KJ Jones Senior Technical Editor
November 5, 2009
Photos By: Courtesy of Anderson Ford Motorsport

Horse Sense:
The electronic returnless fuel system found on Mustang GTs has been around since 1999. It's comprised of a high-pressure pump that delivers fuel directly to the fuel rail, which effectively eliminates two of the most important components of a power-adder Mustang's fuel system: the bypass fuel-pressure regulator and return line. While the move to this type of fuel system came about because of mandates made by the Environmental Protection Agency and the California Air Resources Board that required new vehicle manufacturers to meet ever more stringent levels of evaporative emissions control, its result dealt yet another blow to modern high-performance and left 'Stangbangers searching for a remedy. See More At 50mustangandsuperfords.Com

In a nutshell, returnless fuel systems were designed and implemented to offer improved fuel economy, lower fuel temperatures, and decrease vapor in the fuel tanks of today's fuel-injected vehicles. They're the OEM's best solution for meeting the new evaporative-emissions-control requirements. By avoiding flow through the fuel rail, heat transfer to the fuel is minimized, resulting in fewer fuel vapors in the tank that could enter and pollute the atmosphere.

In the static, returnless fuel system on '99-'04 'Stangs, there is considerably less fuel flow than that of the return-style (bypass) fuel system in earlier fuel-injected Mustangs. Without a return line, the only fuel flowing in the system is that which the engine can use at any given moment. A specialized, speed-controlled (by the ECU) turbine fuel pump is utilized to create and maintain a preset pressure level, somewhere between 30 and 70 psi. The command fuel pressure is determined by many factors, including engine load, manifold vacuum or boost, and even underhood temperature. A sensor mounted on the fuel rail monitors both fuel and manifold pressures, and updates the ECU on both.

By all accounts, Ford's returnless fuel system represents the leading edge of EEC technology. But this technology makes it more difficult to modify or enhance the fuel system to deliver the critical flow necessary to support significant horsepower gains. Time and time again, 'Stangbangers with New Edge cars are finding out the hard way that pushing the limits of the factory returnless fuel system (with 93-octane fuel and the big three power adders) is like playing with matches at a gas station.

From a performance standpoint, return-style fuel systems have their good points-voltage is constant to the fuel pump, adjustable fuel-pressure regulators can be used, additional inline pumps can be added easily-and their bad. We'll focus on the negative aspects. Heat can become a factor in the efficiency of a return-style system. When recycled fuel is heated by the engine, exhaust and road-surface temperatures, or in-tank fuel temps, and if its temperature gets too high, another hot-fuel-handling problem usually arises-fuel-pump cavitation and vapor builds in the fuel lines and fuel rails. As we know, bad things happen when engines don't have fuel. "Lean is very mean," according to Rick Anderson of Anderson Ford Motorsport. "Without a good supplement to any EFI Mustang's stock fuel supply, the road to meltdown is fairly short if you're trying to make horsepower with a blower, turbo, or nitrous."

With a returnless system, since cool fuel isn't fed to the fuel rail at all times, the standing fuel in the rail also can become superheated. So, additional pressure is required to prevent fuel from boiling and vapor locking the rail.

Aeromotive of Lenexa, Kansas, is one company that fully understands how important it is for engines with power adders to have fuel systems that can support major horsepower. The company's original Dynamic Fuel Systems for '87-'95 Mustangs and Digital FMU for '96-'04 Mustangs are great solutions for most cars. Basically, a mechanical FMU (fixed boost-to-fuel ratio) and Aeromotive's DFMU (variable, custom ratios) act as a regulators and fuel-pump controllers that increase fuel pressure when they sense boost, by managing the output of an additional fuel pump that was incorporated in the system. Until now, the DFMU has been an easy solution for owners of returnless-fuel systems who wished to maintain driveability under low-boost conditions and support 500 hp at wide-open-throttle in their street 'Stangs. The DFMU takes to a new level the process of using increased fuel pressure to compensate for a lack of injector size. When it senses boost, the unit controls an auxiliary, variable-voltage, inline pump and allows the creation of a custom fuel-pressure curve that eliminates the fat midrange/lean top-end problems of old.

