When you browse through the history of the Indianapolis 500, you can read about the revolution of the rear-engined car of Cooper and Lotus and the crushing victory of Jim Clarck in a Lotus-Ford in 1965. The entire 1967 field had rear engines except one special car, the STP Praxton Turbocar , the “whooshmobile” or “Silent Sam”.
This nickname was an attempt to characterize the unusual rushing sound of this car, which had nothing to do with the hammering and screeching of the piston engine competitors. Its drive unit was a Pratt & Whitney Canada ST6B-62 gas turbine with 550 bhp and enormous torque.
When STP boss Andy Granatelli presented this car to driver Parnelli Jones. Jones was a little sceptical but after negotiating a large driver’s fee (100.000 dollar!) and a first run, he got quickly enthusiastic about it. Getting used to it was necessary; not only because of the driver's position, but above all because the abnormal sounds of the car and no sound of the engine and of the turbine's response. After stepping on the accelerator, it took up to six seconds for the power to arrive - but then it was immediately powerful. This was not the only reason why the car had four-wheel drive (based on the Ferguson principle). Of course, with 4WD it was easier to control on a track like the Indianapolis Motor Speedway with four straights and four corners than on a street course.
The basic principle of a central tube construction was not new in two-seaters, but illogical in a single seater. On the design, Granatelli used the clever idea of the side-by-side concept: the long gas turbine was suspended on the left side of the chassis and the driver’s seat was placed on the right. The Granatellis realised that 4WD was the key component needed to get the most from a turbine-powered car. This came with the addition of a Ferguson four-wheel drive transmission based on Granatelli’s previous Indy entries with the Novi Ferguson.
Towards the front and rear, the chassis forks into two branches to accommodate the massive double wishbone suspension, whose springs were inboard mounted as well as the disc brakes. The drive shaft to the front axle runs past the left spring on the left. The torsional strength of the chassis was eight to ten times higher than that of previous Indy chassis.
The power unit, a Pratt & Whitney ST6B-62 made by United Aircraft of Canada Ltd. gas turbine said to produce 550 “shaft horsepower” at 6200 rpm. This gas-turbine engine cost four times more than a normal engine, but is very strong with a long lifespan. All in all, it takes the exhaust gases of a jet engine and converts them to energy to drive a turbine wheel connected to an output shaft. In this Turbocar the air enters the engine through a duct and goes through four stages of compression with fuel being injected and then ignited by a glow plug, the resulting combustion driving the output turbine and therefore output shaft which goes to a transmission box or torque-converter from which power is fed to the four-wheels. Only one cooling unit was necessary at the back. Because of the torque converter, the car did not have a gearbox and a clutch. The driver only having to handle the throttle or let’s say “the flow” as it idled at more than 50 percent throttle but also had a three second lag between action by the driver and the car actually accelerating. For help in the breaking a movable flap was mounted on the back that functions as an air-break.
The fuel tank layout for the kerosene was interesting: There were five Firestone safety cells with a total capacity of 166 liters. One-way valves ensured that as the tank became increasingly empty, the remaining amount was always concentrated where it was advantageous for weight distribution. Because consumption was then higher in practice than planned, an additional 38-liter tank was installed in the front.
Designer Ken Wallis had in 1966 offered the project first to Dan Gurney and then to Carrol Shelby, but both of them turned him down. The ideas for installing the turbine on the side and for the four-wheel drive came from Andy Granatelli, CEO of Scientifically Treated Petroleum or let’s say STP. Sponsorship money was no problem so they hired and bought everything needed to create a winner. Manufacturing the chassis was a tough nut to crack. The first version made of stainless steel warped during heat treatment, so that they had to start from scratch and a participation in the 1966 Indy 500 was already impossible. The second, riveted version made of the light titanium alloy 7178-T6 weighed 60 instead of the previous 40 kilograms. Nevertheless, the entire asymmetrical short car only weighed 634 kilograms, with a prescribed minimum weight of 611 kilograms. The second chassis was ready early 1967.
Andy Granatelli had made big promises before the 1967 race: "Just as the central engine cars have pushed the front engine cars out of the field, the gas turbine cars will push out the piston engines."
The Granatellis realised that 4WD was the key component needed to get the most from a turbine-powered car. Sponsorship money was no problem so they hired and bought everything needed to create a winner, to come heartbreakingly close. Few fans liked the car, however, most hated it and every car owner but the Granatellis hated it even more. Curiously, despite the common hatred for the car among the crowd, 200.000 people came to the track to practice and Pole Day because they all wanted to see (and hear) the car with their own eyes and ears. Parnelli Jones was still a mighty popular driver with the majority of the crowd but the Granatellis as persons hadn't been that well liked, despite the fact they had 'given' the crowd the Novis to cheer for. With the Novi cars gone from the team, one must really wonder how much more unpopular the team would have been if they had hired a driver other than Parnelli!
Stories state that Colin Chapman of Lotus helped with adjusting the setup of the car.
In qualifying, Jones placed the car in sixth place in a field of top-class participants from the USA and Europe. Mario Andretti got the pole-position. The alarmed competitors, however, suspected that Jones had deliberately not shown the car's full potential by taking his foot off the accelerator before the braking zones. And all other drivers were complaining of the hot exhaust air with a smell of kerosine.
On Race day, in the first two corners after the start, this theory was soon confirmed when Jones took the lead on the first lap. The STP car was well in front until the rain put an end to the race after 18 laps. At the restart the next day, the same scenario. Even an excursion in the midfield cannot stop Jones. This was not seen before. With still 20 laps to go, he is one lap ahead of AJ Foyt. And then it went wrong. In lap 196 of the total 200, the red car slowed down and retired. A six-dollar part, a bearing on the rear axle, prevented the sensational first victory! Foyt won the race and he could not hold it back during the ceremony. He said: “cheaters aren’t winners”.
But the turbine car of Granatelli had almost won the “500” in 1967, and there is no shadow of a doubt that it was by far the car of the year. The 1967 Indy 500 is not a story of how AJ Foyt won the race but how Parnelli and the Turbine lost it. What followed was a worldwide free publicity for STP.
For 1968, the intake cross-section for the gas turbines was massively restricted to 103,1 cm2 by the USAC. They also restricted the tyre dimensions for a 4WD. Pratt & Whitney responded with a new ST6-70 turbine specifically for vehicles, which again produced 550 hp. In 1968, it was installed in both the STP Paxton and the wedge-shaped 4WD Lotus 56. The Granatelli brothers brought five turbine cars to the Indy 500. The Lotus cars performed well, but the STP Silent Sam is to slow. Parnelli cannot switch to Lotus and decided on the spot to stop his racing career. And his team mate Joe Leonard destroyed the car in practice. In the end on the first row you could see the Lotus of Graham Hill and Leonard. During the race, the Leonard lotus Looked like winning, but nine laps before the end, the turbine failed.
After 1968, gas turbines were banned completely in Indy, as was four-wheel drive. Nevertheless, Silent Sam was a unique car. The crashed STP was restored and ended up in the Indianapolis Motor Speedway museum. It remains a true favourite of the visitors.
