Stu Hilborn: Father of Hot Rod Fuel Injection
Story By David Marin
OK, you should be in your recliner, eyes closed and focus on the name Hilborn. Really now, focus. Can you feel it? The rumbling vibration of torque as it pours through the driveline. Hear it? The throaty sound of horsepower as the tach rises from idle. Are you there? Got it? Hilborn Fuel Injection fogging the cylinders with high-octane fuel. Ahh yes, Hilborn, there’s a look, sound, feel, a mystique, it’s enough to make a grown man misty-eyed.
It’s been more than a half-century since a youthful Stu Hilborn, while in Greenville, Mississippi awaiting discharge from the Air Force, penciled out the first drawing of the fuel injector. The idea had been spawned in the mind of the innovative genius years earlier on the dry lakes of California while racing an A-V8 Ford roadster and later a Streamliner, both with notable success.
“I raced at the dry lakes in California, using carburetors like all of us did,” the legendary rodder says. “We all struggled with the problems of running methanol with carburetors, namely, very unequal distribution of fuel to different cylinders as well as the difficulties of supplying two-and-one-third times as much fuel as with gasoline; and, there was also the matter of dealing with heavy corrosion of pot metal parts.”
Stu diagnosed the worst problem as poor fuel distribution noting, “It could be improved by running eight single throat carburetors, but this was cumbersome and complicated. It could also be solved in a better way with fuel injection; however, all the existing types of fuel injection were pretty much the same. They all ran extremely high fuel pressure, required a precision fuel pump at a very high price, and all were timed to inject fuel
into the engine only on the intake stroke. All in all, it was too complicated and too expensive for hot rodders and our low-budget-modified stock engines.”
Equal fuel flow to each cylinder was the objective and Stu spent a good deal of time thinking of a way to get around the problems.
“That was my main goal,” he says. “Then the idea came to me; since I was only interested in running at 5,000 rpm, why interrupt the fuel flow 40 times a second? What is gained? Nothing! Eliminating this would eliminate the complicated timing system as well. A simple, low pressure, constant flow system is all I needed… a hot rodder’s dream! So that became my goal.”
With his Air Force discharge and fuel injection drawing in hand Stu, who majored in chemistry in college, returned to his job as a chemist with a paint company in the Los Angeles area.He then commenced construction on the fuel injection system and at the same time devoted much of his spare time to getting his car ready to run at the dry lakes.
Drawing on his training as a chemist and the pre-war engine building skills (learned from former Indianapolis race driver Eddie Miller, a skilled metal worker and welder), Stu painstakingly hand fabricated his first fuel injector. The following is excerpted from Stuart Hilborn’s autobiography…
All of us racing at the lakes are using carburetors that automatically control the fuel flow by means of the airflow through the venturi tube. But, the fuel injector doesn’t have a venturi tube and, none of us know the actual amount of fuel the engine is using at full throttle with carburetors. Remember, there are no dynamometers or fuel-flow meters available to measure such things. To solve the problem I had to rely on my training as a chemist. I set up the chemical equation for the oxidation of methanol in air. This allowed me to calculate the stoichiometric, or theoretical air fuel ratio for methanol. Engines don’t perform well at stoichiometric ratios and have to be richened up. About 2/3 of stoichiometric works good. Next, I needed to know the amount of air the engine used. This is fairly easy to calculate from the engine cubic inches and the r.p.m. Flat head engines don’t breathe real well, so I knew if I used 100% volumetric efficiency I would be well on the safe side. I could always lean the system down a little after I started running it. Now, knowing the airflow, I could easily calculate the required fuel flow I needed.
Next, how to measure the fuel flow to be sure I had the right amount. To do this I hooked the fuel pump up to an electric drill motor to drive it. I already had one injector nozzle built, guessing at the size. I connected the nozzle to the pump outlet, turned on the motor, and adjusted the fuel pressure. I then collected the fuel discharged from the nozzle in one minute, in a coffee can. After weighing the contents on a small scale, and some more calculations, I finally had an actual fuel flow number. I hoped it would be 1/8 of my calculated total fuel flow number for the engine. I was off by a mile. The nozzle was much too small. After several more attempts I was able to make a nozzle that flowed exactly 1/8 of my previously calculated total flow for the engine. So far, so good. But I still needed seven more just like it. They must all flow precisely the same amount. I built seven more nozzles and measured the fuel flow. Disaster! None of them matched the first one. In fact, there weren’t any two alike out of the entire seven, even though I had drilled them all with the same drill. This wasn’t going to be as easy as I thought.
I still was spending most of my spare time racing my car at the dry lakes, and with all the preparation needed before each race. I had now gotten up to 139 m.p.h., which was quite pleasing. Along the way I kept building and testing more nozzles. My success wasn’t increasing, but my pile of rejected nozzles sure was.
