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Discussion Starter · #1 ·
I actually made this post for another thread. However, I feel more than just the subscribers to that thread can benefit greatly from this post. There seems to be some misconceptions about the way these things work. So here it is. Feel free to let me know if I need to add/address anything!

What is the function of these valves?
A turbocharger compresses air with the help of exhaust gases... Just because you let your foot off the gas, that doesn't mean boost building is stopped. That excess boost has to go SOMEWHERE. The factory valve reroutes it to the intake. Atmospheric valves, vent to atmosphere, and hybrids will do a bit of both. I will post diagrams I mocked up, and go from left to right through the illustrations, describing each condition with each type of valve. I hope this helps illustrate WHY a BOV is potentially harmful, while BPVs and Hybrids are not.

STOCK BPV:

* When the throttle is open - The piston is not under pressure, so the compressed air from the turbo flows right through.
* When the throttle is closed at low boost levels (Example: Shifting around 2200 RPM) - The piston is under light pressure, and the stock spring is not very strong, so that excess air is effectively routed back to the intake tract. (The MAF sensor expects this excess air to be plumbed back to the intake, so the ECU demands more fuel to balance the A/F ratio.)
* When the throttle is closed at high boost levels (Example: Shifting around 4500 RPM) - The piston is under heavy pressure, so that excess air is effectively routed back to the intake tract. (The MAF sensor expects this excess air to be plumbed back to the intake, so the ECU demands more fuel to balance the A/F ratio.)

ATMOSPHERIC BOV:


* When the throttle is open - The piston is not under pressure, so the compressed air from the turbo flows right through.
* When the throttle is closed at low boost levels (Example: Shifting around 2200 RPM) - The piston is under light pressure, and the since the spring is heavier in aftermarket applications, the excess air creates a back-pressure by having nowhere to go other than back where it came from... The turbo's compressor wheel is forced to spin backwards. This is referred to as Compressor Surge. This can seriously damage your turbo over time. The second ill effect is that your ECU has demanded fuel, but that air is not routed back to the intake, thus causing an extremely rich moment between the shift (Often resulting in a fire ball out the exhaust).
* When the throttle is closed at high boost levels (Example: Shifting around 4500 RPM) - The piston is under heavy pressure, and the piston moves far enough to expose the opening and vents that excess boost off to the atmosphere. This creates a very rich moment during the shift, because your ECU has demanded fuel, but that air is not routed back to the intake, again causing an extremely rich moment between the shift (Often resulting in a fire ball out the exhaust).
**Additional Notes** - If the valve is adjusted to a lighter spring tension, you will effectively prevent the backspin, but it will also allow the piston to push too easily creating boost leak, before you even reach your target boost pressures. Now you may think you have a "perfectly tuned" the valve.. But this is VERY unlikely, because boost pressures vary. Your valve's spring will ALWAYS be too strong or too tight. Causing boost leak, or compressor Surge, and always leading to that rich mixture between shifts.

HYBRID or 50/50 BOV:

* When the throttle is open - The piston is not under pressure, so the compressed air from the turbo flows right through.
* When the throttle is closed at low boost levels (Example: Shifting around 2200 RPM) - The piston is under light pressure, the piston is only pushed far enough to reveal the first opening which routes the excess boost to flow back to the intake. (Just like stock)
* When the throttle is closed at high boost levels (Example: Shifting around 4500 RPM) - The piston is under heavy pressure, and the piston moves far enough to expose both openings: theoretically this satisfies the plumb back to the intake, and then vents off any additional boost off to the atmosphere.
**Additional Notes** - The idea behind a Hybrid valve is simple and effective. It's got two openings. One to plumb back to the intake, and one to vent to atmosphere. The Vent opening is further back than the Intake opening, so that under light load and light boost levels, the valve functions EXACTLY like a by pass valve. But once you're shifting under full boost, there is so much pressure that the piston is pushed even further back revealing BOTH openings. Theres only so much boost that first opening can plumb back in that moment, so the rest is vented off to the atmosphere.

