KJ Jones Senior Technical Editor
December 1, 2007
Photos By: KJ Jones

Horse Sense: American automobiles have used various forms of fuel injection since the '50s. Since it was more of a specialized method of delivery back then and throughout the '60s and '70s, carburetors remained the predominant provider for gasoline engines. Ford ended the 5.0 Mustang's carbureted life in 1985. A year later, V-8-powered GTs sported a new sequential fuel-injection system that would eventually be recognized as the launch pad for nearly all the late-model Mustang performance we've experienced during the past 20 years.

As we've noted in past tech reports, Mustangs and other cars are comprised of many different systems that contribute to making them work. While some may argue that one system is more important than another, we think a consensus has been reached when it comes to assessing the fuel system's role in the mix, especially when taking performance beyond stock is the mission. As such, we're reviewing the ins and outs of fuel systems and featuring some of the new and old parts, theories, and ideas that contribute to improving this vital piece of a Mustang's operational makeup.

Not to belittle anyone reading this, but we think it's important to begin with a brief overview of what an EFI fuel system is. As a side note, since most Mustang street and racing engines run on gasoline of varying octanes, all references to "fuel" in this story are made with the understanding that gasoline is the fuel being discussed.

The fuel system stores and provides gasoline to a 'Stang's engine. It's made up of a few specialized components that work together to supply the engine with its required amounts of gasoline while the car is being driven or raced. These components are comprised of a tank, a pump, a filter, lines and rails, a pressure regulator, and injectors.

In a basic EFI system, at the turn of the key or flip of a switch/12-volt signal (for race cars), gas is pumped under high pressure from the tank through a filter and fuel lines and into a regulator where it's distributed through lines into the fuel rails and injectors. The fuel injectors are important in this equation, as they're the final gates that gasoline passes through before entering the chambers in the heads, setting off the big bang of combustion.

Most naturally aspirated gas engines burn approximately 0.5 pound of fuel per horsepower hour. This amount is also known as brake-specific fuel consumption. It's a good practice to plan and measure a fuel system's operation using this number.

The amount of fuel required and delivered to 5.0, 5.8, 4.6, and 5.4 engines in EFI street Mustangs is dictated by the Powertrain Control Module and a myriad of engine-control sensors. We've found these sensors can be manipulated by plug-in chips or tuning software to make calculations that are necessary for supporting a lot of horsepower.

There's a section in the PCM that stores all the fuel lookup tables. With a chip or tuner, the OEM's lookup tables are overwritten with higher or lower values based on fuel or timing needs in certain driving conditions. The same fuel-control theory is true for higher-horsepower 'Stangs, but they're better served by standalone EFI systems with stronger processors and wider ranges of adjustment, big fuel injectors, and tailor-made wiring harnesses.

A return-style fuel system is, for all intents and purposes, the preferred method of fuel delivery for medium- to high-horsepower street or race 'Stangs of any vintage. In a return-style system, the primary fuel line is run forward from the tank along either side of the car and split into two separate lines with a Y-block that's usually mounted along the front fenderwell. Individual lines from the Y-block feed gas to each fuel rail independently. The gas then travels from each rail into a bypass regulator, which returns it to the tank via a line running next to the primary feed.

With a return-style fuel system, an engine is guaranteed a continuous supply of constantly flowing, evenly pressurized, cool fuel. Ultimately, consistent fuel flow into the injectors and intake ports is one of the most critical elements of making horsepower on an individual-cylinder and overall basis. We believe return-style fuel delivery is the best method for achieving this kind of consistency with EFI or carburetors.

In direct-port or wet nitrous applications, a dedicated return-style fuel system is a recommended method of providing fuel enrichment for the nitrous system that's independent of the primary fuel system. This supplemental fuel system also consists of a tank, a fuel pump, a pressure regulator, and a feed line to the fuel solenoid. Using a dedicated fuel system offers a greater degree of tuneability and eliminates problems that may arise from having a big-horsepower nitrous unit share a fuel pump with the engine.

If you're planning to increase your Pony's ponies in the future, it's best to enhance the fuel system first. Read on for a few suggestions on how to do it the right way.

