Automotive fuel systems have a simple function-they deliver fuel to the carburetor or electronic fuel injection system. But, like many things that seem simple, there's more to getting gas from one end of the car to the other than meets the eye.
An old standby, the T-style...
An old standby, the T-style fuel pump gets its name from its familiar shape. Durable and reliable output pressure of these pumps is preset.
The fuel system has to meet the varying demands of the engine. Obviously, when an engine is running at slow speeds it requires less fuel than it does at wide-open throttle so the system must adjust to those needs. Another factor is the powerplant being used. The capabilities of the fuel system must be appropriate for the engine-be it a vintage Flathead V-8 or a new LS7. These are just some of the issues to be addressed when designing a fuel system for a street rod (or an engine swap), which we're about to tackle with Jesse Powell of Aeromotive.
Types Of Systems
Fuel systems fall into two basic categories: static or deadhead, and dynamic or return type. Static systems typically used with carburetors essentially deliver fuel "on demand." Normally the fuel pump (mechanical or electric) will supply the carburetor's needle and seat with around 7 pounds of pressure, and under normal conditions the pump will keep up with demand and maintain that pressure. As fuel is used by the engine, the float drops and the needle valve opens, the resulting reduction in pressure causes the diaphragm in a mechanical pump to begin moving again, or the bypass in an electric pump to close, building the pressure back to 7 psi. In many cases, the pressure fluctuation won't be noticed without data logging the system or a fuel pressure gauge that you can see while driving.
T-style pumps use a spring...
T-style pumps use a spring and poppet valve to control pressure. When system pressure exceeds the preset limit fuel is bypassed from the outlet back to the inlet through an internal passage.
Although the fuel system's first priority is to keep the floats from running low enough to uncover the carburetor's main jets, the second, more difficult job is maintaining the optimum fuel level in the bowls. It may not seem significant but the weight of fuel above the main jet impacts fuel flow through it and that effects the engine's air/fuel ratio. The stop-and-go delivery of a traditional static fuel system can make a constant fuel level difficult to maintain, which means the engine may at times be robbed of fuel, limiting its potential.
Sophisticated carburetor racers using carburetors know the fuel level in the float bowl(s) must be consistent if engine tune is to be held across the rpm band, and this holds true for street engines in stop-and-go traffic as well. One method of accomplishing this is to use a higher-pressure pump (12-60 psi) with a regulator lowering the pressure to the carburetor (8-9 psi).
This is an example of a return...
This is an example of a return system used on an EFI system; unused fuel is sent back to the tank. Return systems can also be used with carbureted systems to provide consistent fuel delivery.
Another option to maintain consistent fuel levels in the carburetor is a dynamic, or return-type system, like those used on fuel injection systems. The benefits include longer pump life, a marked increase in pump to horsepower ratings (allowing smaller, lighter pumps to fuel more horsepower), and even quieter pump operation is common. With a return-style system the pump pushes fuel at 100 percent of its capacity all the time-it also pulls 100 percent of its capacity from the fuel tank all the time, which is why pumps in the return-style system are more sensitive on the inlet side (we'll discuss that further when we get to filters). With a return-style system, fuel flows through the entire system at the pressure determined by a regulator-be it 7 psi for a carburetor or 60 psi for EFI. The regulator allows fuel to "bypass" and return to the tank, then as the engine begins to use more fuel, less returns to the tank. The benefit is the carburetor does not see any pressure drop because the pump is flowing at full capacity to the regulator all the time.