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post #1 of 15 Old 06-03-2015, 06:08 PM Thread Starter
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BOV for Beginners

I am writing this thread in because “Whats a good Blow Off Valve” Is becoming a more and more frequent question. There are threads about Blow off valves and why to avoid them already but they don’t contain the description that I think many new drivers need.

In this thread I will attempt to explain what a Blow Off Valve is vs a Bypass Valve, why one is better for your car than the other and how you can get the Fast and Furious PSSSHHHTTT sound when you shift without hurting your engine.

I hope this thread can be helpful to Subaru owners of all sorts. Please feel free to link to it and refer to it in the future. As stated in my previous “What To Do First” thread please feel free to comment and make suggestions to make this thread better.

** This article is for all vehicle with tunes that rely on the MAF, All stock of mostly stock Subarus use this setup. If you have a speed density tune for example you can disregard this whole document, you know who you are **

Definitions to start:
Blow Off Valve - aka BOV, Atmospheric Blow Off Valve, Dump Valve - A Pressure regulating device present in turbocharged cars used to vent intake manifold pressure to atmosphere.

Bypass Valve - aka BPV, Compressor Bypass Valve, Recirculating Valve - A pressure regulating device present in turbocharged cars used to route intake manifold pressure to the lower pressure side of the intake path ( ie. turbo inlet side )

The key difference between a Blow off Valve and a Bypass Valve is that the Blow off valve releases air from the system whereas a Bypass valve moves it from a high pressure zone to a low pressure zone within the intake path.

Why you need a Valve at all:
When the engine is running exhaust gasses are running through the turbine side of the turbo. Under load the wastegate is closed and the exhaust is hitting the turbine wheel making it spin at a very high rate of speed, that rotation is transferred through a short shaft to the compressor side of the turbo. The compressor side of the turbo takes air in through the intake path (low pressure side ) and compresses it as it sends it to your intercooler and eventually to your intake manifold ( high pressure side ). When you are driving with your foot on the throttle and the engine is at 4,500 RPM for example everything is great until you take your foot off the throttle, at that moment the throttle body valve closes and the compressor side will continue pushing air into the intake. The wastegate will open routing some exhaust gasses away from the turbine housing but the compressor wheel will continue spinning under the existing rotational kinetic energy. which will either force the intake valve open again or cause things like compressor surge, where the turbo creates more pressure than it can sustain and can cause bearing failure or in extreme cases total compressor wheel failure. To avoid compressor surge we use a valve to dump the extra pressure in the intake manifold and re-route the exhaust gases around the turbine through the wastegate allowing the compressor wheel to continue spinning under low pressure until the throttle body valve is opened again.

Why Bypass Valves are better for your engine:
In many modern factory turbocharged cars ( Subarus Included ) the Engine Control Unit ( ECU from here on ) makes active decisions about how much fuel to send through the injectors based on a number of factors like throttle position, engine RPM and , most importantly AMOUNT OF AIR. The ECU gathers the amount of air in the system by reading values from the Mass Airflow Sensor ( MAF from here on ). As soon as air enters the intake at the filter is accounted for and the ECU knows about it. During normal driving air is sucked into the compressor side of the turbo and spit out at a much higher pressure, the now high pressure air moves into your intercooler and finally down into the throttle body and into the intake manifold.

When you take your foot off the throttle everything changes. The high pressure air that’s in the intercooler slams into a wall because your throttle body valve is closed. Because your throttle body valve is closed the turbo is pushing against an immovable object. To relieve that pressure we add our valve to the high pressure side of the compressor, usually after the intercooler so It is as close as possible to the throttle body. The Bypass valve opens and the pressure is released. If you have a stock ByPass Valve or an aftermarket Bypass valve that high pressure air is dumped back into the low pressure side of the intake, after the MAF but before the turbo compressor intake where it is still accounted for by the ECU. If you have an aftermarket Atmospheric Blow Off Valve then the High pressure air is dumped to the outside world where leaving the ECU expecting air thats no longer in the system. Because the ECU is still expecting that extra air it continues sending fuel through the injectors leaving the system with not enough air and lots of extra fuel. That condition is known as a rich condition, It’s bad for your engine because it does not burn correctly, bad for your catalytic converters because they have extra fuel to burn and bad for your shift recovery because your engine won't be running at peak power when you get back on the throttle.

