The fuel pressure regulator, fuel rails and fuel injectors make up a system that delivers the correct amount of fuel to each cylinder for proper combustion. The pressure in the fuel rail and intake manifold should be maintained at a level to suit the optimum air/fuel ratio. Your Mustang, as well as most fuel injected cars, has a one-to-one ratio fuel regulator. It varies the fuel pressure by being connected to manifold pressure via a vacuum connection. It also utilizes an internal diaphragm to control the pressure.
At idle, the engine is in a manifold vacuum situation and the fuel pressure is around -9psi to manifold pressure ratio. At full throttle, the engine is in a no vacuum situation, in theory it would be +0psi to standard manifold pressure ratio. If you add more positive manifold pressure to your system with turbo or supercharger, the added boost pressure will further increase fuel pressure. In factory regulators, for every pound of boost, it adds a pound of fuel psi since it has a one-to-one ratio. Most aftermarket adjustable regulators are also one-to-one or close to that, however you have the ability to adjust the pressure at idle or full throttle for fine tuning.
The installation of an aftermarket fuel pressure regulator allows you to adjust the fuel pressure to suit larger aftermarket injectors and other engine modifications—most commonly, forced induction. They are also necessary to regulate the flow of increased volumes of fuel pumped by high flow aftermarket fuel pumps.
There is one other type of regulator that is used with aftermarket forced induction. These are rising rate regulators that commonly go by the term FMU (Fuel Management Unit). This style of regulator increases fuel pressure at a greater than one-to-one ratio. So, instead of adjusting fuel enrichment with complicated computers and injection duty cycles, these systems just increase fuel pressure to add fuel. They go inline, down from the factory regulators, and only start to add pressure under boost. So when you are off the boost, you maintain factory tuning and drive-ability. Only as you get into a boost situation does the FMU begin to increase fuel pressure.
Besides upgrading to an aftermarket fuel pressure regulator, you may also need to look at your fuel rails. Many stock rails can have small passages or restrictive bends in the tubing that do not support proper flow of fuel to your injectors. This is especially true when you are requiring more fuel than your stock fuel system was originally designed to deliver. One particular area of concern is the fuel line that connects one fuel rail from one side of the motor to the fuel rail on the other side of the motor. Most of the time you will find an aftermarket fuel rail kit that includes both rails and connecting line and other various fittings to connect it to your fuel lines.
The majority of aftermarket fuel rail manufactures have also designed their fuel rails to look better than a stock OEM part. Since fuel rails sit on top of your motor they have a high visibility factor and many companies make them out of color anodized or polished billet aluminum to improve your car’s under-hood look.
Newer cars use a return-less style fuel system where there is no fuel pressure regulator, only a fuel pressure sensor. In these systems, the cars computer takes readings from the fuel pressure sensor and then adjusts the voltage to the fuel pump to cause it to pump more or less fuel to the motor as necessary to maintain the proper air/fuel ratio.
Proper fuel delivery is vitally important in all stages of engine operation. A car will not start without proper fuel pressure and most certainly will not perform correctly under acceleration without adequate fuel delivery.