When I was racing my 1972 Mustang unblown fuel-injected A/FC, I made a careless mistake in the pits one day with my Hilborn Fuel Injection system. We had just fired up the car to warm it up for qualifying, I pulled the main jet body out of the fuel line to change the jet, and upon re-installing it, I accidentally put it in BACKWARDS!
The Hilborn jet body was a beauty, machined out of brass and looked almost identical at both ends, except for the end that unscrewed to allow you to change the jet. It was mounted in the fuel return line, so that the way it worked was to restrict the fuel returning to the fuel tank. So therefore, a larger hole in the jet would allow more fuel to return to the fuel tank, which would lean out the engine and a smaller jet would hold more fuel pressure and volume in the system to richen up the system, forcing the engine to burn more fuel.
This made tuning a nitromethane-burning engine such as mine very easy. The main jets came in very small changes, just fractions of an inch, such as going from a .060 inch opening to . 065 inch hole would lean out the engine enough to make my car run 7.55 seconds at 180 mph instead of a 7.60 Elapsed Time at 175 mph. Doesn't sound like much, but that small change is sometimes the difference between winning and losing.
So we towed up to the starting line, fired the engine for my first qualifying run and the engine sounded a bit different than usual, but I thought nothing of it until I did the burnout. The engine had more power than usual, it was even louder than normal and when I whacked the throttle after the burnout, it violently shook the body like a hand grenade had gone off inside the car!
I didn't know what to think, but I staged, left on the last yellow as I always did, the car exploded off the starting line with a force I had never felt before! But at the 1000 foot mark of the quarter mile, it laid over like a sick donkey. I thought the mag had gone bad, or a fuel line broke…a hundred thoughts went through my head in a fraction of seconds.
Still qualified on that run, but not in my usual #1 qualifying position. We towed back to the pits, pulled the Mustang body off to go through the whole car and I saw the jet body was in the return line backwards. That means the fuel system had no return capacity. It was the same as running a BLANK JET! The jet body has a spring and seat device inside that keeps fuel from backing up the return line and flooding the engine, so with the jet body backwards, the jet was shut off, no fuel could return to the tank and the engine was getting 100 percent of the fuel pump volume.
And all of a sudden, I had an AHA! Moment! I realized that my car had TONS more power on the low end when it was dead rich, getting 100 percent of the fuel that my Hilborn pump could produce. But it couldn't burn it all on the top end, so it laid over at the 3/4 mark of the run.
We put the jet body back in the return line correctly, qualified #1 and won the race, but I was thinking all the way home in the tow truck about what I had done, and what I had learned from it. Like most all unblown nitro racers back then, I thought you had to run the engine on the lean side to go quick and fast, even if it burned the ends of the spark plugs a little bit. That’s when you would richen it up just a hair, stop burning the plugs and perform the best.
Then I found out about The High-Speed Leanout Valve! It would turn a one-stage fuel system into a TWO-STAGE system! You could run rich on the low end, or first 2/3 of the quarter mile, and then bleed off the excess fuel on the top end that an unblown fuel engine couldn’t burn when the engine RPM’s were up around 7500 and the fuel pump was pumping full volume, since it was driven off of the front of the timing cover by the camshaft.
The High-Speed Leanout had a jet in it also, so that you could control exactly how much fuel to bleed off, and before the the jet was a pop off valve that you could adjust with small shim washers under the spring to increase or decrease the fuel pressure that it took to open the Leanout valve before the fuel could go through the hole in the jet. A very elaborate system that worked flawlessly. It gave me two more variables to tune with…the jet size and the correct number of washers under the spring to open the pop off valve at the right time. I never burned the spark plug tips after that. They would come out a bright beautiful hot blue color but with no more frazzled ends.
There was only one small drawback…I had to buy a new log book for all the new data, since I had more tuning variables to deal with. I recorded all my runs at every track where I raced so that when I went back there again, I had a baseline “best run” to look at with all the data on temperature, humidity, nitro percentage, main jet size, high-speed lean-out jet size and number of spring pressure shims, magneto advance, tire pressure, clutch settings, spark plug heat range, etc.
This was the answer to my dilemma of how to run rich on the bottom end and leaner on the top end. I got a high-speed lean-out from Gene Adams at Hiiborn and started playing around with it, but ended up with an Ed Pink aluminum bodied unit, because it was bigger. the Hilborn unit used a #6 fuel inlet and the Ed Pink unit had a #8 inlet line. The bigger line worked better for me, since I was running the big Hilborn fuel pump, not the small one that most all the other injected cars were running. That way I could run maximum fuel on the low end and bleed off the most on the top end. It worked beautifully! I set a new National elapsed time record with this setup and won the IHRA Nationals in 1973 with it at Dallas International Motor Speedway.
NOSTALGIA DRAG WORLD - By Ben Griffin; Photo courtesy of BG