Where would this report be without the guest of honor? Fuel for 'Stangs comes in different grades and octanes. We're always referring to the unleaded 91-octane gas that we use for tech projects in our SoCal location as "the gas from hell." From a performance standpoint, Cali's fuel is limited as far as its ability to make horsepower without detonating or hurting an engine. With proper tuning, pushrod engines are good for a safe 500 horses, and Two-Valve modulars can achieve low 400s without hurting anything using 91-octane fuel. On the other hand, companies such as VP Racing Fuels offer higher-octane blends such as the ones shown in this photo. They're better suited for getting maximum performance from mildly and highly modified 'Stangs with nitrous, blowers, or turbos that see street and racetrack action. The two white-labeled cans contain 103- and 109-octane gas respectively. They're unleaded fuels that can be used for 'Stangs with catalytic converters and other smog equipment. The yellow-labeled cans are leaded race-specific grades.

Fuel Injectors
Injectors are small, electronically controlled valves that open and close at the PCM's command and spray atomized fuel into the engine. Most engines have one fuel injector per cylinder. Each one atomizes (turns liquid gasoline into a fine mist) and sprays fuel directly at the intake valves. By injecting fuel close to the valves, it stays atomized and burns more efficiently when ignited by the spark plugs.

The true virtue of sequential fuel injection is its ability to respond quickly to sudden changes in the way an engine operates. It only has to wait until the next intake valve opens instead of the next complete revolution of the engine.

Choosing proper-size injectors is always a big concern. If you have a ballpark idea of the amount of horsepower your 'Stang's engine makes, this calculation will ensure you're using injectors that are big enough to support it:

LB/HR = Injector Size x Number of Injectors
Horsepower at {{{100}}} percent Duty Cycle =
Brake-specific fuel consumption (0.5 naturally aspirated/0.6 turbo/0.65
supercharger/0.7 nitrous)

Injectors typically run at 80-percent duty cycle (0.8) and 43.5 psi of fuel pressure. Power at 100-percent duty cycle multiplied by 0.8 is the calculation that must be performed to determine horsepower levels for injectors when they're operating normally.

The amount of fuel supplied to the engine is determined by the size of the fuel injectors and their pulse width-the amount of time the injectors remain open, which is controlled by the PCM.

Notice the major nozzle difference between this 42-lb/hr injector (left) and a stock 19-lb/hr squirter. ACCEL has a cool fuel-injector calculator (PN 74142G) that lets you quickly determine what size you need. There are also several online calculators.

BBK's Universal 5.0 fuel rail kit is primarily for stock 'Stang fuel systems, but we think it's a great idea when you step up to 11/42-inch hose or AN -10 braided line (Tech Inspection, Nov. '06, p. 184). These aluminum pieces are designed to take the pain and suffering out of trying to fit larger fuel rails without contacting the distributor. Specially machined fittings included with BBK's kit allow fuel feed and return lines to be connected to stock-style, spring-lock fittings if they're still being used.

These Siemens VDO Deka IV series long injectors feature pencil-style nozzles that are angled to distribute a steady stream of fuel at the intake valves. We recommend 60-lb/hr units such as these for boosted or nitrous-injected applications that will make as much as 650 rwhp.

Fuel Tanks And Cells
A stock fuel tank needs no explanation. It's technically a container made of steel that stores the gasoline to feed a 'Stang's hungry engine. A stock fuel tank can be modified with a sump to suit higher-performance street and full-on racing applications.

A fuel cell is a replacement many 'Stangbangers choose to install when fuel system upgrades are in progress. They're used as safety equipment for off-road or racing applications, mainly because they're designed to be leak-proof in the event of a rollover accident. If they do leak, it will be slow thanks to bladders and foam inserts inside the cell. Cells are made of plastic or aluminum, reducing the possibility of sparks flying everywhere and causing a fire if it's punctured or dragged on the ground.

Additional fuel demands must be met when nitrous oxide is the power adder of choice. Failure to add compensatory supplemental fuel to a nitrous mixture usually brings on a lean condition that ultimately leads to cylinders full of molten aluminum. Auxiliary enrichment tanks are helpful in this situation. A 1-gallon tank can be easily mounted in a Mustang's engine bay with lines plumbed into the fuel side of a nitrous unit for better nitrous efficiency and performance.

