Guide to BPV's and BOV's

HolyCrapItsFast

Drinks beer!
In this write up I will attempt to explain the purpose of a Bypass Valve/Blow Off Valve (BPV/BOV) and what the differences are and how and when one should be used over the other. I will attempt to shed light on the pros and cons and dispel some misconceptions. We will also discuss some tuning options and provide a guide for selection.

Before I continue with this section I will say this statement first… It is through many years of findings and real world experience, by many members of the STi community, that you should not replace your BPV unless you have a definite need to do so. Because you want a cool sound is not a good reason. The induction system in a Subaru engine is precisely designed and making changes to this system with out clear purpose is counter intuitive. You absolutely get the best performance from the stock BPV on a stock car with a stock turbo. Other mods may require you to upgrade to an after market unit but you will need to choose wisely. Please read on...

1) What is a Bypass Valve (BPV) or Blow Off Valve (BOV)

A BPV/BOV is a valve that is placed in the air induction system of an engine and is specifically located after the outlet of the turbo and before the throttle plate and intake manifold. In the Subaru it is generally placed on the outlet end tank on the intercooler which is just before the throttle body.

A BPV/BOV is meant to relieve excess pressure after the turbo as the throttle plate is shut rapidly to prevent excessive compressor surge or stall. Here is a good explanation of compressor surge in section 1.1 of this paper http://www.gmrc.org/documents/GMRCSurgeGuideline_000.pdf. Compressor surge can be damaging to a turbo and significantly decrease its life. By relieving the pressure you reduce this effect significantly.

Some beneficial side effects of employing a BPV/BOV in the design of an engine is noticeably less compressor speed deltas, which means the turbo will remain spooled longer because you don’t have pre-conceived pressures backing up against the turbine blades. You also get smoother shifting.

2) What is the difference between a BPV and BOV.

Both valve types do exactly the same thing with one significant and important difference.

A BPV releases pressure and circulates it back to the turbo inlet after the MAF sensor. It is circulating air that has already been metered by the ECU. See Fig 1A.

A BOV releases pressure out to the atmosphere which is were the so called “cool” sound comes from. See Fig 1B.

A hybrid is a combination of the two. It circulates 50% of the pressure back to the intake and 50% of the pressure to atmosphere. This minimizes the effects of a BOV on a system designed for a BPV while maintaining the cool sound of a BOV. If you must have a BOV for the sound then this is the preferred method.

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3.0) BPV/BOV usage

As previously stated there should not be a reason to change out the stock BPV for most applications. However in some cases where high pressure and high volumes of air are being used, an aftermarket valve is in order.

The stock valve is a great valve. It is highly responsive and one of the most precise and well constructed BPV’s out. However it does have a limit. In high boost applications it can begin to fail and leak pressure at boost levels as low as 18-19psi. This is rare but it does happen. If you plan on running boost much beyond 20 psi then it is a good idea to change out your stock BPV for an aftermarket BPV/BOV. And No… The Ebay BPV/BOV’s are not adequate. Don’t skimp here because this is a device meant to save your turbo.

You may want to consider obtaining a valve with some kind of adjustment or replaceable spring. This will allow you to tailor the valve specifically to your application. Adjusting the BPV/BOV is a simple process.

You would start out at the lowest possible setting and drive around. You will want to drive using rapid throttle deltas and you want to shift allot. Take note and observe how smooth shift transitions are and watch for stalling during shift. One way to check for this is to go WOT in third gear and suddenly let off the gas and depress the clutch at the same time. If the engine reverts to a smooth idle quickly and does not stall you are good and probably require no more adjustment. If it stalls then you need to increase the spring pressure of the BPV/BOV.

If it does not stall but shifting is not smooth then you need to find a balance between good pressure release and shifting by increasing the spring pressure. You may begin to observe a slight degree of compressor surge but a small amount will not hurt the turbo. In fact some surge is desired on some applications were spooling the turbo takes time, such as in systems with front mount intercoolers and very large turbo applications, you want there to be some degree of pressure to remain in the system to minimize these effects.
 