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Conclusion - The entire point of this point was to dismiss rumors and hopefully provide some technical insight to those who didn't quite know the difference, or just wanted to know more about the way these valves function. Obviously, the more you know about a product, the more of an educated decision you can make while being aware of any potential gain or harm. If there's any biased message, it's that Atmospheric BOVs are unsafe in applications like Mazdaspeed3s (and most other stock turbo cars). Hybrids are, in theory, the best. The stock BPVs can only hold so much boost because the springs are just strong enough to hold factory level boost (sometimes a tad less, and end up causing boost leaks). Atmospheric BOVs are capable of holding higher boost, but are also adjustable to allow more leak or more resistance. (Again, an 'absolutely perfect' compromise is impossible). Hybrids or 50/50 valves try to give you that BOV sound at high boost levels while effectively functioning like BPVs under lower boost. Boost leak is safer than compressor surge. But neither is really a good thing. One loses power, one harms the turbocharger.

Happy Motoring!
 

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never figured that BOV's needed its own topic, however, BA did a great job again.

how about a writeup for CBE's next :p
 

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Very nice write-up for someone like me thats new to the turbo's
Who makes a good Hybrid valve or the best!
I know this is going to be your opinion and thats ok.
Thanks again for the write-up!
 

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Wow bro nice diagrams and write up. Clears up a couple issues for me. Which would be the safest for our stock engine? I don't plan on doing anything other than MS parts till my warranty is up besides a BOV.
 

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Discussion Starter · #9 ·
Boveo, At the moment, I don't know who makes a hybrid valve that would work on the speed without an adapter yet. I have been very busy lately and I haven't had the chance to do any product research.

Cosmic, Yeah I agree... Some of us take the things we know for granted, and I realized that there are so many threads about which is better and people talking crap on other valves when really they knew nothing of the internals. So to clear things up, here it is.

Jcutler, I would suggest keeping the stock BPV until a good hybrid is released. If you still want to go BOV, then just be aware of the possible repercussions. My safest suggestion to you is to enjoy the added induction noise of a CAI, then once a good, proven hybrid valve is released, upgrade. :)

And apparently I'm missing something that Rotus8 pointed out to me in a PM, but I'm still waiting to hear back from him. Maybe I can refresh the topic with more info.

Thanks everyone for the constructive feedback!
 

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Some more useful info. (stolen form tt.net)

Should I get blow-off valves or not? Do they really help?

The Great Blow Off Valve Debate:

First of all to understand the functionality and benefits of a blow off valve you have to have a complete understanding of how a turbocharger works. If you don't understand the basic mechanics of a turbocharger you can forget about trying to understand the benefits of a blow off valve. If you do understand the mechanics of a turbocharger, read on...

An Explanation of Compressor Surge:

The turbine in a turbocharger in its ideal mode is spinning at a very high rate and therefore generating lots of pressurized air that is fed into the OPEN (wide open throttle) intake manifold which translates into more power, but what happens when you take your foot off the gas when shifting between gears? The turbine in the turbo is still spinning at a high rate but now you've taken your foot off the gas and the intake manifold is CLOSED. Now the turbine spinning at a high rate and is trying to push out all this compressed air but since the manifold is closed, that compressed air has nowhere to go. What happens next is that the turbine slows down because of this pressure build-up between the compressor and the intake manifold. Since the pressurized air can't go forward into the engine it heads the wrong way back towards the turbo where it slows the turbine down (compressor surge). After you shift gears and you step on the gas again you want to have the turbine spinning fast so that its producing boost but since you took your foot off the gas and caused compressor surge, it slowed down, so you have to wait for the turbine to "spool up" (spin fast) again. This delay waiting for turbines to spool up is what causes some of the performance degradation that were trying to avoid.