As a rule of thumb for general safety, any fuel tank or cell should always be properly vented.

A sump tray ensures that there's always a solid flow of fuel available for the pump, especially during hard launches or other aggressive driving conditions. Fab-Tech Custom Fabrication and Welding of Chatsworth, California, makes this weld-in sump tray (for the do-it-yourselfers) and complete, sumped '99-'04 fuel tanks for converting returnless fuel systems to return-style. Fab-Tech's New Edge tanks retain the OEM fuel-pump basket and sending unit, and all the evaporative emissions-control connections and tubes.

This AlumaStealth fuel tank ('79-'93 Mustangs) by Rick's Hot Rods was detailed in our Jan. '07 Tech Inspection (p. 180). The 14-gallon tank is a direct-fit replacement for stock fuel tanks. It features Aeromotive's A1000 high-volume electric pump mounted inside and will support serious street horsepower while maintaining a stealthy appearance. Using a continuous-duty relay is imperative when wiring a standalone fuel pump similar to this setup. The relay should be located close to the battery and the pump.

Race Mustangs benefit from fuel cells similar to the one shown in this photo, mainly because they can hold a sufficient amount of fuel at a considerable weight savings. Primary fuel cells are usually mounted inside a 'Stang's trunk or hatch area, but Rick's Hot Rod Shop also makes a trick, pump-inclusive, supplemental tank for power adders that holds a higher-octane fuel and can be custom-made to fit a Fox, SN-95, New Edge, or S197 'Stang's engine compartment.

HP Performance offers this return-style fuel setup for all '87-'04 EFI Mustangs. The kit includes a sumped tank, a 1,000hp pump, a regulator, a 111/44-inch micron filter, a 131/44-inch micron fuel filter, fuel rails, pre-made lines, and all the necessary fittings and hardware. HP's fuel system is return-style with a rear-mounted regulator.

Fuel Lines
Fuel lines are probably the least thought-about components, yet they're among the most important pieces of a Mustang's fuel system.

There's no way a 'Stang's engine can run without fuel lines, because they shuttle gasoline between the fuel tank and the engine.

Most stock fuel lines are made of rubber or hard steel that's 31/48 or 51/48 inch in diameter. This size tubing is relatively small and doesn't support the fuel-volume needs of big-horsepower Mustangs, so upgrading to AN -10 (approximately 11/42-inch diameter) braided-steel hose is a common practice.

Setting up a Fox Mustang with 11/42-inch braided or high-pressure rubber fuel-delivery and return lines is recommended for power-adder-assisted setups. Braided hose is flexible, but it's also strong and can stand up to the vigors of extreme racing conditions and daily street use. Braided lines use AN fittings with bends of varying degrees. The fittings ensure positive, tight connections at the critical unions within a fuel system.

We spoke with Jonny Orme of Orme Brothers Hose and Fittings in Northridge, California, and asked for his insights on the approximate amount of braided line required for creating a typical return-style fuel system for a Fox Mustang. Here's the 42-foot breakdown for a 350-400hp street/strip fuel system:

AN -6/Return: 21 feetAN -8/Main Feed: 18 feetAN -10/Tank-to-Pump: 3 feet

This estimate may vary, depending on the level of custom the fuel system takes on or how far you deviate from the basic return-system layout. Keep in mind the shortest distance between two points is a straight line, so your 'Stang's fuel line should be laid out for maximum efficiency, not maximum flair. Intricate turns and routing of a fuel line looks nice, but it can become expensive as the AN fittings add up.

We've often been asked what the trick is for cutting braided hose. There isn't any magic in it at all. A chop saw or cutoff wheel will easily get through the thickest braided hose. The key to success is to wrap a small piece of masking or electrical tape around the end and cut in a slow, even fashion.

Don't take the term "chop" too literally when you're operating the saw. Smooth cuts will produce clean, non-frayed hoses. We also recommend a set of Koul Tools for assembling your fuel system's hoses and fittings.

Here's a look at the difference in the diameters of the three most popular braided hose sizes. From left to right are an AN -6 (return line), AN -8, and AN -10. The AN -8 line can be used as primary fuel feed to support as much as 500 rwhp, but its more common use is as a primary feed for 'Stangs that put out a maximum 350 to 450 horses. The AN -10 braided is mandatory for engines that produce 600 hp or better at the rear wheels. Fuel volume is much more critical for those applications.

