This is an adjustable fuel pressure regulator (AFPR). There are many types of AFPR’s out there, but the most common type is similar to the one in the picture above. It is installed by removing the factory fuel pressure regulator on the return side of the fuel rail, bolting a fitting in its place, then a line is attached to the fitting and runs to the AFPR.
Though the AFPR is completely adjustable, generally you will set it to the factory fuel pressure value stock fuel pressures per car listed below). The main purpose of using an AFPR in place of the stock fuel pressure regulator is to ensure that the fuel pressure maintains the correct fuel pressure at all times. On any turbocharged DSM, you have a base fuel pressure. Lets say the base fuel pressure for our car is 40psi. Once you begin building boost, the fuel pressure is supposed to raise 1psi for every 1psi of boost (1:1 ratio). At 10psi, our fuel pressure should rise to 50psi. At 20psi of boost, our fuel pressure should rise to 60psi. Get the idea?
The stock fuel pressure do not do a very good job of maintaining that crucial 1:1 ratio, especially with an increase in fuel after installing bigger injectors and/or fuel pump at higher than stock boost levels. This can be very dangerous as it will cause lean conditions because the stock fuel pressure cannot keep up with the boost levels.
