Let’s start by identifying the Pressure Reference Port of a fuel pressure regulator. This is the small “air fitting” on the side of the regulator cover. Or with some carbureted regulators, it may be a small hole in the side or rear of the regulator cover. The Pressure Reference Port provides a point of reference for fuel pressure control. With forced induction systems (turbocharging and supercharging), this port is used to receive a “boost reference” to increase fuel pressure under boost conditions. The boost reference originates at the induction system of an engine, and acts upon the diaphragm in the regulator, thereby enabling the regulator to compensate for boost pressure. If the port is not used to maintain constant fuel pressure, such as with normally aspirated engines, then it is vented to the atmosphere. With the atmosphere as a reference, the fuel pressure is constant with atmospheric pressure. In this case it is important to not plug up the pressure reference ports or holes in the regulator, as slight pressure errors can occur.
For boosted EFI applications; the regulator needs to allow additional fuel pressure to overcome resistance created by boost pressure. For example, let’s say a boosted application runs 45 psi of fuel pressure to the injector when under no boost. Let’s say the engine is then put under 20 psi of boost pressure. That means the intake manifold, where the fuel injectors are mounted and spray into, is pressurized by 20 psi. This pressure then “pushes back” fuel in the injector/fuel rail by 20 psi. That means the fuel pressure in the fuel rail must be increased by 20 psi to compensate. So, the 20 psi of boost reference to the Pressure Reference Port causes the regulator to increase fuel pressure in the fuel rail/delivered to the injector from 45 psi to 65 psi – thereby compensating for the 20 psi of resistance created by boost pressure.
However, it should be noted various conditions can affect regulated pressure readings. While Bypass (Return) Style or Blocking (Traditional) Style regulators that use diaphragms are exactly 1:1, in some situations when measurements are taken, it appears less than 1:1. For example, some with Bypass Style regulators have measured their fuel pressure and boost pressure for a blown application and found it to be less than 1:1. For many of these users, they are comparing the operating conditions between idle and full throttle. During full throttle operation, much less fuel is returning back to the fuel tank than during idle operation. When the regulator is returning less fuel, the diaphragm is “more closed” and therefore in a slightly different position, such that the amount of squeeze on the diaphragm spring is less. With less force from the spring, the pressure lowers. The amount of pressure change as a result of how much flow difference is going through the valve is known as the Regulation Slope. Regulation Slope represents the amount of pressure difference (PSI) expected per change in flow rate (GPM). For example, say an engine consumes 1 GPM (60 GPH) at full throttle and the regulator has a Regulation Slope of 3 PSI/GPM, then we can expect that the fuel pressure will lower by 3 PSI at full throttle compared to when measured at idle.