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HolyCrapItsFast

Drinks beer!
Guide to BPV's and BOV's (continued)

3.1) When is it okay to use a BOV instead of a BPV

It is not If you maintain the Stock ECU and/or the MAF sensor in the stock location.

The car you are now driving was designed with a BPV. This means that the air that has passed the MAF sensor has already been metered by the ECU and has taken it into consideration. That air must remain in the system or you will experience a sudden momentary rich condition if it is suddenly vented to the atmosphere.

So what, you say?

For the most part you are right. Most daily driving never sees the BOV venting to atmosphere and the occasional rich condition is inconsequential. These rich conditions last mere fractions of a second. But who drive there STi like a Grandma. Consistent cycling of the BOV and creating artificial rich conditions can cause your spark plugs to foul, can clog your catalytic converter with soot and can cause significant backfires to occur.

Backfires... Cool!, you say?

No this is not cool. The backfires that occur as a result of a BOV are different from the occasional backfire you experience from an aftermarket exhaust. The backfire that occurs happens because unburned fuel has entered the exhaust stream and is ignited by the heat contained within. The resulting explosion causes a damaging pressure wave that traverses back to the turbo and slams against the turbine blades of the turbo. This causes significant stress on the thrust bearings inside the turbo. You are causing potential damage the one thing a BOV was meant to save.

Truly the only time a BOV is a better choice is when aftermarket ECU?s are used or the stock ECU is modified to that employ speed density method of fuel calculation versus the Mass Air Flow system of the stock ECU. In a speed density system the fuel is calculated based on input from the Manifold Absolute Pressure senor and not the Mass Air Flow sensor of a Mass Air system. It doesn't care how much air there is in the induction system.

The other scenario that sees a BOV as a better choice is in an induction system that moves the MAF sensor after the turbo in a Blow Through configuration. Typically in this configuration the BOV is placed after the turbo outlet down stream of the intercooler but before the MAF sensor. So the MAF sensor is the last device in the stream before the throttle body.

3.2) So then I can never use a BOV on a car with a stock ECU or a MAF sensor in the stock location?

Not necessarily true. Knowing when to use one is key.

First, the more stock the car is, the more you should not use a BOV. The flow characteristics of the induction system effect how a BOV can affect the rest of the system. For example, if you have a very high flowing system utilizing high boost pressures beyond what was originally designed you may actually need a BOV over a BPV even in a vehicle that employs Mass Air to calculate fuel.

With a BPV, when the high pressure builds in a high flowing system, the air that is circulated back to the intake is returning at such velocity and such volume that it will actually pressurize the intake and causes the flow of air to reverse through the MAF sensor. In this situation there is a lack of air flow or the reversing of air flow over the MAF sensor. You run the risk of going lean during shifting transitions and Tip-in will not be able to compensate for the sudden lean condition. This only happens under significant high flow/boost situations and the car will see normal driving most of the time. In this situation it is better to employ a hybrid BPV/BOV. This will minimize the circulating air and richen things back up a little bit. If you have a 35R or bigger turbo with a FMIC and your car is being built to flog constantly, then by all means go with a full BOV.

If using a BOV, you experience excessive rich condition during shift, you might be able to employ some tuning techniques to counter this. Though it is not possible to tune specifically for a BOV it is possible to minimize its effects. When a BOV releases pressure you are effectively creating an artificially induced Over-Run richening condition. Your ECU does this by default to keep the system appropriately enriched while shifting or during moments of turn-in during HPDE. By adding a BOV you are just making that enrichment greater.

Some parameters can be adjusted together to achieve a desired result. You need to look at Over-Run Enrichment, Throttle Enrichment and Tip-in Enrichment together along with load compensation tables. Exactly how to do this is up to you but the greatest impact will come from adjusting your Load Compensation (MAP) Tables. More importantly, increasing the spring rate of the BOV to activate just before the threshold of surge also has a great positive effect.

