Richard Holdener
October 1, 2012

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.

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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.

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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.

The problem with this type of rising-rate fuel management system (FMU) is that it made life much harder on the fuel pump. Not only were we asking the fuel pump to support the additional power potential of the force-fed Ford, but also to do so at a greatly increased fuel pressure. Not surprisingly, many early attempts were met with less-than-stellar results. The cure was to increase the flow potential of the fuel pump.

Naturally, it was possible to install a better-flowing pump in place of the stock 88-lph 5.0L pump. The aftermarket soon supplied fuel pump upgrades ranging from 110 to 255 lph, but now in-tank pumps flow as much as 340 lph. It was also possible to install an inline fuel pump to work in conjunction with the stock (or upgraded) pump.

Aeromotive, Aerospace Components, Holley, Weldon, and a few others have designed high-flow inline (and in-tank) pumps to meet a variety of performance levels. The benefit of the inline pump is that it can be installed without dropping the gas tank, but there are other issues (like noise) associated with external pumps.

High-flow inline pumps help improve the fuel flow of the feed pump (stock or otherwise) by reducing the system pressure between the two pumps. Since the inline pump outflows many stock-type in-tank pumps (though many aftermarket pumps are now available to solve this), the pressure between the two pumps decreases. The reduced pressure helps increase the flow potential of the feed pump, thus improving the overall system. It is important to note that the limiting factor of the inline system will still be the pump with the lowest flow rate.

Another popular method, made famous by Kenne Bell, is to increase the supply voltage to the fuel pump. The now famous Boost-A-Pump improves the flow rate of an electric fuel pump by increasing the supply voltage. Most fuel pumps are (flow) rated at 12-13 volts. If you have a fuel pump rated at 255 lph at 13 volts, know that the flow rate will dramatically increase if the voltage supply is increased to 17 (or even 21) volts.

The supplied Flow Chart illustrates the increase in flow rate offered by the Kenne Bell Boost-A-Pump at various voltages and pressures. The Kenne Bell unit will always maintain a minimum of 13 volts, and can be dialed in to produce the desired (increased) voltage, and activated under boost or zero vacuum (wide open throttle). The benefit to desired activation of the Boost-A-Pump (especially on an early return-style fuel system) is that the pumps are not running full speed all the time.

Despite Internet rumors that the increased voltage will somehow diminish the life of the fuel pump, Kenne Bell has never had a single pump failure related to the proper installation of a Boost-A-Pump. Ease of installation, the ability to control onset and supplied voltage, and to increase the flow rate of nearly any electric fuel pump by 30-150 percent makes the Boost-A-Pump an attractive option, and all but mandatory on high-horsepower ('11-up) Mustangs.

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BSFC & Delta Pressure

While most enthusiasts understand the basic math that illustrates how much fuel is required to produce a given amount of power, things get complicated when talk turns to forced induction. Simply put, a supercharged engine producing 600 hp will require more fuel flow than a normally aspirated engine producing the same output. Furthermore, a supercharged application will be more taxing on the fuel pump. Why? The reasons are twofold, including a change in brake specific fuel consumption (BSFC), and a change in the (delta) fuel pressure (and attending drop in flow from the fuel pump).

The primary change in BSFC comes from a richer mixture. Turbo and blower motors require a rich mixture for cooling and to help suppress detonation. More fuel flow for a given amount of power equals a higher BSFC. Blower and turbo motors always require a drop-in ignition timing to ward off detonation, making power production safer, but slightly less efficient.

Another area of concern is the type of supercharger being used. According to Kenne Bell, a less efficient Roots blower will require more fuel than the twin-screw (or turbo) because of the increased parasitic loses associated with driving the blower. This is similar to to the rwhp difference on a chassis dyno between a manual and automatic transmission on the same engine.

On a blower or turbo engine, it is necessary to increase the fuel pressure at a 1:1 rate with boost. The reason is that the fuel flow out of the injectors and into the engine is a function of what is referred to as delta pressure across the injector. On a normally aspirated engine with a fuel pressure of 50 psi, the delta pressure is 50 psi, since there is no boost present. On a forced-induction application running 10 psi of boost and 50 psi of fuel pressure, the delta pressure will be 40 psi. Thus, only 40 psi of fuel pressure will be supplied to the engine since the 50 psi of fuel pressure must work against the 10 psi of boost.

If we supplied the system with a 1:1 regulator (or transducer on a return-less system) that increased fuel pressure in relation to boost, we would have 60 psi of fuel pressure working against 10 psi of boost pressure for a delta pressure of 50 psi. The problem is that the fuel pump must now work against 60 psi of fuel pressure to provide just 50 psi worth of fuel. The fuel pump flows measurably less (6-7 percent) at 60 psi than it does at 50 psi, so increasing the delta pressure has a positive effect on the fuel flow only if the fuel pump can keep up.

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Pressure Down & Injector Up

In terms of the pump, fuel flow and pressure are inversely related. Raising the fuel pressure increases the fuel flow through the injectors, but decreases the flow capacity of the fuel pump. The ideal situation for elevated power levels is to decrease the fuel pressure, but increase the fuel flow. How is this possible? The answer is to run larger injectors.

