Fancy Fun Facts about Fuel Systems

4 barrel holley carburetor

Ever since an internal combustion engine has been around, the need to add fuel to create a proper BURN has been present. This was accomplished back in the day with carburetors and now with direct injection.  Hopefully after reading this, you’ll have a sound knowledge of the different types of fuel injection that have been used over the years .

All fuel system set ups will be a variation of this layout: a gas tank, fuel pump, fuel lines and the way in which the fuel is injected into the engine. One of the biggest differences in these set ups is the fuel pump, the pressures it puts out, and the capacity it needs to flow. Pressures can range from 6 psi up to 80 psi leading into a high pressure fuel pump that puts out 3,000 psi.



A Carburetor operates on Bernoulli’s principle. In this principle, the faster air moves, the lower its static pressure, and the higher its dynamic pressure. Meaning that as the speed of the air increases, the pressure decreases. A carburetor in it’s most basic form is a tunnel that narrows at a point and then widens again. As the tunnel narrows, it causes the velocity of the air to increase. As the air’s velocity increases while it passes the jets, it draws fuel out of the float chamber. The amount of fuel that flows out is determined by the size of the orifice in the jet. This is why as people increase the amount of air they allow to flow into their motors (whether it’s by forced induction or a better flowing naturally aspirated system), they increase the size of their jets. All of that is just so that as the velocity of the air increases at the narrowing point of the tunnel, more fuel is drawn out of the bigger orifice of the jet.

In the float chamber is the float and the float arm. As the fuel is used up, the chamber empties and the float lowers. Then the float arm opens the float valve until the chamber is filled enough to push the arm back up and close the valve. Due to the way that the float chamber operates, a carburetor system operates on a low pressure fuel pump. This system only requires 6-9 psi. Any more than that and the float valve could be blown past, and you could pressurize the float chamber.

Looking at this picture, you should be able to see how the tunnel narrows to increase the velocity of the air. You also can see how as the level of the fuel decreases, the float arm lowers with the fuel level and opens the valve at the top.

carb tunnel

throttle body injection

Throttle Body Injection

Throttle body injection came out as an improvement to the carburetor in the late 80’s. Fuel is added to the intake manifold at the same place as the carburetor is. This system operates electronically as opposed to working off a law of physics (like the Venturi Effect). As you can see in the picture above, it can resemble a carburetor in some aspects, but it’s function is completely different.

Just like every electronic fuel injection system, it operates off of the inputs of several different sensors. Things like coolant temperature, throttle position, oxygen sensor, and an idle air control valve just to name a few. It can see when the engine has reached operating temperatures to where it can run without issues of fuel not vaporizing properly due to colder temperatures. It can determine the amount of throttle given and compensate with the amount of fuel to add. Then it can read the burn and determine if too much or too little fuel was added.

A throttle body injection system is called a wet manifold system because the fuel has to pass all the way through the manifold just like it would when operating with a carburetor. This system brought reliability and better emissions to the vehicles equipped with it.

fuel rail

Port Fuel Injection

Port fuel injection brought an even higher expectation of reliability and emissions. As the name suggests, there are individual ports for a fuel injector to be placed in the intake manifold. If we take the TurboTimer wagon and it’s four cylinder flat four engine for example, there is a fuel injector placed at the intake valve of each cylinder. Unlike the throttle body injection system, this is a dry manifold setup as the fuel doesn’t pass through the manifold. It is placed right at the intake valve so as that valve opens it can be inducted in to the cylinder and be ready for combustion.

Because each cylinder gets it’s own injector, a finer and smoother process can be achieved. It is harder for the fuel to lose its way through the intake manifold and find its way in the cylinder when it is right by the valve. This offers more potential for a precise burn and good emissions.

I know that they are called fuel injectors; I’ve called them an injector the whole time in this post, but the function of an injector isn’t necessarily to inject.  My professor, Matt, at my university was adamant we didn’t call them an injector as they are more a solenoid than anything. This “Injector” opens and closes at the pulse of a signal. It has an orifice that will allow a certain amount of fuel to flow per unit of time. You’ll see the ratings of their flow such as 500 CC’s of fuel per minute. There is pressure in the lines, say roughly about 80 psi, and then the ECU sends a signal to the “injector” (Matt I hope you aren’t reading this!) and it opens like the solenoid it is. The duration that it stays open is called duty cycle. As the duty cycle of an injector begins to max out, you’ll need to increase the CC’s the “injector” can flow in order to have a properly flowing system.


Direct Injection

When we talked about port fuel injection, it was so cool that we could inject it right at the intake valve. This offered so many opportunities to tune to a precise degree, but the newest technology out is a direct form of fuel injection. Instead of waiting for the valve to open and have to draw the fuel through the valve, we have fuel injectors that wait until the last moment for the cylinder to need fuel and then inject it into the cylinder. If we wait until the piston is already traveling to top dead center, then cylinder pressures are starting to rise. There won’t be any induction to pull the fuel in as there was when we used port fuel injection. For direct injection, there needs to be a high pressure fuel pump that will allow the fuel to over come cylinder pressures. This fuel pump is run off the cam shaft; as it spins, it actuates a piston that pressurizes fuel. These fuel pressures will range from 500-3,000 psi. In the picture below, you can see that the factory fuel pump piston on the right is smaller. At a certain RPM the piston could only build so much pressure. So by increasing the size of the piston it can build higher pressures.

HPFP internals

Now that we have the fuel pressures to overcome cylinder pressures, we can now inject fuel directly into the cylinder. Think about the advantages tuning-wise that can be found with this technology. There is improvement to be found, just like we saw from throttle body injection to port injection. In direct injection, there is no wet or dry manifold, no lost fuel, and also no fuel sitting around heating up. There are no opportunities for the fuel to ignite before it is intended because it is injected exactly when it is needed. Direct injection is how current cars have found such good fuel economy. Not only is it making your everyday commuter happy, but it is putting smiles on tuners out there that can enjoy the ability this technology has to make safer horsepower in cars.

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