The most important feature of a recirculating bypass valve (Recirc Valve, BPV, BOV) on your Subaru is to evacuate the pressure out of the charge circuit when the throttle is closed. This prevents a surge of boost from hitting the throttle and then reflecting back toward the compressor wheel where it can slow or even stop the movement of the compressor wheel and corresponding turbine wheel. Ideally, the function of a bypass valve is to open this circuit and dump that pressure and drag on the turbocharger compressor wheel out to the unpressurized intake downstream of the Mass Airflow Sensor. This ensures that the vehicle still has the air accounted for to avoid afterfire and extreme rich running conditions on transition from one gear to the next. The faster the valve can open and the more the valve can flow, the better it is at evacuating that drag on the turbocharger. Unfortunately, with most diaphragm based designs, that comes at a trade off. You either have a valve that acts slowly (opening too slow resulting in some really interesting sounds as it opens) or a valve that can open quickly but has a small cross-sectional area of flow in order to allow it to act quickly but to hold boost when you want it to remain closed. Unfortunately the factory is a perfect example of the latter valve. It opens reasonably quickly, but the valve itself is the size of the head of a 16 penny nail, measuring roughly 5/16" across. That's right, it's just a hair over 1/4" in diameter.
The Nameless Performance FA20DIT Wave Spring Bypass Valve ensures that your charge circuit purges quickly, without restriction, and holds boost better than even the factory restrictive valve.
We have run these successfully with and without tuning, but the behavior of the valve is identical to factory, so the only need for a re-tune is if you've got the vehicle tuned with an already leaking valve and are replacing it with one that does not leak.
All prices are in USD