An example works well here. If you are trying to make 600 hp with 36-lb/hr injectors (flow rated at 43 psi), it will require considerably more fuel pressure than 43 psi. Math (injector size x 16) tells us that 36-lb/hr injectors can support roughly 575 hp at 43 psi with a 0.5 BSFC. Raising fuel pressure to 50 psi, 60 psi, or even 70 psi, will increase fuel flow through the injector, but remember that we must subtract the boost pressure from the fuel pressure to reach the true delta pressure to the motor.

The problem with this scenario is that the elevated pressure will increase fuel flow, but only if the fuel pump is able to support the additional flow at the desired pressure. A review of the fuel flow chart illustrates that dramatic drop in fuel flow with increased pressure. The cure is simply to increase the size of the fuel injector to 42-, 50-, or even 65-lb/hr and drop the fuel pressure to make life easier on the fuel pump. This is especially true of the system run on the '11-up Mustang, in which the internal bypass of the pump limits fuel pressure at the injector to around 55 psi. More pressure drop is evident a higher boost (and horsepower) levels.

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The injector flow rating can be increased (or decreased) with a change in the fuel pressure. Most injectors are rated at 43 psi. The maximum power level is obtained by dividing the flow rating by the BSFC number then multiplying by the number of cylinders. In the case of the 19-lb/hr injectors, the formula looks like this:

Max power = 19/0.5 BSFC x 8, or 19/0.5 = 38 x 8 cylinders = 304 hp

It is possible to increase the power output of a given flow rating by improving the BSFC number. If we use the 19-lb/hr injector and run a (more efficient) engine with a 0.4 BSFC. We get the following:

Max power = 19/0.4 BSFC x 8 = 47.5 x 8 = 380 hp

The reverse is also true, where the power potential of the injector is decreased with an increase in the BSFC number (from 0.5 to 0.6). This is typical of forced-induction applications that run reduced timing and richer air/fuel mixtures. Once again running the 19-pound injectors, we see the following:

Max power = 19/0.6 BSFC x 8 = 31.66 x 8 = 253 hp

System Flow & Pressure Loss

Fuel pump flow is all well and good, but the fuel flow from the pump must reach the injectors before it can supply the engine. Unfortunately, there are a number of obstacles along the way that can diminish fuel flow and make life harder on the pump.

The first might be a dirty screen located before the pump. This filter (or sock) keeps debris in the tank (like rust or other particles). Next on the list are the fuel lines and fittings associated with the pump assembly. The size of the line(s), number of fittings, and number of turns between the pump and exit of the pump assembly (hat), all combine to diminish fuel flow. We see the diminished fuel flow in the form of a drop in fuel pressure.

Next up are the actual fuel line(s) used to run fuel from the rear- (tank-)mounted pump up to the fuel rails. Once again, the length of line, fittings, filters, and bends all contribute to drop the fuel flow and pressure. The rails are the final hurdle for the fuel system. Testing has demonstrated the losses associated not just with the entire system, but with the individual components.

The Kenne Bell fuel flowbench is a state-of-the art flow system that allowed us to test the flow rate of the various fuel pumps at different pressures and voltages, and also individual components of the systems, including hats, fittings, lines, and rails. Testing the complete fuel system (pump, hat, lines, and rail) on the flowbench illustrated a drop in fuel pressure of 7.5 psi (registered from the outlet of the pump to the rail).

Obviously, this would decrease the maximum fuel flow potential of the system on an '11-up Mustang GT, but don't be too quick to run out and purchase a whole new fuel system for your Stang. Despite the drop in fuel pressure and the internal bypass valve of the stock pump limiting maximum system pressure, it is still possible to exceed 800 rwhp with the stock fuel pump, lines, and rail.

Running with a Kenne Bell 3.6L supercharger on an otherwise-stock Boss 302, Kenne Bell was able to eclipse 800 rwhp using nothing more than larger 80-lb/hr injectors and a Boost-A-Pump. If larger (12-14 ohm) injectors were available, even more power would be possible with the stock system, but 800 hp covers a minimum of 95 percent of the street Mustangs. If you are looking to take your new GT beyond this power level, look for multiple pumps and a complete (and expensive) revision of your fuel system.

Another area of concern is the wiring, or more accurately, the voltage supply to the fuel pump. We all take for granted that having the fuel pump running means it has plenty of voltage, but testing has shown that even turning on lights and other electrical accessories can diminish the voltage to your fuel pump. While your supercharged engine might love the cool night air, the fuel pump might not keep up due to reduced voltage.

The same goes for the actual wiring, especially when switching over to larger or multiple pumps. Wiring (and associated fuses) must be sufficient to carry the required current to the pump. Speaker wires aren't going to get the job done for that Aeromotive A1000 or dual (or triple) in-tank pump assembly. Testing on a supercharged 5.0L illustrated a drop in air/fuel ratio of over 0.5 points from simply turning on the lights and stereo.

