The Subaru Impreza WRX/STI has unique blow off valve requirements particularly when fitted with a large turbocharger. A blow off valve is a device that releases pressure from the turbocharger when the throttle is released.

APS designed the APS high volume twin vent blow off valve specifically for the Subaru Impreza WRX/STI. It is an ideal enhancement for applications using the standard turbocharger up to large 600 hp units.

With its huge capacity the APS blow off valve gives you the ultimate in turbocharger durability and improved turbo response particularly during gear changes. A mammoth 1.417" (36 mm) internal bore size delivers huge capacity and at full piston travel, all ports are fully exposed for maximum blow off volume.

The APS twin vent BOV features 30 micron hard anodized, T6 billet aluminium body, low friction bronze piston, all stainless steel hardware including spring and Viton 'O' rings.

The large primary vent is designed to plumb back to the standard entry point which ensures the best driveability during cruise conditions and cures the erratic idle and engine's rich condition (on gear changes) problems seen with after market external single venting blow off valves.

And for large turbochargers running high boost pressure levels, the APS twin vent blow off valve continues to vent through to the second stage port.

Fine adjustment to blow off valve operation can be performed through the actuation adjustment screw for optimum configuration.
 

1. Ports Closed - Under open throttle conditions where turbocharger boost pressure is required, the bronze BOV piston is held shut in order to supply the maximum amount of charge air to the engine
2. Primary Port Open - Under light load conditions where the throttle opening has been reduced or shut, the piston moves partially up the bore travel and opens the primary port to vent charge air back into the inlet tract. This ensures smooth engine operation and alleviates the rich condition and subsequent backfire when utilizing a BOV construction that vents100% of charge air to atmosphere.
3. Both Ports Open - Under high load conditions where the throttle has been shut, the piston travels fully so that both the primary and the wide mouth secondary ports open to vent the maximum charge air possible. Air vented through the primary port is routed back into the inlet tract. In addition, additional excess charge is air vented through the wide mouth secondary port to atmosphere.
   
   

With a large turbocharger, releasing pressure from the turbine compressor wheel when the throttle is snapped shut is critical not only for turbocharger response, but for turbocharger longevity. A huge volume of air must be passed through the blow off valve as soon as the throttle is closed.

Most blow off valves have limitations in their design that inhibit the level of performance we demand. Piston/bore diameter and venting ports are often too small and surprisingly, most will not fully expose the exhaust ports when fully opened.

Ultimately, the need for a truly high performance blow off valve specifically designed for the WRX/STI needed to be satisfied.

The pictures below show various aspects of blow off valve design and compare particular features of the APS dual vent blow off valve with other respected blow off valves(s).