If you have an aftermarket fuel pump or are interested in proper tuning, you need an adjustable fuel pressure regulator. Until now that meant having to adapt and cobble up some fittings that look bad and are complicated to install, may require some welding or epoxy (are you going to trust a fire to some epoxy?). Most aftermarket pumps will outflow the stock regulator, this overrun condition will cause idle fuel pressure to run high and effect the tune up and performance of your car. It can also effect mileage and drivability. Fits with the perrin evo fuel rail. Fits Lancer Evolution 8.