Welcome to the North American Subaru Impreza Owners Club Monday July 14, 2014
Home Forums WikiNASIOC Products Store Modifications Upgrade Garage
NASIOC
Here you can view your subscribed threads, work with private messages and edit your profile and preferences Home Registration is free! Visit the NASIOC Store NASIOC Rules Search Find other members Frequently Asked Questions Calendar Archive NASIOC Upgrade Garage Logout
Go Back   NASIOC > NASIOC General > Newbies & FAQs

Welcome to NASIOC - The world's largest online community for Subaru enthusiasts!
Welcome to the NASIOC.com Subaru forum.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, free of charge, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!

If you have any problems with the registration process or your account login, please contact us.
* Registered users of the site do not see these ads.
Reply
 
Thread Tools Display Modes
Old 04-13-2012, 10:41 PM   #1
the suicidal eggroll
Scooby Guru
 
Member#: 51961
Join Date: Jan 2004
Chapter/Region: RMIC
Location: Broomfield, CO
Vehicle:
2005 STi
2012 WRX

Default How Blow Off Valves Work

The third in the series:
http://forums.nasioc.com/forums/show....php?t=2339831


Blow off valves


Also see Unabomber's Manifesto on the topic:
http://forums.nasioc.com/forums/showthread.php?t=468038


For some reason a lot of people get confused between about wastegates and blow off valves. Let me start by saying that wastegates and blow off valves are not the same thing. They are not installed in the same place, they do not have the same function, they do not make the same sound, they do not make sound at the same time, and the operation of one is not affected in any way, shape, or form by the operation of the other. They are as different as your eyeball and your elbow. Now that that’s out of the way, let’s continue.

To understand what a blow off valve does, imagine you’re driving. You’re on the throttle hard, the turbo is spinning at 100,000 RPM and is sucking in air through the filter like a cracked out vacuum cleaner and pumping it out faster than a leaf blower on steroids. This air is flying into the intercooler at 100 mph, into the engine, and forcing itself into the cylinders. The engine is barely able to suck down all of this air, and the wastegate is bypassing exhaust around the turbo to keep it spinning at the right speed. Got the picture? Now you let off the gas, the throttle plate closes, the air flow into the engine drops to almost zero, what happens? Without a blow off valve, as far as the turbo is concerned it just ran full throttle into a brick wall. It takes a long time to spin down from 100,000 RPM, and the turbo has approximately 0.00001 seconds to do it before ramming into pressure spike so high it would make your head cave in. Well, it can’t, it rams into the pressure spike, air starts flowing backwards through the compressor wheel, the wheel all but stops in its tracks, and you get a loud pulsing/chugging sound as it does so. There are all kinds of arguments on both sides of the debate about whether this is actually bad for the turbo, I’m not going to get into that, but I will say that while it might not be bad for the turbo, it certainly isn’t good for the turbo.

Now introduce the blow off valve. This is a valve not unlike the wastegate or FPR, which is placed somewhere in the intercooler piping between the turbo and the throttle body. Its sole purpose is to stay closed until the pressure difference between the intercooler piping and the manifold exceeds some threshold, at which point it opens. Throttle plate open, the pressure in the intercooler piping before the throttle plate, and the pressure in the manifold after the throttle plate is more or less equal, so the blow off valve is closed. Throttle plate closed and boost is high, the pressure in the intercooler piping before the throttle plate is high, the pressure in the manifold after the throttle plate is very low, so the blow off valve opens. The blow off valve gives the turbo an outlet through which it can pump air while it slows down, since you closed the throttle plate and the engine can’t suck in any air anymore.

Let’s repeat the above scenario now that we have a blow off valve. You’re on the throttle hard, turbo is spinning at 100,000 RPM, pumping air into the engine. You let off the gas, throttle plate closes, manifold pressure drops, blow off valve opens. The turbo continues to pump air into the intercooler piping, through the blow off valve, until it has slowed down to the point where the intercooler piping pressure drops enough for the blow off valve to close (or until you hit the gas and open the throttle plate back up, giving the turbo an outlet again and closing the blow off valve). This gives the turbo time to slow down gently. While it does vent off all of the pressure in the intercooler piping between shifts, it also keeps the turbo spinning faster through the shift, so boost will come on quicker when you get back on the gas in the next gear.

