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You just got your new Subaru, and you are dying to let everyone know that you have a turbo under the hood. First thing that comes to mind? Let me get a BOV that'll show em! I'm here to explain to you the Dos and Donts of swapping out your factory recirculation valve (BPV).

First, you need to know a bit about what a bpv/bov is, how it works, and why you have one. The entire purpose of this valve is to eliminate compressor surge when you let off the throttle and your turbo is still spinning. Without it (or with one set so stiff it wont open) you'll backspin the turbo. This is the compressor surge noise that people tend to think is so cool. In reality, its like hitting your turbo's compressor wheel with a hammer. Under boost the valve is held shut by boost pressure itself, that's why it has a reference line on the top of it. Once you let off, the combination of vacuum on the reference port and pressure on the bottom of the valve forces the valve open and vents the excess air charge.

BOV - Blow Off valve, this is an atmospheric valve generally. Some can be reconfigured to recirculate with extra parts or adapters.
BPV - Bypass Valve, this valve recirculates back to the pre turbo intake tract.
50/50 Valve - This type of valve is used as a compromise in metered air system setups, the 50/50 works better because it leaks LESS air, and they are less prone to problems since they recirc at the bottom of their stroke so when they do float open its not a vacuum leak. At the top of their stroke they vent to atmosphere, providing the noise everyone enjoys.

Many valves have an adjustable preload by spinning the housing, or a set screw adjustment that compresses the spring, and they pretty much all have different spring rates available. The primary function of changing the spring rate or preload has almost NOTHING to do with how much boost the valve will hold, but mostly to do with keeping the valve shut under idle and vacuum.

Now lets get subaru specific. Your car has a Mass Air Flow sensor (MAF). This meters the air as it comes through your intake. Once its in the system, the only place it should go is in the engine. Similarly, the only way air should enter the system is through the MAF. You can see where I'm going with this can't you? No matter what anyone tells you, venting out metered air is wrong. At best, it creates a rich condition on shifts. At worst, it can cause idle dips, stalling, stumbles, lean conditions, rich conditions, death. Ok not death, but you get the idea.

If your Subaru came with a metal BPV, you are in luck! You don't need to upgrade this for most configurations. If it has a plastic one, well, that's unfortunate. They are prone to leaking, which overspins your turbo and lowers performance. You should upgrade to a QUALITY aftermarket valve like the Turbosmart or a properly sprung Forge. Very high boost / high airflow setups have a possibility of leaking even on the stock metal valve but by then you most likely will have gone to an SD setup and will be able to use a quality Tial or Turbosmart solution.

The only time its appropriate to have a VTA style valve on your Subaru, is when you have converted to a Speed Density style tuning system, which no longer uses the MAF sensor.

Wicked M@