Radium Fuel Pressure Regulator Kit | 2008-2021 Subaru STI (20-0965)

The GR/GV STI's quasi dead-end fuel routing is behind more stumble complaints than most owners realize.

From the factory, the 2008-2021 STI circulates fuel through the regulator and only sends it to the engine on demand, a layout Subaru tuners have long worked around. The Radium Engineering Fuel Pressure Regulator Kit replaces the OEM regulator and damper assembly and re-routes fuel to cycle through the fuel rails before reaching the regulator and returning to the tank, the routing tuners prefer for chasing out stumble issues. The RA-series regulator at its core brings tool-less pressure adjustment with 1 psi detents and a 360-degree swiveling vacuum port. It installs in the factory regulator location, no relocation bracketry needed.

What You'll Feel

• Smoother, more consistent running as constant flow through the rails replaces the dead-end routing
• Base pressure changes by hand in seconds, with haptic detents instead of a jam nut and set screw
• A more stable vacuum signal to the regulator, re-referenced from the plenum instead of the #4 runner
• A cleaner engine bay with the regulator sitting exactly where the factory unit lived
• Easy pressure checks if you add the optional gauge to the spare port

Features

• Replaces the OEM fuel pressure regulator and damper assembly on the USDM EJ257 STI
• Re-routes fuel through the rails before the regulator, the preferred routing for addressing stumbling
• RA-series regulator with tool-less knob adjustment; each detent changes pressure roughly 1 psi (+/- 0.5 psi)
• Single stainless steel return orifice generates laminar flow for enhanced pressure stability
• 360-degree swiveling, double O-ring sealed vacuum reference port
• Unused 8AN ORB port accepts the optional Radium 20-0386 pressure gauge kit, sold separately

The Details

The factory system's weakness is that fuel loiters at the regulator instead of continuously sweeping through the rails, which invites pressure inconsistency at the injectors. This kit reverses that: feed enters the rails, flows through them, then meets the regulator before returning to the tank, so the injectors always sit in moving, pressure-controlled fuel. The included vacuum hose and fittings relocate the regulator's reference signal from the #4 cylinder runner to the intake manifold plenum, trading a pulsing single-runner signal for a stable average that the diaphragm can actually track. Adjustment is where the RA-series stands apart: turn the knob and feel each detent click off roughly 1 psi, no wrenches, no jam nut, no set screw. Installation happens in the stock regulator location with the OEM lines, and if you want eyes on your base pressure, the spare 8AN ORB port takes Radium's 90-degree gauge kit. This kit is specific to the USDM EJ257 STI, so 2015-2021 WRX FA20DIT owners need to look elsewhere.

Kit Includes

• 1x RA-series direct mount fuel pressure regulator (black)
• Mounting and adapter hardware for the OEM regulator location
• Vacuum hose and fittings to re-route the reference line to the intake manifold plenum

Applications

• 2008-2021 Subaru WRX STI (USDM, EJ257 engine only)
• Not compatible with 2015-2021 Subaru WRX (FA20DIT)