The STP-Paxton Turbocar was a racing car, designed by Ken Wallis as the STP entry in the Indianapolis 500.
Wallis, a distant relative of famed British engineer Barnes Wallis, had developed a workable plan for harnessing a gas turbine to a race car. He first presented the idea to Dan Gurney, who passed on the idea. Wallis then offered the plan to Carroll Shelby and Shelby said (according to later court testimony), "Hogwash." Finally, Andy Granatelli of STP expressed interest in the concept. Wallis and his crew moved in with Andy's brother Joe at STP's Paxton division in Santa Monica, and they began work on the turbocar in January 1966. It was Granatelli who introduced a side-by-side concept—that is, putting the engine at the driver's left. Granatelli also added four-wheel drive to the design.
The aluminum frame of the car was badly warped during heat treating in early 1966, eliminating any possibility of the car racing in the 1966 Indianapolis 500. Work started over again and the car was ready for the 1967 Indianapolis 500. Parnelli Jones drove the car during tire testing in Phoenix early that year and was impressed with the car. He agreed to drive the car in the Indianapolis 500 after being offered $100,000 and half of any prize money he won.
Jones qualified the car at Indianapolis in sixth place at 166.075 mph. At the start of the race, he quickly took the lead and rarely relinquished it. However, with just eight miles left to go, he coasted into the pits with a transmission bearing failure. The car was refurbished and entered by STP in the 1968 Indianapolis 500. Driven by Joe Leonard, the car crashed into the wall during qualifying and never raced again.
The car was originally donated to the Smithsonian Institution's National Museum of American History by the STP Corporation. It is currently on loan to the Indianapolis Motor Speedway Hall of Fame Museum. The design was unique enough for Mattel to include a model of the car as the "Shelby Turbine" as one of the popular Hot Wheels toy cars. The Lotus 56 used a modified version of the same engine and 4 wheel drive in a more advanced wedge-shaped body with new USAC intake restrictions, but one car crashed and the 3 entered into the race did not finish either before USAC banned turbine and 4-wheel drive cars entirely.
The STP-Paxton Turbocar was built around an aluminum box-shaped backbone. The driver was seated on the right side of the backbone, while the engine, a Pratt & Whitney Canada ST6B-62 turbine engine, was mounted on the left side of the backbone. Though never successful as an automobile powerplant, the small aircraft engine it was based on would become one of the most popular turboprop aircraft engines in history. The engine drove a Ferguson 4-wheel drive system, which transmitted the power to the wheels. A torque converter eliminated the need for a clutch pedal and gearshift. The engine idled at 54% of full throttle, which meant that the driver didn't even have to press the accelerator pedal to pull away; all he had to do was ease his foot off the brake pedal. A movable panel was mounted behind the cockpit, which acted as an airbrake. The suspension's coil springs were located inside the backbone and the suspension A-frames had airfoil cross-sections. The car weighed 1,750 pounds, a few hundred pounds more than the Indy minimum weight of 1,350 pounds.
USAC had limited the engine intake area to 23.999 square inches to limit the turbine's power output, but the engine still produced 550 hp. However, drivers reported that it had a three-second throttle lag. In less than a month after the 1967 Indianapolis 500, USAC cut the allowable turbine air intake area from 23.999 to 15.999 inches and imposed the ruling immediately, although it had been customary to give two years' notice of engine changes. With the reduced inlet area, the maximum lap speed that could be achieved was 161 mph.