Your car, in fact all turbo Subarus since god knows when, already has a blow off valve from the factory, and it does its job well. There are two styles when it comes to blow off valves: atmospheric, and recirculating (aka: bypass valves). Their names are pretty self-explanatory; at atmospheric blow off valve vents the air to atmosphere, while a recirculating blow off valve (bypass valve) plumbs this vented air back into the turbo inlet before the turbo. The OEM valve is the latter.

What is the difference between a BOV and a BPV? Other than where the vented air goes, nothing. They do the same thing in the same way. Where the vented air goes does matter though, a lot. If you understand how a MAF-based fueling system like ours works (if not, read the article on the fuel system, primarily the section on the ECU logic and how it uses the sensors to control fueling), then this should make sense. Our cars use a draw through MAF system. This means that the order of operations goes: filter -> MAF -> turbo -> intercooler -> engine. Notice that the MAF is located before the turbo, the turbo has to draw the air through the MAF sensor, hence the name “draw through MAF”. Similarly, a “blow through MAF” system goes: filter -> turbo -> intercooler -> MAF -> engine. I’m sure you can figure out why it’s named blow through.

Now let’s revisit our earlier example with the draw through MAF setup in mind. The turbo is spinning at 100,000 RPM, sucking air through the filter, through the MAF (where the ECU is measuring it to figure out how much fuel to add), and then blowing it through the intercooler and into the engine. You let off the gas, throttle plate closes, blow off valve opens, charge air from the turbo is diverted through the blow off valve rather than into the engine (since the throttle plate is closed and the engine is drawing in very little air). With a recirculating bypass valve, this air is routed back to the intake between the MAF sensor and the turbo. This creates a loop, the turbo sucks in say 100 grams per second of air, it compresses and blows out 100g/s of air, 2g/s enters the engine, the other 98g/s passes through the bypass valve back into the intake. There it’s joined by 2g/s being drawn in through the MAF sensor, and re-enters the turbo as 100g/s again. The turbo is pumping 100g/s, but only 2g/s is being metered by the MAF, and only 2g/s is entering the engine, everything is happy. Now what happens with an atmospheric valve? The turbo sucks in 100g/s, it compresses and blows out 100g/s, 2g/s enters the engine, the other 98g/s vents out into the atmosphere. The turbo is still drawing in 100g/s though, and since nothing is coming in from the valve, the full 100g/s is passing through the MAF sensor. This means the ECU is measuring 100g/s of air flow, and injecting fuel for 100g/s of air flow, but only 2g/s of that is actually entering the engine. The AFR swings stupid rich, so rich it can’t even combust, the engine misfires, you get a huge fireball out of the exhaust, and the fueling system takes a while to recover, delaying spool up in the next gear.

Not only that, but atmospheric valves can cause problems at idle as well. Remember that blow off valves operate on the pressure difference between the intercooler piping and the manifold? Well when you’re sitting at idle, the intercooler piping is close to 0 psig, while the manifold is somewhere around -10 psig. Depending on the valve, this could be enough to open it, just sitting there at idle not doing anything. With a recirculating valve, that’s not a problem. If it’s open, that just creates another path to the intake where it can grab metered air. With an atmospheric valve, if the intercooler piping isn’t EXACTLY at 0 psig, some air will either be entering or leaving through the open valve, making you run lean or rich accordingly. This can cause stumbling, hesitation, or even stalling at idle.

Some people claim that their atmospheric valve didn’t harm their driving experience much at all. I do not think that these people are lying, at all, but I DO think that they are more the exception than the rule. These claims should be taken with a grain of salt, because for every one person who doesn’t have a problem with an atmospheric valve, there is at least one person who does. It all depends on the specific valve being used, the spring in the valve, the specific car, and the temperature and pressure where they live. If you want to avoid problems, stick with a recirculating bypass valve.

There are some exceptions to the rule. Earlier I mentioned blow through MAF setups where the MAF sensor is located after the turbo before the throttle body. In these setups, as long as the valve is located before the MAF sensor, it can be atmospheric or recirculating, it makes no difference, because the air being vented has not been metered by the MAF sensor yet. Another exception is speed density, in which there is no MAF, the ECU uses the temperature and pressure of the air in the intake manifold to estimate the amount of air entering the engine (using a volumetric efficiency table instead of a MAF scaling table). As long as the air is being vented before it reaches the intake manifold (which it has to be, otherwise it would defeat the purpose of the blow off valve in the first place), then it doesn’t matter if it’s atmospheric or recirculating.