The Blow off Valve:

What if we could figure out a way of eliminating compressor surge? Then the turbine in our turbo would have a much easier time keeping its speed up and we wouldn't get the lag in power waiting for the turbine to spool up between shifts. Enter the Blow off valve. At that moment in time between shifts when compressor surge is slowing the turbine down we'll just poke a hole in the tube running between the turbo and the intake manifold and let all that air leak out somewhere. The turbine can keep spinning fast & we'll have lots of boost when we step on the gas again. We'll also want to plug up that hole we made because we have a purpose for that pressurized air now, where as when we let off the gas we did not. The blow off valve is a gizmo that's constantly opening and closing giving you that cool sneeze sound when it "pokes" that hole in the intake manifold tube to keep the turbine spinning fast. We've eliminated compressor surge but we did confuse the engine just a bit since the ECU already told the injectors to issue fuel to be used for the air that we've now thrown out. Other than that sounds like a good idea right? so what if were running a bit rich for a moment, no big deal. Before you place your order read on...

The Re-circulating Valve... a better idea:

How about instead of poking a hole in the turbo to intake manifold tube and just letting the air leak out wherever it wants to, we could direct that pressurized air somewhere else where we could hold on to it until we need it again? The intake side of the compressor housing (between the intercooler and the turbo) would be a good place wouldn't it? that way we wouldn't slow the turbine down and we would have that pressurized air that would have otherwise been thrown out, ready in a really good place when we need it. On top of that eve got still got the correct air/fuel ratio going since we're not throwing and air out, just moving it from one place to another within the intake system. Instead of having to suck in air for the turbo to pressurize, we've got some pressurized air with the correct fuel amount already waiting. Now we've got all the benefits of a blow off valve and then some. That's a re-circulating valve. Let's all order re-circulating valves then and skip the blow off valves completely. They're probably gonna cost a bit more than the blow off valves since they work a little better right? Now the big surprise... Re-circulating valves were engineered into the design of your 90+ twin turbo z on the drawing board... you've already got them...

In conclusion: You could always disconnect your stock re-circulating valves (which you have to do to install blow off valves) and spend 500-600 bucks for blow off valves and what will you get? better performance? No, basically you do get the cool sneeze sound though you aren't going to be any faster than you were before (but you might sound like you are). If by chance you are running boost levels that exceed the airflow capacity of the re-circulating valves (~600+ hp?) then you've probably spent enough money and have enough experience and knowledge to know where the benefit reverses. For the rest of you who weren't in the know, I hope I've helped you save some time and money...

zxttsteve
 

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[quote author=surfwagon link=topic=80641.msg1511987#msg1511987 date=1182662302]
Some more useful info. (stolen form tt.net)

Should I get blow-off valves or not? Do they really help?

The Great Blow Off Valve Debate:

First of all to understand the functionality and benefits of a blow off valve you have to have a complete understanding of how a turbocharger works. If you don't understand the basic mechanics of a turbocharger you can forget about trying to understand the benefits of a blow off valve. If you do understand the mechanics of a turbocharger, read on...

An Explanation of Compressor Surge:

The turbine in a turbocharger in its ideal mode is spinning at a very high rate and therefore generating lots of pressurized air that is fed into the OPEN (wide open throttle) intake manifold which translates into more power, but what happens when you take your foot off the gas when shifting between gears? The turbine in the turbo is still spinning at a high rate but now you've taken your foot off the gas and the intake manifold is CLOSED. Now the turbine spinning at a high rate and is trying to push out all this compressed air but since the manifold is closed, that compressed air has nowhere to go. What happens next is that the turbine slows down because of this pressure build-up between the compressor and the intake manifold. Since the pressurized air can't go forward into the engine it heads the wrong way back towards the turbo where it slows the turbine down (compressor surge). After you shift gears and you step on the gas again you want to have the turbine spinning fast so that its producing boost but since you took your foot off the gas and caused compressor surge, it slowed down, so you have to wait for the turbine to "spool up" (spin fast) again. This delay waiting for turbines to spool up is what causes some of the performance degradation that were trying to avoid.