4) What types of BPV/BOV are there and what is the advantage to one versus the other.


Aside from the Valve configuration (BOV vs BPV vs 50/50) there are a number of different types of valves on the market today. Each one of them with their own advantages and disadvantages. Among the most common are...

- Push Type
- Pull Type
- Diaphragm Actuated
- Piston Actuated
- Sequential
- Synchronic

Many of these types come in combination with each other meaning you could have a Push or Pull type Piston or a Push or Pull type Diaphragm or a Pull type Sequential or a Pull type Synchronic. I will simply explain each type individually to keep things simple.

Push Type. In this configuration, pressure is pushing against the valve and inherently wants to open it. The spring pressure is what keeps it closed and it's operation is triggered by both the pressure on the valve and the vacuum assist to overcome the spring pressure and ultimately pushes the valve open, hence the name Push Type. This type of valve is less desired in high boost/flow applications but is ideal for stock level and moderately modified applications. With this type of valve you don't need a strong vacuum source to successfully operate the valve and its reaction time is very quick. It lends itself well to applications utilizing large cams and is useful in driving situations that see many large throttle deltas in a short amount of time such as in an autocross event. It is also least likely to cause compressor surge but you risk leaking boost under high boost pressure applications.

Pull Type. In this configuration, pressure is pushing against the valve and inherently wants to close it. It's operation is triggered by the vacuum assist combined with the spring pressure and ultimately pulls the valve open, hence the name Pull Type. This type of valve requires a stronger vacuum signal to operate and its reaction time is slower. This type of valve lends itself well to high boost and flow applications and is better suited for 1/4 mile and 0-60 where shifting is quick and concise. There is a higher propensity for compressor surge with this type of valve how ever the higher levels of residual pressure that remains inside the intercooler piping is desired when racing the quarter mile. This will decrease spool time because the piping does not have to fill with so much pressure during the onset of power. Some of it is already there. With a pull type valve you have a balance between pressure release and surge

Diaphragm Actuated. These can be seen in both push and pull type configurations. It is constructed using a shaft attached to a disk that seals against a o-ringed surface or a precision machined surface and is actuated by a sealed chamber diaphragm at the other end. This type of valve tends to be durable and will maintain a seal over a good deal of time. The vulnerability is in the diaphragm. This component is subject to ware.

Piston Actuated. This valve is generally only push type though pull type piston valves exist such as in the Synchronic valve by Synapse.. It is constructed using a hollowed out piston like actuator that, when closed, seals against an o-ringed surface while the body of the piston blocks the outlet port. A spring is applied to the opposite side of the piston that sits inside a sealed chamber. when pressure is present in the intercooler and enough vacuum is present to overcome the spring pressure, the valve is actuated. Over time the piston tends to ware due to lack of lubrication and the valve may bind and/or leak.

Sequential. In this configuration the construction is much like a pull type diaphragm valve with one exception. The disk that makes up the valve is a two stage device consisting of two concentric disks with one inside the other. It is essentially one valve with in the other and consists of a main primary valve and an initial low level valve. During normal and moderate performance driving, the initial valve activates first to relieve the low volumes of pressure. As your level of performance increases, the primary vale then opens to relieve the higher pressures generated. This is a great idea because it behaves much like a push type valve but with the advantages of a pull type. The valve will remain sealed under extreme pressure and the sequential design is less susceptible to surge. So you can have your cake and eat it to.

