Today I’m going to talk about constant flow fuel injection. There are mysteries around this fuel system that very few people across the country really understand. With any given engine, it’s crucial to have the appropriate air-to-fuel ratio for the horsepower that you are producing.
There are many ways to get to that ratio. I started my career in NASCAR, so I built carbureted engines. The carburetor is a smart piece of equipment that does a good job of regulating the amount of fuel in relation to the amount of fuel that is going into the engine. In contrast, constant flow fuel injection is very stupid. It can’t regulate ratios. It only does what you tell it to do, and that’s where the mystery and the magic starts.
Overview of the fuel system
As a basic overview of the fuel system, you have fuel in the tank. You have a line coming in and a line going out to the fuel pump. To clarify, you have one line going in and either three or four coming out, depending on whether you have a stumble valve. This depends on your bypass system, and I will provide more detail on that shortly. So, one line goes to the barrel valve on top of the engine. The barrel valve is a regulating device that is also hooked up to the butterflies and varies its orifice size depending on throttle position.
As the throttle opens, it allows more fuel into the engine. The barrel valve has two ports—one going in and one going out. It is connected by high-pressure lines to the nozzles themselves. The nozzles are in the cylinder head in the intake port on all the 410 engines. In some other classes like the ASCS, the nozzle has to be in the injector runner itself.
The other lines outside the fuel pump are designed for bypasses—for fuel not being used by the engine to be redirected back to the fuel tank itself. So you have your main jet for the system and a high-speed bypass, which bypasses fuel as the fuel pressure increases, past the point of the engine needing more fuel. You also have a barrel valve bypass for when you are on the throttle full blast and then let off quickly. What happens then, when you are at high pressure and you let off quickly? The barrel valve bypass lets the fuel bypass back to the tank instead of flooding the engine.
Next, I will explain the different parts of this system by working my way backward from the fuel nozzle back to the gas tank. The fuel nozzle has a specifically sized orifice tube which helps regulate the amount of fuel going into the port. There are two types of nozzles that we use. We call the first one a sprayer and the second one a bullet.
The sprayer nozzle is one where the stream of fuel goes into a chamber inside the nozzle body and starts the process of mixing with air. As that begins, the fuel continues down a tube. It hits a screen which helps diffuse the fuel, further enhancing atomization for a more complete burn. The sprayer nozzle must be able to breathe. There are aspiration holes inside the nozzle that must be connected to atmospheric air. Usually, there is a filter to keep the dirt out. Still, if the aspiration hole is plugged, there will be a tremendous decrease in the efficiency of the nozzle. The performance will degrade from a fine mist spray going into the port to a dribble, and you will notice the difference in how the engine runs.
The bullet nozzle is similar to a sprayer but doesn’t have a diffusing screen on the end of the nozzle. Instead, the nozzle shoots a small solid stream directly into the port. Right after I joined Shaver, at least 20 years ago, we were testing with a customer. Stu Hillborn brought these bullet nozzles in and asked me to try them in an engine. I told him it would not work, that we needed to diffuse the fuel. He persisted, and I agreed to give his idea a try. When we tested it, we picked up 15 HP. His idea was that if the stream was pointed in the right direction, it wouldn’t block the air coming down the port like the mist of fuel from the sprayer injector would. The mist going into the port slows the air marginally, producing a bit less power. Instead, when the pinpoint stream hit the valve in the right place, it dissipated on the intake valve, allowing more air into the engine, and we picked up power. That is the evolution of the bullet nozzle.
Now I’m going to talk about the barrel valve.
The barrel valve is attached to the butterfly linkage of the throttle assembly. As the butterfly opens, it opens a slot in the side of the barrel valve that allows more fuel to go in. It tries to regulate the amount of fuel depending on the butterfly position.
The other important part of what the barrel valve does is in the closed position. In the closed position, a port is connected to an exit tube that leads to the low-speed bypass. As the engine goes from idle to full rpm wide open throttle, the fuel pressure can vary from 10 to 130 psi.
On the other hand, let’s say you are running with a wide-open throttle and need to come into a corner and shut the throttle really quick. You might be at 110 or 115 psi, and the fuel needs to go somewhere when you close the throttle. Without a second hole in the barrel valve, the fuel would go back into the engine and flood the motor. The secondary bypass allows the fuel to bypass back to the tank as soon as you close the throttle.
Next, we want to follow the fuel line back to the cockpit and the shutoff valve. This is where the driver or the dyno guy can shut the fuel off to the engine. This is critical. On mechanical fuel injection engines, there is a lot of fuel being dumped into the engine. We always want to shut the engine down by shutting the fuel off first before we shut off the ignition. The reason is that we need the engine to burn the fuel out of the ports and chambers, so it doesn’t sit there. If you shut the ignition off, the fuel will still be pouring into the engine. People will flood their engine out and try to start it again and bend the rods.
Let’s return to the fuel pump itself, which is the heart of the constant flow fuel injection system. Each fuel pump is designed with tight tolerances to pump a specific amount of fuel at a specific pressure at a specific rpm. A vital part of our success is knowing how much fuel is going into the motor. The fuel pump is driven off the cam at half-crank speed. The fuel pump is designed to pump more fuel than the engine needs. That’s where the other bleeds come in, starting with the main jet. The main jet has two purposes. First, it is designed to get the correct air-fuel ratio for your engine. Second, it is crucial for your engine to start. A spring-loaded valve in the main jet doesn’t open until it reaches a certain pressure. As you try to start your sprint car, if that spring is too weak, the fuel pump is going too slow and won’t pump enough fuel into the engine. We set our valves at about 10 psi, enough to start most vehicles at the racetrack.
The next bypass is called the high-speed bypass. This is a pressure-regulated system with jets that bypass fuel as the engine’s RPM reaches the point where the main jet is not enough to bypass the fuel produced by the fuel pump.
Finally, I want to talk about the importance of testing equipment. One of the most critical tools in my arsenal is the equipment I have here at Shaver Specialties to precisely measure the fuel flowing through the system in pounds per hour. Both dynos here have very accurate fuel flow meters. To determine how much fuel each motor needs, I must accurately measure the fuel flow. I also have instruments that will test the fuel pump by itself to tell me exactly how much fuel it’s putting out. I even have a testing instrument to tell me how much fuel flows through a jet at a specific pressure.
I’d like to end this post with a note for the history buffs, and a thank you to those who led the way to where we are today. We can trace the origins of constant flow mechanical fuel injection back to 1948 when Stuart Hilborn designed the system specifically for the Indy 500. In 1949, the car using his system set fast time and a new track record. Unfortunately, the car broke and was unable to complete the race.
The technology advances that have taken place since then are based on his work. Ron Shaver is one of a small group of visionaries that has been able to adapt and perfect this system throughout the years as engine horsepower continues to increase. I am very fortunate to have learned so much from Ron, and now I am enjoying being able to share my knowledge with all of you.