The Blow off Valve:

What if we could figure out a way of eliminating compressor surge? Then the turbine in our turbo would have a much easier time keeping its speed up and we wouldn't get the lag in power waiting for the turbine to spool up between shifts. Enter the Blow off valve. At that moment in time between shifts when compressor surge is slowing the turbine down we'll just poke a hole in the tube running between the turbo and the intake manifold and let all that air leak out somewhere. The turbine can keep spinning fast & we'll have lots of boost when we step on the gas again. We'll also want to plug up that hole we made because we have a purpose for that pressurized air now, where as when we let off the gas we did not. The blow off valve is a gizmo that's constantly opening and closing giving you that cool sneeze sound when it "pokes" that hole in the intake manifold tube to keep the turbine spinning fast. We've eliminated compressor surge but we did confuse the engine just a bit since the ECU already told the injectors to issue fuel to be used for the air that we've now thrown out. Other than that sounds like a good idea right? so what if were running a bit rich for a moment, no big deal. Before you place your order read on...

The Re-circulating Valve... a better idea:

How about instead of poking a hole in the turbo to intake manifold tube and just letting the air leak out wherever it wants to, we could direct that pressurized air somewhere else where we could hold on to it until we need it again? The intake side of the compressor housing (between the intercooler and the turbo) would be a good place wouldn't it? that way we wouldn't slow the turbine down and we would have that pressurized air that would have otherwise been thrown out, ready in a really good place when we need it. On top of that eve got still got the correct air/fuel ratio going since we're not throwing and air out, just moving it from one place to another within the intake system. Instead of having to suck in air for the turbo to pressurize, we've got some pressurized air with the correct fuel amount already waiting. Now we've got all the benefits of a blow off valve and then some. That's a re-circulating valve. Let's all order re-circulating valves then and skip the blow off valves completely. They're probably gonna cost a bit more than the blow off valves since they work a little better right? Now the big surprise... Re-circulating valves were engineered into the design of your 90+ twin turbo z on the drawing board... you've already got them...

In conclusion: You could always disconnect your stock re-circulating valves (which you have to do to install blow off valves) and spend 500-600 bucks for blow off valves and what will you get? better performance? No, basically you do get the cool sneeze sound though you aren't going to be any faster than you were before (but you might sound like you are). If by chance you are running boost levels that exceed the airflow capacity of the re-circulating valves (~600+ hp?) then you've probably spent enough money and have enough experience and knowledge to know where the benefit reverses. For the rest of you who weren't in the know, I hope I've helped you save some time and money...

zxttsteve
[/quote]

+1

There are no benefits in BOV's for this car. I've seen some serious problems on other cars because of them though.

Did you get that from tt.net? I think I've read this before.
 

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Discussion Starter · #14 ·
The article you posted seems a bit slant.. Which is why I try to keep a balance on my posts/writeups. I do them from scratch, because I'm trying to present the topic from a neutral and informative standpoint.

Its not "just the sneeze" you gain with an aftermarket BOV.

Generally, they are a good idea for people who plan on boosting higher levels than stock. In general stock valves will leak or crack under higher boost pressures.

So an aftermarket valve isn't completely useless. That's why I try to present both sides of the argument in my posts, so that people get the whole scoop, not just arsenal to fire away AGAINST something, but also FOR something if they choose to.
 

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-It is from TT.net. That's awesome you caught that.

-Understood Boost Addict. It seems that a lot of guys are getting BOV's when still running at stock boost. It's a mods that I would consider but there's a long list of stuff I'd rather spend my money on first. Thanks for creating these great write-ups. They're really informative.
 

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Discussion Starter · #16 ·
Oh you're absolutely right.

And regardless of the bias, I'm glad you posted it... It's just more good information for the most part, while it may be trying to encourage someway over another.