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Upgrading Your System

Upgrading the fuel system on nearly any Mustang (from '86-present) is pretty straightforward. For early (return-style) applications, the installation of a 255-lph fuel pump will provide sufficient fuel flow for nearly 650-800 horsepower, assuming a system pressure of 40 psi, a supply voltage of at least 12 volts, and 0.5-brake-specific fuel consumption (BSFC).

Actually, testing has shown that most normally aspirated fuel-injected 5.0L and 4.6L applications exceed a 0.5 BSFC number and often dip down into the mid-0.4s. This means that the engine will require less fuel to produce a given amount of power. While your first thought might be that this entails a lean mixture, it is possible to increase the power output without affecting the air/fuel ratio, thus improving (decreasing) the BSFC number. Even a dedicated (normally aspirated) race motor could get by with a 255-lph, or certainly an Aeromotive Stealth in-tank pump (assuming proper wiring and fuse capacity).

Most normally aspirated engine can get by with 40 psi, but the same doesn't hold true for forced-induction applications unless overly large injectors are employed. Judging by the flow numbers in our pump test, combining a 255-lph in-tank pump with a Kenne Bell Boost-A-Pump should handle just about any 5.0L or 4.6L combination you are likely to throw at it, including high- horsepower, turbo, or blower engines.

Upgrading to a Stealth Pump from Aeromotive or TI pump from Walboro with the Boost-A-Pump will provide more fuel than you could even use. There are some minor exceptions to this rule, like the '11-up Four-Valve Coyote motor. Since the fuel system is self-regulated at just 55 psi (at the pump), a high-flow pump must be combined with large injectors if you plan to exceed 850 rwhp. The '03-'04 Cobra and Shelby GT500 both utilized dual pumps from the factory, so pump replacement requires a pair of pumps. Adding a Kenne Bell Boost-A-Pump to the stock pumps will take you beyond 800 hp with the right size injectors, making pump replacement unnecessary for all but dedicated race motors. Shelby currently employs the Boost-A-Pump on its Super Snake GT500 vehicles.

The Walbro GSS340 was the typical 255-plh pump employed on a variety of different Mustang and other applications. The Walbro TI F262 was a new in-tank pump designed for gas, while the TI F267 in-tank pump was the only one tested specifically designed for use with alcohol-based fuels. The Stealth from Aeromotive and Walbro TI pumps were high-flow in-tank pumps that easily out-performed the venerable 255, while the A1000 (offered in both inline and in-tank configurations) offered the most flow of any of the pumps tested. On some applications, the A1000 required the entire fuel system be upgraded with new plumbing, regulator, and wiring. Due to the extreme flow rates, the F262 and F267 TI pumps would require attention to the wiring and fuse components, as would other after-market/non-OEM pumps in dual (or triple) configurations.

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Common Injector Flow Rates vs. Power Output High-Impedance Injector Flow Ratings


Max HP

Injector(0.5 BSFC)(0.6 BSFC)
19 lb/hr304 hp253 hp
24 lb/hr384 hp320 hp
30 lb/hr480 hp400 hp
36 lb/hr576 hp480 hp
40 lb/hr640 hp533 hp
42 lb/hr672 hp560 hp
50 lb/hr800 hp667 hp
55 lb/hr880 hp733hp
65 lb/hr1,040 hp867 hp
75 lb/hr1,200 hp1,000 hp
85 lb/hr1,360 hp1,133 hp

Fuel Pump Flow Chart in Liters/Hour
Various Pressures and Voltages

VehicleVolts40 psi50 psi60 psi70 psi
5.0L & SN-951270666258
5.0L & SN-9513.588837874
5.0L & SN-9517.511210610094
5.0L & SN-9521140133125118
4.6L 2V Late12167158149 141
4.6L 2V Late13.5207195184174
4.6L 2V Late17.5301284268253
4.6L 2V Late21388366345325
4.6L 4V Late12224211199188
4.6L 4V Late13.5255241227214
4.6L 4V Late17.53743533314
4.6L 4V Late21464438413390
'03 4V Cobra12245216194172
'03 4V Cobra13.5265243223206
'03 4V Cobra17.5406386360342
'03 4V Cobra21524505483456
Lightning1211510810296
Lightning13.5143135127120
Lightning17.5203192181171
Lightning21254240226214
'07-'12 GT50012377335290246
'07-'12 GT50013.5469429387346
'07-'12 GT50017.5666633587560
'07-'12 GT50021797772732688
2011 GT12261238217193
2011 GT13.5318294268245
2011 GT17.5500463435409
2011 GT21609583549518
Wal GSS34012248227206193
Wal GSS34013.5276263257240
Wal GSS34017.5371 363351336
Wal GSS34021452446440433
Wal F26212352318281248
Wal F26213.5422388352318
Wal F26217.5571540520494
Wal F26221624594568524
Wal F26712384351320292
Wal F26713.5451421389 360
Wal F26717.5607579549517
Wal F26721707676641574
Aero Stealth12285264242223
Aero Stealth13.5348327304287
Aero Stealth17.5493472449431
Aero Stealth21562547531515
Aero A100012415395375355
Aero A100013.5495475450427
Aero A100017.5686660634609
Aero A100021829803777751

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