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Discussion Starter · #1 · (Edited)
Hi everybody! Glad to have you here! Today we are talking about intake pressure release valves.

BOV - Blowoff Valve
BPV - Bypass Valve
RBV - Recirculating Bypass Valve (same as bpv)
VTA - Vent to Atmosphere
DV - Diverter Valve or Dump Valve (euro terminology for BPV / BOV)
CHRA - Center Housing Rotating Assembly
MAF / MAS - Mass Airflow Sensor
MAP - Manifold Absolute Pressure
B3wbs - Boobs

What are BOVs and RBVs for??
Turbos build intake pressure using heated exhaust gasses to spin a turbine wheel which is connected via a shaft to the compressor wheel. The compressor wheel compresses the intake charge which increases the amount of air that is fed into the cylinders.

Upon a shift or sudden throttle decrease (wide open throttle to no throttle) several thing happen. There is now no load on the engine, less air is flowing through the engine because the RPMs are decreasing and the throttle plate is closed, and there is less exhaust gas on the turbine wheel.

What now happens is almost like water hammer. If you've ever been squirting a hose with a nozzle, then let off the handle rapidly to close the nozzle, you've seen the hose jump a bit. This is called water hammer. You are flowing a ton of air through a turbo (hundreds of cubic feet a minute) and then all of a sudden you decrease the flow rate of the air because you have closed your throttle plate. This causes a sudden spike in pressure in the postcharger intake system (everything between the turbo and the throttle plate) and creates cavitation of air across the blades of the compressor wheel. This causes a phenomenon known as "compressor surge".

Is compressor surge dangerous?
Well... Kind of. Old turbo applications did not use BOVs / RBVs. In "Maximum Boost" author Corky Bell describes how valves were not used in early turbo systems. On low boost applications the pressure spike isn't as big of a deal. On today's turbos (even stock turbos) the boost and CFM is higher than they used to be. The cavitation of compressor surge can cause the bearings in the CHRA to fail which will cause a massive turbo failure and possibly engine failure.

A BOV or a RBV releases the pressure at the point where compressor surge would exist on sudden decreases in engine load (ie shifts)

Wow Kevin... You're talking about all this stupid fluid dynamics crap that I don't care about... Can I get a noise maker for my turbo or not?
Hold on buddy and I'll get there!

How does a BOV / RBV work?
Inside the valve body there is a diaphragm which is connected to the valve piston. On the top side there is a vacuum signal which delivers pressure from the intake manifold after the throttle body. The underside of the piston is in contact with the postcharger air pressure. During throttle, the pressure on the inside of the manifold and the postcharger piping is the same. This keeps the valve closed. When the throttle plate closes, a vacuum is created in the intake manifold and the top side of the diaphragm doesn't have enough pressure to keep the piston closed. The piston opens now and bleeds air off, hopefully preventing compressor surge.

In new model turbocharged gasoline cars that come from a manufacturer, a recirculating bypass valve is used which plumbs the air back into the intake tract pre turbo and post MAF. This is because the MAF sensor has already accounted for the air blowing out of the intake tract.

If the air is not plumbed back post MAF and instead is VTA (vented to atmosphere) like on a standard BOV, then a rich condition is created during the shift.

Is that rich condition a problem Kevin?
Yes... Well no... Well... it depends on your opinion. If it's a rich enough condition, then you can stall out... rarely happens on a WRX (unless at idle.. but that's a different cause). The rich condition can cause you to foul your spark plugs and/or your catalytic converter. You can end up "throwing a fireball" if you don't have a catalytic converter in place. That's illegal, dangerous, and can cause a wildland fire. There is also the theory of what is called "cylinder wash". A thin layer of oil is on the cylinder walls. The theory is that excess fuel will degrade the layering of that oil on the cylinder wall by washing it away. In theory this will wear your piston rings and cylinder walls prematurely, and increase the engine operating temperature. I haven't seen factual data to support or debunk the theory.

So enough with the B.S. Kevin... I want a BOV.
Should I get a BOV??
Most of the time, the answer is no. Your stock RBV is sufficient for a decent amount of boost. The 02-07 WRXs / STis had a metal valve which could hold well over 20psi (the peak numbers vary from valve to valve). The 08+ design went to a plastic valve that can fail earlier under less boost. I have no experience first hand with this valve, and can't tell you when to replace it.

If you want or need a valve, the safest bet is an upgraded RBV. They'll function similarly to your stock valve, and won't cause a rich condition.
The second safest bet is a 50/50 valve. They supply some air back to the intake tract so that the rich condition isn't as severe, but still allow some air to go to the atmosphere to get the "PSHHHhh" sound that you want.
The least safe bet is a VTA BOV. No air comes back to the intake tract, and the rich condition problems will be apparent.

What other problems are there with a BOV?
A lot of times they're not set up with the correct spring, or spring tension. The car can hesitate or die while at idle because the spring is too stiff to allow the RBV to open at idle, or you can still get compressor surge because the BOV's spring is too strong and won't allow the pressure to escape. The spring could also be too weak, which would allow boost to be "leaked" out of the valve.

I want "PSHHHH" what should I do?
Buy an aftermarket short ram intake, or cold air intake. These delete the resonator, and give you a surprisingly loud "woosh" sound between shifts because you can hear the air rushing back through the exhaust side of the RBV.

I still want a valve... What should I get??
I have opinions just like everyone else. I'm not going to say one valve is better than another, but the ones I've had good experiences with are APS, TIAL, and Cobb's XLE. The ones I don't care for are the HKS SSQV, and anything TurboXS makes (I mean ANYTHING)

Can I use a Duck Call on a BOV?
Yes. See here:

Can I pay you for your time typing this up?
Yes. I'd love it. PM me with your bank account number.

In Æternum
16,316 Posts
In for Kevin's steamy BOV talk :D
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