But that PSSHHHTTT sounds so cool:
I agree, there is a high school boy somewhere in me that loves the sound of an atmospheric blow off valve. I have spent a great deal of time working with film sound designers and I can assure you that sound is often copy/pasted onto any scene where there are import cars racing for the same reason there are gun cocking noises every time a police officer opens a door. It sounds cool to the general public.

I still think it sounds cool:
If you want more sound from your stock or aftermarket ByPass Valve I would start by smoothing out your intake path. The stock intake path has sounds reducing baffling that helps keep the car docile feeling for people interested in a reliable sports sedan. By replacing your stock rubber baffled intake with a smooth aluminium or rigid polymer intake you will inherently make the intake louder and when the stock ByPass Valve opens you will hear it dumping into the now smooth the intake . Short ram intakes are particularly loud because they have fewer bends to capture ByPass Valve release sound.

There is a middle ground:
If you are dead set on having a louder Bypass Valve there are companies that make “Hybrid” or “Adjustable” Bypass Valves. These valves vent some of the air to atmosphere and send some back into the intake path. When installing a valve that is hybrid you will still cause a rich condition but the severity of said condition will be controlled by how much you choose to vent them to atmosphere. More vent to atmosphere = more volume of air lost, louder sound and more rich condition.

For hybrid valves that dump a predetermined percentage to atmosphere there are ways to safely tune the ECU to handle the venting pressurized air. These tunes would need to be discussed with your tuner and they would require you to tune for each position of the valve. The safest way to tune for a hybrid valve would be to take it to your tuner and have them run a road tune where they can compensate for the conditions that the ECU is seeing. Making adjustments after the fact for any changes ( like turning that valve for more or less noise ) would be detrimental to the system. These tuning options exist for fully atmospheric blow off valves as well but for reasons discussed earlier we will assume you want to keep that atmosphere in the system to be reused.

TLDR - Blow Off Valves are bad for cars that rely on the Mass AirFlow Sensor to manage air fuel ratios. If you want more noise try an intake with an approrpriate tune or talk to a reliable Subaru tuner for more options.

- Jason
Slow is Smooth > Smooth is Fast

Last edited by arcticscythe; 06-22-2015 at 12:55 PM.
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post #2 of 15 Old 06-04-2015, 10:24 AM
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Good reading and nice info! Thanks for posting this. These are some of the reasons why I chose my GFB Mach 2 fully recirculating BPV. A write up on spring tension adjustment for BPV/BOV I think would be very helpful as well if there isn't already one out there.
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post #3 of 15 Old 06-18-2015, 06:43 PM
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Thanks for the write up! As someone who just purchased a WRX (my first turbo car, though I have done research online in the past) that has a greddy bov installed already from the previous owner, I was curious as to how much using one as opposed to a bpv can effect performance. I love the sound, but was considering switching over depending on the extent of the performance hindering. Would you say that it's only a relatively small difference that can be addressed a little further down the line, or would you consider it to be a big enough deal so that I should look into it being one of my first changes?

Thanks,
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post #4 of 15 Old 06-18-2015, 08:01 PM Thread Starter
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Personally I would look at the other modifications to the vehicle and adjust accordingly. In many cases drivers who choose a blow off valve will typically have a number of other common bolt on parts as well. I would check to be sure you have an appropriate tune for the existing hardware and if it is in your budget I would replace the blow off valve with a recirculating model. The performance gains are not so much in the way of power but that the ECU does not need to react to a rich condition every time you shift so you should build boost faster and run cleaner after each shift.

Depending on your model year and the current condition of the car there are other things I would handle first but I would put a stock BPV or an aftermarket BPV pretty high on the list. Just under "an appropriate tune"

Welcome to the forums, thanks for taking the time to read my post.

Don't forget to wave : )

- Jason
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post #5 of 15 Old 06-19-2015, 03:28 PM
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I didn't see this, nice job! Tell you what if you put some more cautionary language around the hybrid choice I'll sticky it! I've heard too many very smart people on here refer to hybyrid BOV's as "the worst of both worlds". At least atmospheric you can tune around if you're insistent on doing it. You can program the ECU to dump fuel in when it opens to some extent.

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post #6 of 15 Old 06-19-2015, 06:23 PM Thread Starter
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Quote:
Originally Posted by mosc View Post
I didn't see this, nice job! Tell you what if you put some more cautionary language around the hybrid choice I'll sticky it! I've heard too many very smart people on here refer to hybyrid BOV's as "the worst of both worlds". At least atmospheric you can tune around if you're insistent on doing it. You can program the ECU to dump fuel in when it opens to some extent.
- Thanks for the props, I added a bit of language around the hybrid valves I would still like drivers to avoid atmospheric blow off valves if possible. An interesting side note I did fine some evidence that haltech has some live adjustable tunes that work well with the GFB Deceptor line of electronically adjustable hybrid BOV/BPVs. They were able to adjust their tunes live for the percentage of atmospheric burst.