Yeah, I agree. A Blow Off Valve, is definitely low to none on the priority list in my personal view. Unless a great hybrid valve was introduced and I planned to increase boost significantly.

Thanks for sharing, man.
 

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Very informative info on he BOV. I was told that if we do any EMS that increases boost levels at all, than the stock BOV ill not hold. But according to the SurfWagons post that seems unnecessary or is it. The article claimed that after 600 hp we will have an issue. I am currently very happy with the stock BOV even though it is plastic and do not want to change it unless i have to. So what is the census here, with the EMS thats coming fromm CP-E and Cobb this fall(hoprfully) that will probably raise boost levels a little, do we stay stock or just wait and see, or i imagine that CP-E and Cobb will probbaly make some recommendations on what to do about the BOV. I will sit tight for the moment, I just ordered rims so im broke anyway....
 

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[quote author=Boost Addict link=topic=80641.msg1526495#msg1526495 date=1183093546]
Oh you're absolutely right.

And regardless of the bias, I'm glad you posted it... It's just more good information for the most part, while it may be trying to encourage someway over another.

Yeah, I agree. A Blow Off Valve, is definitely low to none on the priority list in my personal view. Unless a great hybrid valve was introduced and I planned to increase boost significantly.

Thanks for sharing, man.
[/quote]

I wouldn't call it a bias. This is the truth about BOV's. There is no benefit in going to a BOV over a recirculation valve. People do it because they like the sound. I've seen more than a few aftermarket BOV's that would leak at stock boost on other cars. If you're going to BOV's because of higher boost you'd have to buy a pretty pricey BOV. And you'd still be better off if you installed it as a recirculation valve. As boost gets higher, the negative effects of a BOV get much worse.

Not knocking you at all. I think your write up was great. I just think that people really don't understand how a BOV affects things. Even a hybrid valve will have those issues. Theres nothing good about venting metered air.
 

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Discussion Starter · #19 ·
[quote author=Toso link=topic=80641.msg1528085#msg1528085 date=1183146822]
[quote author=Boost Addict link=topic=80641.msg1526495#msg1526495 date=1183093546]
Oh you're absolutely right.

And regardless of the bias, I'm glad you posted it... It's just more good information for the most part, while it may be trying to encourage someway over another.

Yeah, I agree. A Blow Off Valve, is definitely low to none on the priority list in my personal view. Unless a great hybrid valve was introduced and I planned to increase boost significantly.

Thanks for sharing, man.
[/quote]

I wouldn't call it a bias. This is the truth about BOV's. There is no benefit in going to a BOV over a recirculation valve. People do it because they like the sound. I've seen more than a few aftermarket BOV's that would leak at stock boost on other cars. If you're going to BOV's because of higher boost you'd have to buy a pretty pricey BOV. And you'd still be better off if you installed it as a recirculation valve. As boost gets higher, the negative effects of a BOV get much worse.

Not knocking you at all. I think your write up was great. I just think that people really don't understand how a BOV affects things. Even a hybrid valve will have those issues. Theres nothing good about venting metered air.
[/quote]

Omitting information relevant to the function and main purpose of a blow off valve is far from being the "truth". While the motivation behind buying most of them may come down to sound, the PURPOSE of a BOV is not the sound. The only reason a BOV will leak at stock boost levels is because the spring tension is not tight enough, and hence bleeds off boost. The risk of over-tightening the spring is compressor surge. It will never be tuned perfectly. And if I must point out, the stock valve is not perfect either.

And I disagree.. Just because you want to boost more than factory levels does not constitute needing a PRICEY valve. Almost all aftermarket valves are engineered to hold far more boost than a stock bpv. Mostly due to the materials used and overall build quality.

In a hybrid valve, only excess air is vented off... the first helping of metered air is actually plumbed back. So in theory, a hybrid valve is as perfect as you can get.
 
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