Vapor lock occurs when part of the fuel system reaches a temperature above the fuel's boiling point while at idle, in stop-and-go driving, or after the engine is shut down. To avoid having this condition affect your 'Stang, make sure fuel lines are routed away from the exhaust or other heat-producing elements below the car or in the engine compartment, typically to the outermost side of frame connectors.

Have you ever seen engines with custom-colored AN fittings at the ends of braided lines and wondered how they came to be? This isn't a critical tech element, but it's interesting nonetheless. Dunking fittings in a plastic container of household oven cleaner will strip away the anodize finish and leave them ready for new, custom color-anodizing.

Fuel Pumps And Regulators
Fuel pumps send low- or high-pressure (EFI) gasoline feeds from the tank through lines and up to the engine. They maintain enough pressure to keep the fuel from boiling.

The fuel-injected engines in stock and street/strip 'Stangs are usually fed by in-tank electric pumps. 'Stangs with considerable amounts of power and torque use high-pressure electric fuel pumps such as Aeromotive's A1000 or Eliminator to get an engine's fire fueled. Big pumps such as these are efficient and capable of continuous-duty operation on the street.

An interesting note on fuel pressure that may come as a surprise to many of you; fuel pressure is actually at its highest when the engine is at idle and lowest when an engine is running at top speed. It's also worth noting that higher pressure results in reduced flow, so the more base pressure you run, the harder your pump has to work. Flow rate is critical for high-rpm and boosted applications, as high rpm reduces time to get fuel into the cylinders and high boost pressure actually pushes back against the fuel.

In order to maintain fuel-pressure consistency, a bypass regulator must be used to control the amount of fuel pressure in the lines. In boosted applications, regulators draw a vacuum signal from the intake manifold to increase fuel pressure as vacuum decreases and rpm increases during acceleration.

As you would expect, performance can be enhanced or hindered by fuel-pressure changes that are made through what seem to be simple turns on a regulator. We found this interesting and informative rundown of do's and don'ts for tuning EFI engines with fuel pressure on Aeromotive's Web site. We recommend you use them when dialing in the new fuel system in your 'Stang.

1) Install a fuel pressure gauge before removing the stock fuel pressure regu-lator. Leave it installed until the final adjustments are made to the new one.

2) Don't use liquid-filled fuel pressure gauges on any automotive tuning application. By design, they can't provide consistent readings as the gauge temperature changes.

3) Begin by setting your adjustable regulator to the same fuel pressure as the stock one. Remember: Always remove the vacuum/boost line when checking or adjusting the base fuel pressure. Don't forget to reconnect it before driving.

4) Initial changes in fuel pressure for performance tuning should always begin with adjustments toward a higher pressure than stock. This helps find where the engine wants to be regarding fuel while avoiding an engine-damaging lean condition in the beginning stages of tuning.

5) Make small, incremental changes and measure the results after each change. Tuning on a chassis dyno or at the track makes any affect on performance easy and safe to observe and evaluate.

6) Stop adjusting the pressure once noticeable improvements stop, especially if you're gaining horsepower by going to lower pressure (leaner) settings. For engine durability, it's recommended that the final fuel pressure setting be 1-2 psi above the pressure that produced the best lean power.

7) The objective of changing fuel pressure is to optimize the engine's air/fuel ratio for best wide-open-throttle (WOT) power. Tuning with the aid of a wide-band air/fuel meter is strongly advised.

8) Adjusting the base fuel pressure of most modern EFI engines will initially affect the WOT and idle/cruise air/fuel. The permanent effects are mainly to full-throttle air/fuel only. Make fuel-pressure changes based on the ratio at WOT, and ignore driveability/cruise air/fuel until WOT is correct and safe.

9) During low-load cruising, most PCMs run in closed loop, using the O2 sensors' feedback to constantly trim injector-pulse width toward the optimum, lean air/fuel ratio of 14.7:1. This is commonly referred to as "stoich" or "stoichiometric" and is the chemically correct balance of air and fuel for a complete burn. The computer learns what's necessary to maintain stoich and stores this information until the learned memory is cleared by disconnecting the computer from the battery for at least five minutes.