FAQ

Q: You spoke about the disadvantages, but are there any advantages to an atmospheric blow off valve?
A: No, none. At BEST an atmospheric valve will make the vehicle run the same as with a recirculating valve. At worst, the vehicle won’t even run. In the real world, it’s going to be somewhere in between. The only thing, and I mean the ONLY thing that you gain with an atmospheric valve, is that it’s louder. If that matters to you, I would suggest installing an aftermarket intake first, as the sound from the recirculating valve will resonate in the intake and make it significantly louder. Remember though, that any aftermarket intake MUST be tuned for (see the fuel system article for why).

Q: Will I gain power with an aftermarket valve?
A: Possibly. In an ideal world no, but we don’t live in an ideal world. The OEM valve does leak at high boost (old ones start leaking around 20 psi, new ones start around 17 psi). If your valve is leaking at high boost, replacing it with one that doesn’t leak will obviously help.

Q: Can the rich swing on shifts with an atmospheric valve be tuned out?
A: No. Contrary to popular belief, you can not “tune” for an atmospheric blow off valve. How would you? There is absolutely no way to know how much air is being vented by the valve versus how much is actually entering the engine, so there is no way to correct for it. There is also no table in the ECU that says “remove X g/s from the measured MAF for Y seconds after I let off the gas when the boost is above Z psi”. None that are even close.
* Registered users of the site do not see these ads.

Last edited by the suicidal eggroll; 04-13-2012 at 11:28 PM.
the suicidal eggroll is offline   Reply With Quote
Old 04-13-2012, 10:49 PM   #2
Thunderpaw
Scooby Specialist
 
Member#: 242205
Join Date: Mar 2010
Location: Dallas
Vehicle:
2009 wrx
blue

Default

I see a teaching lesson involved here. Good job

Last edited by Thunderpaw; 04-14-2012 at 10:48 AM.
Thunderpaw is online now   Reply With Quote
Old 04-13-2012, 10:56 PM   #3
Soobiecabra
Scooby Newbie
 
Member#: 259527
Join Date: Oct 2010
Chapter/Region: TXIC
Location: Wylie, TX
Vehicle:
2011 WRX Sedan
PD Satin Black

Default

But I want to hear the *psssssstttttt* everytime I shift?!
/joking

Good read dude.
Soobiecabra is offline   Reply With Quote
Old 04-13-2012, 11:12 PM   #4
Dr. octagon
Scooby Specialist
 
Member#: 171127
Join Date: Feb 2008
Chapter/Region: MAIC
Location: Fredericksburg
Vehicle:
02 Kinugawa
psm

Default

are you pissed off, or just felt like creating mad threads? I and others appreciate the read, but you know someone is going to start a thread asking about bov's tomorrow.
Dr. octagon is offline   Reply With Quote
Old 04-13-2012, 11:14 PM   #5
the suicidal eggroll
Scooby Guru
 
Member#: 51961
Join Date: Jan 2004
Chapter/Region: RMIC
Location: Broomfield, CO
Vehicle:
2005 STi
2012 WRX

Default

I wrote them while I was bored at work, that's about it. It's been a slow week...

I'm sure somebody will start threads about these tomorrow, and the next day, and the next day. This gives me a place to link them rather than rewriting the same explanation over and over. That's the main goal.

I hate rewriting things I've already written, especially when it's been a while and I can't find them anymore to quote myself because it was in some obscure thread with a subject that has nothing to do with the question.
the suicidal eggroll is offline   Reply With Quote
Old 04-13-2012, 11:17 PM   #6
cinnaman
Scooby Newbie
 
Member#: 298063
Join Date: Oct 2011
Chapter/Region: E. Canada
Location: Oakville, Ontario
Vehicle:
2005 legacy 2.5i
DIP

Default

can you stick this at the top so people can find it as well
http://forums.nasioc.com/forums/showthread.php?t=468038

or just about anything from unabombers manifesto.
cinnaman is offline   Reply With Quote
Old 04-13-2012, 11:17 PM   #7
Thunderpaw
Scooby Specialist
 
