This kit is designed for use in the USDM 2008-2020 Subaru WRX STi (EJ257 only) and replaces the OEM fuel pressure regulator and damper assembly, shown below. The factory STi fuel line routing uses a quasi dead-end system that circulates fuel through the regulator, bypassing the engine. Fuel is only routed to the engine when there is demand.

The Radium FPR kit re-routes fuel to cycle through the fuel rails before reaching the regulator and returning to the tank. This is the preferred fuel line routing by Subaru tuners trying to address stumbling issues. The kit installs in the same location as the OEM fuel pressure regulator. Also included are vacuum hose and fittings to re-route the vacuum reference line from the #4 cylinder runner to the intake manifold plenum for a more stable vacuum signal. The optional pressure guage kit 20-0386 can be installed into the unused 8AN ORB port on the regulator.

The regulator features the first tool-less pressure adjustment. A jam nut and set screw are no longer required. With a simple turn of the knob, pressure will be increased or decreased.

To aid in adjusting pressure, there are incremental detents that provide haptic feedback to the user. Every detent permits 1psi (+/-0.5psi) of adjustment depending on current fuel pressure. A newly designed single stainless steel return orifice generates laminar flow that enhances stability control.

The regulator also features a 360 degree swiveling vacuum reference port that is double O-ring sealed. This is the first in the industry. It permits the installer to quickly point the vacuum fitting in any direction with no tools required.

There are 4 interchangeable 10-32 threaded vacuum port fittings provided to cover all scenarios.
1. Barbed Fitting: A black zinc-plated 3/16″ (5mm) barb machined from high strength steel that is compatible with the following vacuum hoses: 5/32″ (4mm), 3/16″ (5mm), and 7/32″ (6mm).
2. 3AN Male Fitting: A black zinc-plated adapter machined from high strength steel that is compatible with 3AN hose ends.
3. Push-To-Connect: A nickel-plated brass fitting with Buna-N O-rings that is compatible with hard plastic 1/4″ OD tubing.
4. Vacuum Plug: A black oxide stainless steel screw that is used to maintain a constant (non dynamic) fuel pressure. 

This FPR kit is suitable with all gasoline types including unleaded, leaded, oxygenated, and pre-mixes. Furthermore, it is compatible with alcohol blends of fuel including methanol, ethanol, and E85.