10) Air/fuel numbers can be confusing. The ratio is represented as the number of parts air per one part gasoline. The bigger the first number, the more air is in the engine per part of fuel (lean). The smaller the first number, the less air per part of fuel (rich).

11) The best WOT air/fuel ratios vary with the engine combination. Naturally aspirated engines make their best power between 12.0:1 (richest) and 13.2:1 (leanest). Forced-induction combinations like to be richer than their naturally aspirated counterparts. They still should never be as lean as 12.5:1, but they can be as rich as 11.0:1 for high-boost efforts on pump gas. On the dyno, always begin with a rich air/fuel mixture and gradually go leaner while closely monitoring for detonation.

12) Changing fuel pressure with a stock computer to solve driveability problems may yield only temporary results. Adjust fuel pressure to achieve a desirable WOT air/fuel, then leave it alone.

13) Once fuel pressure is set to produce an optimum air/fuel ratio, unplug the PCM or disconnect the battery so it will clear its memory. Reconnect power after five minutes and drive the car for several days, allowing the computer enough time to learn a new strategy for best driveability and performance in closed loop. If driveability problems exist after several days, consider a custom chip or reprogramming of the stock processor to regain good closed-loop performance. If the best fuel pressure for driveability is different than optimum pressure for full-throttle performance, establish WOT fuel pressure, but note the best pressure for driveability. The information can be programmed into a custom chip or tune.

14) Remember, as fuel pressure is increased, the amount of fuel flow available from the pump decreases. This applies to forced-induction combinations with an FMU. If fuel pressure must be raised excessively, be certain you have more fuel system than you think you need in order to ensure enough flow when the pressure is at its peak.

This is exactly the type of old-school-meets-new-school technology we really dig. This high-volume, billet-aluminum fuel pump and regulator from Race Pumps features a variable-displacement design, which adjusts output based on demand for fuel as opposed to pumping and bypassing excess. The Race Pump can support up to 2,000 hp, but unlike electric fuel pumps, this cam-driven deal is mounted in the same manner as a mechanical fuel pump: attached to the side of the engine block. We're using an EFI version of this setup on an upcoming engine project. We'll keep you posted on how it works.

We encountered this stainless steel, "pump on a stick," stock-style fuel pump holder at WFCX. Fox Body Fuel Injection makes this trick piece that features AN -6 fittings (AN -8 also available) for high-pressure fuel feed and return lines. The unit mounts inside the stock gas tank of '79-'97 Mustangs and can hold a Walbro 255-lph or similar fuel pump.

An easy-to-install, locking, hatch- or trunk-mounted aluminum fuel-tank access door is included and allows easy access to the pump without having to drop the tank. Company owner Kevin Konstant has also developed a 17-gallon bolt-in aluminum replacement tank for Fox 'Stangs that includes a high-volume, internal electric pump (Walbro 255-lph or Bosch 044), and tank-mounted AN -10 and AN -8 outlet ports. It's configured to allow ample clearance for tailpipes.

This 1,100hp-capable (with power adder), return-style fuel setup by Aeromotive is the landmark, all-inclusive fuel system for EFI 5.0 Mustangs ('83-'95), inspired by our own Editor Turner in 2000. The fuel system features a sumped fuel tank, A1000 fuel pump, bypass regulator, micron filters, 5.0 Street Rail kit, Y-block, and all the braided lines, hose ends, fittings, couplers, wiring, and hardware required for do-it-yourself, bolt-in installation.

The crew at Anderson Ford Motorsport has a couple of fuel pump solutions for augmenting your existing system or completely revamping it. The bolt-on solution is the AFM Big Pump, which installs between the factory fuel pumps and filter. The Big Pump (PN AF-6286 for '86-'93 Mustangs; PN AF-6294 for '94-'98 Mustangs; $288) is said to deliver 61 gph at 60 psi when supported by a 255-lph in-tank pump.

More than doubling that output is the AFM/Mallory '86-'93 5.0 Fuel Starter Kit (PN PK 160-60; $1,120), said to pump out 160 gph at 60 psi. This system includes the Mallory pumps, a regulator, fuel rails, a Y-block, and all the needed Russell ProClassic lines and fittings.