Member#: 242205
Join Date: Mar 2010
Location: Dallas
Vehicle:
2009 wrx
blue

Default

Quote:
Originally Posted by Dr. octagon View Post
are you pissed off, or just felt like creating mad threads? I and others appreciate the read, but you know someone is going to start a thread asking about bov's tomorrow.
Maybe part of that herd will be culled
Thunderpaw is online now   Reply With Quote
Old 04-13-2012, 11:19 PM   #8
the suicidal eggroll
Scooby Guru
 
Member#: 51961
Join Date: Jan 2004
Chapter/Region: RMIC
Location: Broomfield, CO
Vehicle:
2005 STi
2012 WRX

Default

Quote:
Originally Posted by cinnaman View Post
can you stick this at the top so people can find it as well
http://forums.nasioc.com/forums/showthread.php?t=468038

or just about anything from unabombers manifesto.
Done

I'll do that in the others too, thanks for the reminder

As I mentioned in the main post, Unabomber's Manifesto is really about the aftermarket options available and how they differ. This series is more about what the part does and how it works. Different aspects of the same topic, I feel they compliment each other.

Last edited by the suicidal eggroll; 04-13-2012 at 11:26 PM.
the suicidal eggroll is offline   Reply With Quote
Old 04-13-2012, 11:22 PM   #9
Jubert69
Scooby Specialist
 
Member#: 167739
Join Date: Dec 2007
Chapter/Region: Tri-State
Location: CT
Vehicle:
04 WRX
ej207 spec C

Default

tl;dr

Can I run a bov on my wrx?
Jubert69 is offline   Reply With Quote
Old 04-13-2012, 11:22 PM   #10
Jubert69
Scooby Specialist
 
Member#: 167739
Join Date: Dec 2007
Chapter/Region: Tri-State
Location: CT
Vehicle:
04 WRX
ej207 spec C

Default

lol jk nice right up sir
Jubert69 is offline   Reply With Quote
Old 04-14-2012, 01:49 AM   #11
dillonlightman88
Scooby Newbie
 
Member#: 287889
Join Date: Jul 2011
Location: Hays, kansas
Vehicle:
2008 Wrx

Default

Well written. Technical information simplified without loosing any of the details.
dillonlightman88 is offline   Reply With Quote
Old 04-14-2012, 02:46 AM   #12
i_c_the_light
Scooby Guru
 
Member#: 88618
Join Date: Jun 2005
Chapter/Region: SCIC
Location: New Zealand
Vehicle:
1994 Legacy
"The Silver Slug"

Default

Davenow beat you to the punch you realise.
i_c_the_light is offline   Reply With Quote
Old 04-14-2012, 11:37 AM   #13
the suicidal eggroll
Scooby Guru
 
Member#: 51961
Join Date: Jan 2004
Chapter/Region: RMIC
Location: Broomfield, CO
Vehicle:
2005 STi
2012 WRX

Default

Quote:
Originally Posted by i_c_the_light View Post
Davenow beat you to the punch you realise.
That may be. This is just 1 of 5 articles right now, with more to come, most of which I have never seen descriptions for in any single place (it's just a hodge-podge of different people throwing out different analogies that don't make much sense when put together). The atmo vs recirc BOV debate has been talked about extensively, but not many people have described how a BOV actually works, that's what this is about. As I said in the main thread:

Quote:
If you disagree with something, or feel you have a better way of describing it, or feel it has already been described better somewhere else, post it. More info (as long as it isn't wrong) is always helpful to somebody trying to learn.
The BOV article is arguably the least necessary of the set, I just threw it in there for completeness.

Last edited by the suicidal eggroll; 04-14-2012 at 11:57 AM.
the suicidal eggroll is offline   Reply With Quote
Old 04-16-2012, 10:34 AM   #14
d_r0ck
Scooby Newbie
 
Member#: 259253
Join Date: Sep 2010
Chapter/Region: South East
Location: Tampa
Vehicle:
2011 Impreza WRX 4dr
WRB

Default

Wow - just wow. Thank you so much.
d_r0ck is offline   Reply With Quote
Old 04-16-2012, 12:02 PM   #15
tom312
Scooby Newbie
 
Member#: 261936
Join Date: Oct 2010
Location: CT
Vehicle:
2006 WRX STi
WRB

Default

I thought with cobb's speed density support that bov's can be tuned for now? I asked EFI out of curiosity and got this back:
Quote:
Generally speaking, running the speed density on any car making less than
400whp probably won't be necessary at all. The BOV would not get you
anything extra besides sound - but it is true that the BOV would then no
longer effect how the car ran.

