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.