I think the best caution I can give people is to avoid atmospheric blow off valves all together and in the rare case where your tuner recommends one for your specific build then take their advice because you are paying them for their expertise.

- Jason
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post #7 of 15 Old 01-06-2016, 04:17 AM
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Very informative! Thank you!
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post #8 of 15 Old 11-16-2016, 06:10 PM
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Hi sorry I am a newbie and don't know much about cars, are you saying it's better the vehicle to get an after market by pass valve and then get an after market blow off valve, or just don't get the blow off valve. I like the bov noise but I rather have my car run safe.
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post #9 of 15 Old 11-16-2016, 08:57 PM
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https://www.maperformance.com/produc...sti-cob-712660
Is that what you were talking about? and do I have to get a tune after I install that?
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post #10 of 15 Old 11-23-2016, 02:06 PM Thread Starter
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That product is exactly what we are suggesting.

Keep in mind that there will be little to no performance increase in replacing your stock bypass valve with an aftermarket unit if you are at or near stock power levels. In face in many cases becaues drivers have the option to tune them they tend to over or under tune them causing negative side effects.

If you are interested in making more noise I would suggest a snorkus delete or an intake upgrade but keep in mind that any change to your intake or ehaust path should be done with a proper tune.

- Jason
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post #11 of 15 Old 11-30-2016, 05:52 PM
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I'll share this, and it is not to disagree with anything said above *it's all true*. This has just been my experience with a 50/50 "hybrid" BPV.

After a lot of pre-load on the spring and months of tinkering, I have a sequential 50/50 valve working... OK. It's sequential only by having the recirc orifice lower down than the atmospheric vent so under regular driving conditions it (usually) behaves as a recirc valve... but under higher boost it does vent a good portion into the atmosphere.

So the car does run well... but there are still the obvious, but fairly mild side-effects:
-Single but LOUD pop (like, gunshot loud) each shift when WOT. Personally I've grown to like it... but until I had the valve dialed in it was pretty much just embarrassing because it would happen on random, especially during lower gears (i.e. decelerating downtown lol).
-Since the spring is pretty damn stiff, at very low boost levels the valve does not open so there's a bit of a compressor surge (but this is VERY low boost).

What my car does not have since I was finally able to dial in that I had to deal with for months
-Stalling when pressing the clutch in
-Stalling when trying to reverse
-Uncontrollable popping ALL the time, in pretty much ALL driving conditions
-Hesitation
-Idle Issues (idle dips down when pressing clutch, so on).

Personally I'm happy with it now that it's properly dialed in. But that took a long while and it was very embarrassing until I got here.
I have not experienced any performance issues with the current setup because of the BOV. Boost comes on quick during shifts with my current setup (about as quick as my TD-04 did and I have a VF39 now).

I *do* like the sound... I also like the popping and banging/gurgling and all other noises that my car makes including the fireballs that come out of the exhaust... after all this is a wrx. Maybe it's a little immature (the wrx is the opposite of mature, but that's just me ) but I do not believe any of what I'm doing is causing damage to my motor or degrading my performance in any noticeable way.

BUT... and this is a VERY big but:

I understand what my car is doing. I do my own tuning and have plenty of hours dedicated to make the system work in that way and tweak/maintain the valve regularly to keep it operating as it should.

I would not recommend doing this to others because I had all sorts of issues when I first put it on not knowing what I was doing and what the side-effects actually did. I was not able to dial it in until I developed an understanding of tuning as well as the rest of my car's systems and how they work together.
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post #12 of 15 Old 02-11-2017, 04:40 PM
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Thanks so much for the info! I do have a TurboXS BOV but was curious on how they actually affect the air/fuel ratio. This is my first WRX and turbo car so this was very informative to. I'm going to look into an aftermarket BPV now I think. Glad I kept the old part lol! Might be switching it back out soon.
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post #13 of 15 Old 02-11-2017, 04:42 PM
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Here's a very useful video I ran across earlier on the topic:







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post #14 of 15 Old 03-07-2019, 06:47 PM
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Thank you

Great post, I was always curious on how these worked.
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post #15 of 15 Old 03-19-2019, 12:59 AM
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Very informative! Thank you!
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