settling on a final design.[6] In an effort to conceal the innovative nose air extractors, the bodywork immediately behind the front wheels was painted black.[5]
Racing car designer and aerodynamics expert Michael Fuller wrote that "the most striking feature of the MkIII was the single nose air inlet that channeled air through massive ducting to midmounted water, water-to-oil and turbo intercooler radiators."[4] By using a single aperture to feed several cooling systems, surface turbulence and drag created by air inlets was minimized. This contributed to a clean, uncluttered design that provided for smooth airflow to the rear wing, enhancing its efficiency. The front cross-section of the monocoque was also minimized as much as possible within the rules, in order to reduce drag.[4]
The turbocharged Toyota engine was largely carried over from the HF90, with the addition of a new electronic engine management system. By applying a high level of boost to the 2.1-litre stock-block engine,[5] Toyota Racing Development's shop in Torrance, California was able to wring more than 800 horsepower out of what was the smallest-displacement powerplant used in the GTP series at that point.[4] "I was told that (TRD) blew up four dynamometers... (they) didn't spare the horses," Gurney later said.[4]
For its first on-track test in the summer of 1991, AAR took the MkIII to Willow Springs Raceway, a frequent testing ground for the team. It was immediately apparent that the new MkIII was vastly superior to the HF90/MkII. "Why, it just blew off the MkII. It was obvious," Gurney said.[4]