Synchronic. Synchronic valves are pull type multi tier piston actuated valves and are operated by both a vacuum and boost pressure signal. This valve is designed to leak (remain slightly open) during vacuum only conditions. This is meant to significantly improve throttle response by bypassing latency restrictions of the turbo and piping prior to the throttle body. Instead the air entering the engine enters through the valve and not the intake. If using this type of valve on a MAF car you must use it as a BPV and must route the valve to the intake. This way when the valve is open during vacuum only conditions, the air will still pass the MAF sensor and be metered by the ECU. If you simply must have the sound and want to vent to atmosphere, Synapse has developed a Synchronic valve that can be configured as a BOV with the use of an Anti Stall Adapter. Though adding this adapter kind of defeats one of the main purposed of a Synchronic valve. This type of valve is fully capable of eliminating any and all compressor surge by precisely manipulating the vacuum and pressure signals in combination with spring pressure and the multi tier piston. This is the ideal solution for any valve application and can be ideally set up in both BOV and BPV configurations. It is one of the most precise and responsive valve configurations out and is the only type of valve that can actually increase performance by significantly improving throttle response.

Synchronic BOV Configuration with anti stall on a WRX

Synchronic BOV explained

End: George
 
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icudruln

New member
Nice FAQ. I think it'd be a good idea to clarify what BPV's and BOV's on the market are, as well as what types of systems they use.
 

Batmobile_Engage

Squirrel Meat Aficionado.
Staff member
Great writeup, George. One thing that I would add though, is a section on "Blow-thru MAF configurations". I know that many people do not like this configuration for various reasons, but I have been running it for 4 years now with no ill effects.

I'm sure you're aware, but for everyone else, here's a brief description:

*Blow-Thru MAF Configuration*

The MAF is moved from it's position before the turbo, to after the turbo. With this setup, the MAF is located after the intercooler. This allows the BPV/BOV to be mounted before the MAF, allowing the valve to function in a full VTA (vent to atmosphere) configuration. This way, the valve is not dumping air that has already been measured by the MAF.

I hope this makes sense. I can expand on this subject if need be.
 

HolyCrapItsFast

Drinks beer!
I do mention it but perhaps I can reword it...

The other scenario that sees a BOV as a better choice is in an induction system that moves the MAF sensor after the turbo in a Blow Through configuration. Typically in this configuration the BOV is placed after the turbo outlet down stream of the intercooler but before the MAF sensor. So the MAF sensor is the last device in the stream before the throttle body.
 

Batmobile_Engage

Squirrel Meat Aficionado.
Staff member
:fuji: damn, how did I miss that?!

Oh well, shall I delete the posts I made in reference to Blow-Thru MAF configurations?
 
[MENTION=9]HolyCrapItsFast[/MENTION] so I'm trying to decide between the synapse synchronic BPV & BOV. It will be on a speed density tune so which would you recommend? Do you also recommend plumbing it back to the intake or just vent to atmosphere since I'm open to both options? Plumbing it back in will require me to get another hose and venting to atmosphere is just easier. I could care less about it being a noise maker and I just want what's best for the vehicle and your tuning.

PS sorry for the necrothreading.

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HolyCrapItsFast

Drinks beer!
I would do vent to atmosphere. Don't bother with the extra hardware.
 
I would love to change mine to vent to atmosphere. I love the sound but i fear I we would have to re-tune everything am I right?
 
I would love to change mine to vent to atmosphere. I love the sound but i fear I we would have to re-tune everything am I right?
If your running a MAF tune yes you will and if you are going to do a new tune with a vent to atmosphere you might as well go speed density so you don't see the rich dips when the blow off valve vents the MAF measured air to the atmosphere.

It's probably only worth doing VTA if your redoing a build but you can get a blow off valve and plumb back to the intake like the stock BPV to increase the sound with out a tune.

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If your running a MAF tune yes you will and if you are going to do a new tune with a vent to atmosphere you might as well go speed density so you don't see the rich dips when the blow off valve vents the MAF measured air to the atmosphere.

It's probably only worth doing VTA if your redoing a build but you can get a blow off valve and plumb back to the intake like the stock BPV to increase the sound with out a tune.

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I'm speed density



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