The racing season finally came to an end and I could now devote all my spare time to finishing the fuel injector. Besides the nozzle problem, I still had one other major problem left. I had to find some way to mount the aircraft fuel pump I was planning to use, to the engine, in such a way that it could be driven by the engine. It was essential that the pump speed vary directly with the engine speed. The best solution seemed to be to extend the oil pump shaft right through the flywheel housing to outside the engine and then build a mounting pad for the pump right over the extended shaft. Simple enough. However, the execution was far from simple. The old problem of no machine tools was the trouble. This would be easy to do on a large milling machine, but all I had was an electric drill and some hand tools. I took an old oil pump and removed the shaft and gears and installed it in the engine in place of the regular oil pump. Then, using a long drill the same size as the oil pump shaft, I went down through the pump bushings and slowly drilled right through the flywheel housing. The pump bushings kept the drill in alignment so that everything stayed centered. The rest was fairly easy, requiring only a longer oil pump shaft, a housing and bearing to fit the shaft. The pump bolted right on the main fuel injector plate, which was already completed.
Everything is finished now and on the engine, except I still have no injector nozzles. I am reluctant to continue to make them the same way I had been because I had no real control over the flow. Maybe, if I made 100, I might get 8 matched nozzles. Or, maybe I would only get 3 or 4. I decided to try a different approach. I would take the nozzles I had already made and separate all the nozzles that were low on flow and see if I could find a way to alter them to increase the flow. Drilling them out to the next larger drill size didn’t work. It was far too big a jump. I tried a few different ideas, but none of them worked. Then I tried a big chamfer on the inlet side of the fuel orifice. Bingo! The fuel flow jumped up. Too much in fact. I found by adjusting the size and shape of the chamfer, I could trim the fuel flow right into the number I wanted. Within an hour I had 8 perfectly matched nozzles. The first fuel injector was finished.
A private test at Rosamond Dry Lake went well as Stu took his car to a comfortable 120 mph. That preceded the opening of the 1947 race season at Harper Dry Lake where a loose fuel line fitting brought a near disastrous fire. Bouncing back the following morning (Saturday), Stu clicked off a very respectable 134 mph, but was unable to improve on it mostly because of course conditions and a persistent fuel richness.
Back at Harper the next month, a loose lake surface combined with a broken wire wheel caused Stu’s car to go airborne and barrel roll numerous times. Confined to a body cast for months, Stu was still able to go through his engine, while Eddie Miller rebuilt the body and chassis. Although their 1947 season was over, the car would be ready for 1948.
It’s now April 1948 and Stu is at El Mirage Lake, his personal favorite. He’s “pretty well recovered” from his crash but at his mother’s request (insistence?) he has promised not to drive and enlists fellow Southern California Timing Association club member Howard Wilson to handle the behind-the-wheel duties.
Stu listened as Howard sped down the first mile. The engine sounded really sharp! The speed was 144.96, five miles faster than the best time with carburetors and the fastest speed/time ever recorded by any car in the history of dry lakes racing. Later that day, racing in the opposite direction to preclude any tail wind advantage, the streamliner made its second run at 145.640, faster than the morning run and a new Official Record!
During the 1948 season Stu’s streamliner set and broke records and became the first car to break the 150 mph barrier with a 150.50 clocking. The critics who had said the Hilborn system wouldn’t work were proven wrong. The system not only worked, it worked magnificently. So impressive was the performance of Hilborn’s system Ford Motor Company’s in-house magazine, Ford Times, described Hilborn’s 150.50 speed as equivalent to breaking the sound barrier in aircraft.
There you have it… a thumbnail sketch of the first fuel injection system for hot rods as it evolved from the mind of Stuart Hilborn to the drawing board to reality. Soon he would be building injectors for Offenhauser engines and would eventually go on to feed the engines in 34 winners at the Indianapolis 500, success that forced the hot rodder to quit racing and start his manufacturing career.
Today Hilborn Fuel Injection manufacturers both mechanical and electronic injection systems and Stu, who will celebrate his 90th birthday in October 2007, still handles all the EFI flow bench duties. About five years ago he personally developed the Hilborn EFI along specific guidelines.
“There were many systems out there that didn’t work for our customers,” he says. “The problem was too many sensors and complications. Our customers aren’t that oriented into electronics and we wanted something they, with simple instructions, could easily follow and hook up themselves.”
Thus Stu developed a two-sensor system that requires nothing more than a simple screwdriver to adjust. “The two-sensor system allows our customers to tune their engine without having to call in an electronics expert.
Far from content to rest on his considerable laurels, Stu’s latest innovation is a “rotary injector” This new fuel injector features an internal rotating cylinder instead of traditional throttle shafts and butterflies. The rotary design produces unrestricted air flow with maximum torque and horsepower throughout the full range of throttle openings.
“There’s always someone coming up with a new project,” he says. “I have no problem staying busy.”
Editor’s Note — Special CruZin’ “Thanks” to Stu Hilborn and the folks at Hilborn for their assistance in the preparation of this article. For comprehensive info on Hilborn’s “NEWstalgia” EFI System visit their website at www.hilborninjection.com.
