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Fuel injection know-how for late-model Fords
By now, everyone knows that the fuel-injected Ford ushered in a whole new era of Ford Performance. Sure, the carbureted '82-'85 versions started the 5.0L revolution, but things took off once Ford introduced EFI. Fast-forward to 2012 and a new 5.0L that offers more than twice the power of the original. Despite use of a DOHC, Four-Valve head design, cross-bolted block, and 11.0:1 compression, the new engine only shares one thing in common with its ancestor--the high-pressure fuel system.
Unlike with carburetors (which use about 7 psi), fuel injection relies on much higher fuel pressure (usually between 38-50 psi). The elevated pressure helps atomize the fuel as it flows through the fuel injector. Even with high pressure, sometimes the stock system is not enough. To provide detailed analysis of the fuel system, we ventured to Kenne Bell to play with its new fuel flowbench. Capable of testing every aspect of the fuel system separately or together, the bench provided us the perfect opportunity to provide readers with the ultimate fuel system test.
Fuel Systems: Return vs. Non-Return
While early and late Ford motors employ high-pressure fuel injection, the two systems differ in approach. Conventional return-style fuel injection features an in-tank fuel pump, basically a feed line (and fuel filter) running from the fuel pump to the fuel rails. The fuel rail was designed to distribute fuel flow to the eight injectors, and incorporates a fuel pressure regulator to control the system pressure by restricting a certain amount of fuel that goes back to the tank.
Most factory fuel pressure regulators are not adjustable, but many enthusiasts replace them with adjustable versions. The swap is easy and allows a measure of tuneability. Recirculating the fuel helps keep the temperature down, as the elevated pressure in a fuel-injection system has the undesired effect of heating the fuel. Recirculating it back to the fuel tank eliminates the pressure and allows the heated fuel to join the relatively cooler fuel in the tank. The downside is that constant circulation can elevate the temperature of the fuel in the tank as well. According to testing by Kenne Bell, the fuel temperature can increase by 30 degrees after just 30 minutes.
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 not only the size and design of the pump itself, but also by two very important external factors: supply voltage and system pressure. Know that in a return-style 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 varies with the engine speed and load. At idle, the fuel demand is much lower than when running at 6,000 rpm at WOT. The fuel pump must be sized to support the greatest fuel demand, but not be oversized to tax the return system. 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 in that it deletes the regulator and return part of the system. With this, the fuel is controlled in two ways: one is by the supply voltage to the fuel pump. Like any electric fuel pump, the output of these pulse-width-modulated fuel pumps is dictated by the supply voltage. The fuel pumps used in a return-style fuel system are usually supplied 13 volts, depending on the effectiveness of the charging system and accessories drawing on the electrical system. 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 don't 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. The beauty is that the pump is not constantly supplying maximum fuel flow, only what is needed. The late-model '11-up fuel system employs an internal bypass (or regulator) in the tank to dump fuel past 55 psi. The downside to this system is that pressure can never exceed 55 psi, and will be lower at the injector due to the pressure loss inherent in the lines and fittings.
Fuel Pumps & Boost-A-Pump
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. The size of the fuel pump is somewhat determined by the available space in the pump assembly. This is especially true of in-tank pumps. Inline or external pumps obviously have more space for increased size.
Let's take a look 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 increases with voltage and decreases with system pressure. Simply stated, more voltage spins the pumps faster, and it's harder for the pump to flow against higher pressure than lower pressure. Need verification? Check out the supplied chart on flow rate versus system pressure and voltage.
As mentioned previously, fuel-injected engines typically run fuel pressure ranging from 38-60 psi. This all changed once we discovered how easy it was to install forced induction. Using an early-return-style fuel system, additional fuel was supplied to the boosted motors by 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. Greater fuel to the injectors increased fuel to the engine, hopefully in proper proportion to the airflow supplied under boost.