Richard Holdener
September 1, 2004

The advent of the fuel-injected, 5.0 Mustang back in 1986 ushered in a whole new era of Ford performance. The 5.0 H.O. created an entire industry, one continued to this day since the introduction of the Modular-powered fast Fords.

Though the current overhead cam engines differ from their traditional pushrod counterparts, the two do share a number of things in common. In addition to sharing the V-8 configuration and even the firing order, the injected 5.0s and 4.6s share the high-pressure fuel system common to most fuel-injected motors. Unlike traditional carbureted engines, fuel-injected versions rely on much higher fuel pressure. The elevated fuel pressure helps atomize the fuel as it flows through the fuel injector.

While carbureted motors traditionally run fuel pressure near 7 psi, (port) fuel-injected combinations run best around 40 psi. We specify port fuel injection here as throttle-body injection systems employ fuel pressure ranging from 9-15 psi, but our beloved SEFI Mustangs are not so equipped.

While both EFI engines employ high-pressure fuel injection, the two systems differ in their approach. The conventional return-style fuel-injection system (used on the 5.0 and some early 4.6 motors) featured an in-the-tank fuel pump--a feed line (and fuel filter) running from the fuel pump to the fuel rail. The fuel rail was designed to distribute fuel flow to the eight injectors and incorporated a fuel-pressure regulator designed to control the system pressure.

Though similar in shape, the early positive displacement (5.0 and early 4.6) electric fuel pumps and the later turbine (late 4.6) pumps differ in design. The two are not interchangeable, though turbine pumps can be used in positive displacement applications.

Most factory fuel-pressure regulators were not adjustable, but many enthusiasts soon replaced the stock non-adjustable unit with one of the many adjustable regulators available from the aftermarket. Using a spring and diaphragm, the regulator controlled fuel pressure in the system and allowed excess fuel flow to return back to the fuel tank.

Re-circulating the fuel helped keep the temperature down, as the elevated pressure in a fuel-injection system has the undesired effect of heating the fuel. Re-circulating it back to the fuel tank eliminates the pressure and allows the heated fuel to join the relatively cooler fuel in the tank. Obviously, constant circulation can elevate the temperature of the fuel tank as well.

A better understanding of the role of the fuel pump is necessary before going on to the non-return-style fuel system. In a return-style fuel system, the fuel pump is constantly flowing the maximum amount possible. The flow rate of a fuel pump is determined by the size and design of the pump itself but also by two very important external factors, namely supply voltage and system pressure. We will take a closer look at these two variables later, but for now, know that in a return-style fuel system, the fuel pump is flowing at the maximum flow dictated by the pressure and voltage supply.

While the fuel supply (pump) is flowing at a constant rate, the fuel demand obviously varies with the engine speed and load. At idle, the fuel demand is much lower than when running at 6,000 rpm at wide-open throttle. The fuel pump must be sized to support the greatest fuel demand. The job of the fuel-pressure regulator is to bypass the difference between the fuel supply and the fuel demand while maintaining the preset fuel-system pressure.

The non-return-style fuel system differs from the conventional return-style fuel system by way of the fuel-pressure regulator or, more specifically, the lack thereof. In a non-return-style fuel system, the fuel pressure is controlled by the supply voltage to the fuel pump. Like any electric fuel pump, the output of these pulse-width modulated fuel pumps are dictated by the supply voltage. The fuel pumps used in a return-style fuel system are supplied a constant voltage, usually near 13 volts--depending on the effectiveness of the charging system and accessories currently drawing from that system.

All of our production in-the-tank pumps were tested on the Kenne Bell computerized fuel-pump test bench. The bench can be set to test a pump at any system pressure or supply voltage--just the thing for our needs.

The fuel pumps in a non-return-style system are not supplied a constant voltage and instead receive pulses much like a fuel injector. The pulses do not actually start and stop the fuel pump, but act much like limiting full system voltage to control the fuel pressure. The pulse width is determined by the computer and is based on fuel-pressure information supplied by a pressure transducer in the fuel rail. It makes things easier if you think of the system as an electric version of the mechanical fuel-pressure regulator. The beauty of the system is that the pump is not constantly supplying maximum fuel flow, only that which is needed to produce the desired pressure specified by the computer.

We mentioned previously that the fuel flow rate of an electric fuel pump is determined by the size (and design) of the pump, as well as the supply voltage and system (fuel) pressure. Physical size obviously helps determine the flow rate of a fuel pump. Spin a larger set of gears the same speed as you do a smaller set and the larger set will flow more. Picture the difference between a standard and high-volume oil pump if you have a hard time coming to grips with the gear size. The size of the fuel pump is somewhat determined by the available space in the pump assembly; this is especially true of in-the-tank pumps. For most Mustang applications, the physical size is predetermined, but not so for the voltage supply and system pressure. Let's take a look first at how the system pressure affects the fuel flow and where elevated pressures may become necessary.

As a rule of thumb, the flow rate of a pump decreases with system pressure. Simply stated, it is harder for the pump to flow against higher pressure than lower pressure. Need verification? Check out the changes in flow rate as we increased the fuel pressure from 40 to 60 psi and finally to 80 psi. Like everything else in nature, fuel pumps like to take the easy route.

As mentioned previously, fuel-injected motors typically run fuel pressure somewhere near 40 psi. This all changed once we discovered how easy it was to install forced induction onto the new fuel-injected 5-ohs. Using the early return-style fuel systems, additional fuel was supplied to the boosted motors by simply increasing the fuel pressure. This was accomplished by shutting off the return line to temporarily override the fuel pressure regulator, thus (sometimes) dramatically increasing the fuel pressure supplied to the injectors. Greater fuel to the injectors increased fuel to the motor, hopefully in proper proportion to the airflow supplied under boost.