Regards,
Pete Sohl
Sales Manager - EFILogics LLC
Granted I still thing getting a bov is a waste of money... but if someone really wants that sound they can at least do it safely
tom312 is offline   Reply With Quote
Old 04-16-2012, 12:12 PM   #16
the suicidal eggroll
Scooby Guru
 
Member#: 51961
Join Date: Jan 2004
Chapter/Region: RMIC
Location: Broomfield, CO
Vehicle:
2005 STi
2012 WRX

Default

Quote:
Originally Posted by tom312 View Post
I thought with cobb's speed density support that bov's can be tuned for now? I asked EFI out of curiosity and got this back:

Granted I still thing getting a bov is a waste of money... but if someone really wants that sound they can at least do it safely
Yes, you are correct. As I said in the last paragraph:
Quote:
There are some exceptions to the rule. Earlier I mentioned blow through MAF setups where the MAF sensor is located after the turbo before the throttle body. In these setups, as long as the valve is located before the MAF sensor, it can be atmospheric or recirculating, it makes no difference, because the air being vented has not been metered by the MAF sensor yet. Another exception is speed density, in which there is no MAF, the ECU uses the temperature and pressure of the air in the intake manifold to estimate the amount of air entering the engine (using a volumetric efficiency table instead of a MAF scaling table). As long as the air is being vented before it reaches the intake manifold (which it has to be, otherwise it would defeat the purpose of the blow off valve in the first place), then it doesnít matter if itís atmospheric or recirculating.
But speed density is a completely different mode of operation which doesn't even use a MAF. While switching to speed density does mean you can use an atmo valve without a problem, I don't view it a way to "tune for" an atmo valve. You're completely switching the operation of the fueling system in the car to a mode in which any leaks (including an atmo valve) located before the manifold don't affect AFR. Just semantics I guess.
the suicidal eggroll is offline   Reply With Quote
Old 04-16-2012, 12:14 PM   #17
tom312
Scooby Newbie
 
Member#: 261936
Join Date: Oct 2010
Location: CT
Vehicle:
2006 WRX STi
WRB

Default

Quote:
Originally Posted by the suicidal eggroll View Post
Yes, you are correct. As I said in the last paragraph:


But speed density is a completely different mode of operation which doesn't even use a MAF. While switching to speed density does mean you can use an atmo valve without a problem, I don't view it a way to "tune for" an atmo valve. You're completely switching the operation of the fueling system in the car to a mode in which any leaks (including an atmo valve) located before the manifold don't affect AFR. Just semantics I guess.
guess I should've done more than just skim it and read the end FAQ... great post
tom312 is offline   Reply With Quote
Old 04-16-2012, 12:27 PM   #18
slayer44wagon
Scooby Specialist
 
Member#: 300834
Join Date: Nov 2011
Chapter/Region: MAIC
Vehicle:
03 wrx wagon
silver

Default

Nicely put
slayer44wagon is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
APS blow off valve for sale! Or swap for 04 stock WRX blow off valve. SonomaCaliREX Private 'For Sale' Classifieds 1 05-01-2006 04:15 AM
FS: HKS SSQV Blow Off Valve, & HKS Type II Racing Blow Off Valve! adamsrs Private 'For Sale' Classifieds 7 09-12-2004 11:41 AM
FS: HKS SSQV Blow Off Valve, & HKS Type II Racing Blow Off Valve! adamsrs North West Impreza Club Forum -- NWIC 7 08-22-2004 10:48 PM
which blow off valves work with turbo xs v2 TMIC? M32WRXin3seconds Factory 2.0L Turbo Powertrain 20 01-05-2004 05:33 PM
Which Blow off valve works? WRX? SeattleWRX Factory 2.0L Turbo Powertrain 26 09-21-2001 08:29 PM


All times are GMT -4. The time now is 11:31 AM.


Powered by vBulletin® Version 3.7.0
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Powered by Searchlight © 2014 Axivo Inc.
Copyright ©1999 - 2014, North American Subaru Impreza Owners Club, Inc.