In 1993, Bill Riley began work on a design for a sports prototype to meet the International Motor Sports Association's (IMSA) World Sports Car regulations. These regulations, announced during the 1993 season, were intended to debut low-cost open-cockpit prototypes to replace the expensive closed-cockpit GTP models. Riley & Scott, experienced constructors in the Trans-Am Series, aimed to enter the sports prototype category.
The original Mk III featured a radical design with tapered rear sidepods flowing into the rear wing mounts, exposing much of the chassis floor. Radiators were placed at the front of the car, under a sloping nose. Interest in this design was low, and no car was constructed for the 1994 season. During that year, Riley & Scott aided Dyson Racing in improving their Spice DR3 chassis with a Ferrari engine. After completing this work, Bob and Bill Riley returned to their Mk III design with new knowledge. Dyson Racing then contracted Riley & Scott to replace their Spice-Ferrari with two new Mk IIIs for the 1995 season.
Bill Riley’s redesigned Mk III was simpler, allowing for customer variety and lower cost. Although the extreme aerodynamics of the original design were gone, the car was conceived to be fast enough to compete for overall wins. The redesign took only four months. The chassis featured a steel tube frame with carbon fiber panels, designed to accommodate a variety of naturally aspirated V8 engines from Ford, Chevrolet and Oldsmobile. All cars used a 5-speed transmission, double wishbone suspension with coil springs, and a power steering system.
The Mk III’s carbon fiber and kevlar bodywork was developed by aircraft designer John Roncz, who used early computational fluid dynamics programs to refine the aerodynamics. Final testing was performed at a Lockheed windtunnel. The nose sloped downward towards a splitter, with a wide intake for the radiator. Radiator exits were positioned on top of the nose and in front of the cockpit, with optional headlight installations for night races. Circular ducts allowed air to the front brakes for cooling. The bodywork was square and flat behind the front wheels, with full sidepods running the car’s width. A full-width roll hoop was positioned behind the cockpit, with an arched intake for the engine airbox underneath.
Square holes were created in the sidepods to allow teams to adjust cooling and aerodynamics. At the rear, the bodywork ended behind the rear wheels, with only the rear wing beyond. The aerodynamics were designed for low drag and high downforce. The chassis cost approximately $285,000 without an engine.
In 1999, Riley & Scott evolved the Mk III's designs to adapt to newer Le Mans Prototype (LMP) regulations. An all-new design, officially the Mark III Series C, debuted in 2001. This followed the company’s contract with General Motors to construct a Le Mans Prototype for their Cadillac brand, the Northstar LMP, which featured elements of the Mk III’s design and layout. The Northstar LMP featured a carbon fiber monocoque, a driver-specific roll hoop, and relocated radiators.
Once the Northstar LMPs were completed, Riley & Scott turned their attention back to the Mk III, which was still earning victories. Rather than further evolving the existing design, they started from scratch, incorporating advancements from the Northstar LMP into a simpler design.
The Mk III Series C retained the basic layout of the Mk III, with a slanted nose housing a radiator fed from a vent above the front splitter. Sidepods were straight and flat, with exchangeable panels for cooling and aerodynamics. An engine intake was mounted behind the cockpit, accommodating multiple naturally aspirated engines. The chassis used a steel tube frame with carbon fiber panels, rather than the all-carbon monocoque of the Northstar LMP.
Notable changes included the cockpit and nose areas. The Mk III C shared the Northstar LMP’s roll hoop, positioned behind the driver’s seat, although an optional extension was available for the Rolex Sports Car Series. An aerodynamic windshield was added in front of the driver. The nose was more rounded, with tapered fenders and redesigned headlights. New ducting allowed air from the front diffuser to exit over the bodywork.
Mechanically, the Mk III C featured a new 6-speed gearbox from X-Trac, with an optional pneumatically actuated gear change system with paddles behind the steering wheel. Five Mk III Cs were constructed, with chassis #01-002 the last in use, retired at the end of 2005.
Dyson Racing’s first Mk III took its first laps at Daytona International Speedway in January 1995, setting the fourth fastest time in testing for the 24 Hours of Daytona. At the race, the team qualified sixth, but the Ford engine failed after eleven laps. At the 12 Hours of Sebring, the car finished 37th. James Weaver achieved the car’s first victory at the Grand Prix of Atlanta. Dyson Racing then won at Watkins Glen, Sears Point and Mosport, and finished first and second at New Orleans. James Weaver finished second in the Drivers Championship.
After a successful debut, several teams purchased Mk IIIs for 1996. Doyle Racing and Dyson Racing used Oldsmobile Aurora V8s. Doyle Racing won both the 24 Hours of Daytona and the 12 Hours of Sebring, and three further races. Dyson Racing won three consecutive races to end the season. Wayne Taylor won the WSC Drivers Championship, and Oldsmobile won the Manufacturers Championship. Doyle Racing also entered the 24 Hours of Le Mans, but retired after fourteen hours due to a transmission problem.
The 1997 season saw Dyson Racing win the 24 Hours of Daytona. They won five of the nine remaining races, winning the WSC Teams Championship. Butch Leitzinger won the Drivers Championship, and Ford secured the Manufacturers Championship.
In 1998, the Mk III competed in both the IMSA GT Championship and the new United States Road Racing Championship (USRRC). Dyson Racing won the USRRC Teams Championship and Ford the Manufacturers Championship. James Weaver won the Drivers Championship. In IMSA GT, Dyson finished second in the Teams Championship, and Butch Leitzinger won his second consecutive Drivers Championship. Team Rafanelli of Italy also won an IMSA GT race with a Mk III. In Europe, Solution F won a race in the International Sports Racing Series (ISRS).
Riley & Scott launched their Series 2 Mk III in 1999, winning the 24 Hours of Daytona. Dyson Racing won two USRRC races, earning the Drivers Championship. In the newly formed American Le Mans Series (ALMS), Dyson finished second at the 12 Hours of Sebring, and Team Rafanelli won at Road Atlanta.
In 2000, Riley & Scott’s attention shifted to the Cadillac Northstar LMP program. Despite limited upgrades, the Mk III remained competitive. Robinson Racing won a race in the Rolex Sports Car Series, while Dyson Racing won four races and the Teams Championship. James Weaver earned his second Rolex Series Drivers Championship.
The Mk III C debuted in 2001. Dyson Racing earned a podium finish at the Grand Prix of Mid-Ohio in the ALMS. Robinson Racing debuted the car at the end of the season. Dyson Racing won four Rolex races, and James Weaver earned his second straight Rolex Series Drivers Championship.
In 2002, Riley Technologies took over care of the Mk III Cs. Dyson Racing won four races in the Rolex Series and the Teams Championship, while James Weaver won another Drivers Championship. Autocon Motorsports entered the ALMS, finishing seventh at the 12 Hours of Sebring.
The Mk III C continued to compete in the ALMS through 2005, but never achieved victories. Autocon Motorsports’ Mk III C, chassis #01-002, was the last car in use, retired at the end of 2005.