Knowing that Mustang enthusiasts are always pushing the envelope for more power, Aeromotive engineers returned to the lab and developed a new, improved returnless-fuel supplement that's really impressive. Targeted for those applications where larger (55-160-lb/hr), typically low-impedance injectors are used, this system bridges the canyon between 600 and 1,000-plus horsepower for returnless fuel systems-on the street. Until now, adding twin SVT Focus fuel pumps and '03-'04 Cobra fuel tanks to returnless Mustangs has been an effective-if labor-intensive-solution for higher-horsepower GTs.

Now Aeromotive has an On-Demand Fuel System, which actually complements the stock fuel system-including the pump-by enabling the car to be driven in non-to-low-boost conditions on the stock fuel system alone. The system is comprised of Aeromotive's Eliminator fuel pump (activated by a manifold boost sensor or other method), a one-way check valve, and a regulator that delivers a constant fuel-pressure value under load, maintaining a 1:1 ratio with boost if necessary. As higher engine load and fuel demand are created by boost or nitrous, the Eliminator pump and boost-reference regulator contribute fuel volume and sustain system pressure. Of course, fuel injectors that can support the intended max horsepower level are required for the On-Demand system. If injector size is not sufficient enough to meet fuel-flow requirements, Aeromotive's DFMU must also be used.

"Any engine equipped with enough injector to support the target horsepower needs fuel volume sufficient to maintain the desired base pressure and a 1:1 rise with boost," according to Bret Clow, Aeromotive's director of Technical Service. "Our On-Demand Fuel System basically augments fuel flow by supplying vastly increased volume at a constant delta pressure. Delta pressure is the differential or difference between the fuel pressure in the fuel rail and the air pressure in the intake. Base or static pressure is considered the desired delta, and with a 1:1 boost reference, the fuel-rail pressure will always stay the correct number of psi above the manifold pressure, ensuring a constant fuel-flow rate through the injector any time it's open."

Anderson Ford Motorsport recently installed a prototype of the On-Demand Fuel System on a ProCharger-blown, '00 GT and gave it a workout on its dyno. Sure, the plan was to see how much horsepower and torque the system would support, but AFM also wanted to keep a close eye on air/fuel ratios (with and without the On-Demand kit in action) and to hopefully determine the overall effectiveness of Aeromotive's new setup, with respect to driveability.

Aeromotive's new, On-Demand Fuel System could be the key to boosting a 'Stang's all-important "cred" on the streets by enabling it to roll into the cruise spot with 900 hp under the hood, without requiring a chase vehicle to follow closely behind it. Sanctioned racing's cruise-type classes (Wild Street/True Street) could become a lot more interesting with this deal-a whole lot more.

Dyno Results
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AFM was on a mission to make 600 rear-wheel horsepower with Aeromotive's new On-Demand fuel supplement and using 93-octane pump gas. When the final numbers were tallied, data showed the new support product for returnless fuel systems was made of the right stuff. Equally impressive information, however, was found in air/fuel-ratio data that was captured with and without On-Demand.

For the first pull, without activating the Eliminator , the SVT pump ran out of steam and air/fuel numbers shot to dangerously lean levels (14.00 at 5,500 rpm) quickly. The richest non-Eliminator ratio was 11.79 at 4,261 during the initial run. The second test, with On-Demand in action, brought a much safer air/fuel value (12.44) at 4,261 rpm (AFM also saw an 8.68 gain in horsepower at this rpm). The mixture became progressively richer as the engine's speed increased (peak air/fuel ratio was 11.80 at 5,500 rpm) and the motor produced a much smoother overall power curve that ultimately peaked at 567.92 at 6,300 rpm.