From Muscle Mustangs and Fast Fords
It was time to take the truck to the next level, and the only way we could do that was with a larger fuel system. Unlike other '99-up modular-powered Ford vehicles, the Lightning utilizes a return-style fuel system. That means the rails are fed fuel, and the injectors use only what they need with the unused gasoline returning to the fuel tank. The fuel rails have constant pressure due to a fuel-pressure regulator, which restricts the fuel flow from returning back to the tank. The fuel rails are pressurized to 39 psi in stock trim, and-more importantly-most fuel injectors are rated at that pressure. One trick to get more gasoline to the injectors is to raise the fuel pressure. A 60-pound injector's pressure increases when the fuel-rail pressure is greater than 39. Usually under boost, the fuel pressure jumps 1 psi per 1 psi of manifold boost.