By Lee Stohr: During the winter of 1926 and 1927, at the age of only 23, American racing car driver Frank Lockhart developed an intercooler for his Miller 91 c.i. racing engine. The straight-eight engine designed by Harry A. Miller, featured a centrifugal supercharger and produced about 154 h.p.
Lockhart, who came to Indianapolis in 1926 without a ride, amazingly went on to win the race in a Miller as a substitute driver for Peter Kries who became ill. He later worked at improving his own Miller and first turned to increasing its supercharger’s efficiency for the 1927 race, but soon realized that a more productive way of adding to its power output could be by cooling the pressurized fuel-air mixture.
Lockhart’s intercooler design was entirely contained within the frame rails, which was no mean feat, as the car Harry Miller designed was a pencil-thin single-seater, built without the thought of including such a large device. With help from the University of California Berkeley graduate Zenas Weisel and his brother, Caltech graduate John Weisel, the intercooler was designed with a U-shape to maximize the surface area. Lockhart filed a patent for it in 1927, which was granted in 1931 as U.S. Patent No. 1,807,042.
Lockhart’s Miller 91 engine responded to the new intercooler, and it is possible that 285 h.p. was reached because of it and his many other modifications. In 1927, he qualified on the pole at the Culver City board track at 144 m.p.h. using the new device and it added eight-m.p.h. to his lap speed. On the Muroc Dry Lake, he set a new record of 164 m.p.h. with the 91 c.i. racer. He next qualified on the pole at Indy again setting a new record of 120 m.p.h., a full five-m.p.h. faster than 1926, but retired on lap 107 with a broken connecting rod after having led the race for 81 laps.
After Lockhart’s unfortunate death while chasing the Land Speed Record at Daytona Beach in 1928, one of his Miller’s went on to win the 1929 Indianapolis 500 driven by Ray Keech. Lockhart actually had two Miller 91 racers, and he made at least two intercoolers for them. One was later used on a front-drive Miller 91.
As can be seen above, Lockhart’s creation is made up of five separate parts, from five different aluminum castings. It is held together with sixty-four ¼-inch studs and nuts. The whole assembly weighs in at fifty-four pounds. The outside surfaces of the four main pieces of the assembly are covered with approximately fifty fins.
The photo below shows the intercooler that is being copied for this project on the Perfect Circle Miller 91 owned by Tom Barbour. Editors Note: Part II of this story will show how Lee Stohr of Stohr Design accurately creates a digital archive of the original that will ultimately be used to make two exact reproductions of these castings by using a state-of-the-art 3D printing technique. You can also learn more about Lockhart in a book covering the life and times of the racer at Frank Lockhart – American Speed King.