From NASIOC 
VTA=Vent to atmosphere
-this is the noisey style that is the most popular. 90% of BOVs out there are fully VTA (meaning they vent 100% into the air)
Recirc
A BOV that dumps the vent back into the intake stream. Before the turbo, after the MAF sensor.
50/50
Vents half into the intake, half to atmosphere. Makes some noise, doesnt swing as rich. Typically these are set up sequentially, so that on light vents, it dumps 100% back into the intake, this is so that at light boost/part throttle you get no rich condition, and no/less noise. At higher boost, the valve opens all the way, and dumps into the intake and atmosphere.
Now lets clear a couple things up. If it seems like I am pro-bov, just read everything. I get to the down sides further down.
1. BOVS make you run rich.
--No they do NOT! Here is why. When you are idling, driving around, and under boost, the BOV is CLOSED just like stock. The only way to be running rich is if the BOV was somehow letting air out post maf, WHILE RUNNING, constantly. The exception to this would be a valve that is leaking REALLY badly, and in a case like this, you will know there is a problem right away anyway, as the car will probably barely run. For it to leak this badly, it would have to be physically damaged or just REALLY f'd up
2. But what about when you shift? Arent you rich then?
--Yes. BUT, you need to understand a couple things.
First-The rich condition is not THAT rich. On most tunes the target A/F under boost is @11.1-1 or so. Say you are at 11.1-1, then you shift and it vents. It will swing rich, typically to around 9.5-1. That is NOT that rich.
Second- The rich condition does not last very long. On a typical datalogger, the rich condition takes up no more than 2 rows. You are rich for well under a second.
3. I heard its bad for my cats!
--Its not great for them, but its not really enough to shorten their lifespan in any meaningful way. Exactly how long are you planning to keep the stock cats or even keep the car anyway? Seriously, there are guys running around with 100k+ on their car, totally stock exh, with a bov. No cat problems. If you have a VTA blow off valve, and you lose a cat, you would have lost it anyway, because either the cat was bad to begin with, or your vehicle has some other issue
4. But people will think I am a ricer!
--Did they pay for your car? Are they making your loan payments? Will them thinking this stop you from getting laid? Exactly, then tell them to die in a fire and enjoy your BOV if that is what you want.
It should be noted however, that if you are the type of person that doesnt like people trying to race them, you may want to stay away from a BOV simply because so many ricers think that everyone with a BOV wants to race.
5. I heard that if I want one, I should do a 50/50 venting BOV.
--These are somewhat pointless. You still get the rich condition, just not quite as badly. However, it isnt that bad anyway to begin with. These are however good for people that want a little noise when they are on it, and little to no noise when they arent on it hard. Dont fool yourself into thinking that you wont run rich (because you wont "run rich" with a fully VTA valve either), you still dump post MAF air when it vents, so you still get a rich spot when it vents.
6. But I need one because I run more boost than stock.
--In most cases this is not true. GENERALLY the stock valve can be trusted till @20PSI. Some leak a little bit after about 18PSI, but 90% hold rock solid till about 22PSI, and 99.99999% hold solid at 20PSI. If you are planning to run more than 24PSI, you may want to consider going aftermarket. Over 26PSI, you need to either mod your BOV, or go aftermarket.
7. So BOVs are fine? I can run one with no real downsides?
--Not always. MOST aftermarket valves leak. A leaking BOV, DOES slow you down, and can over time, damage things. To make matters even worse, 99% of people who buy BOVs, dont know how to, or bother to, take the time to set it up properly. So even if they bought a quality valve, it may be leaking.
8. I have had XXXX brand valve for years, it has never leaked.
--How do you know? By looking at your boost gauge? Looking at the boost levels in your datalog? Niether of those prove that the valve isnt leaking.
9. Ok so how do I set my BOV up, so that it doesnt leak/stands less chance of leaking?
--Read the manual first off. Learn how to adjust it. If it doesnt have a manual, just look at the valve. You can almost aways figure out how to adjust it. Some have screws and such, some use washers under the spring. Others use replacement springs.
Now that you know how to adjust it, here is what you do.
First off, start the car, get out and open the hood. If you can see the piston of the valve (look in the hole it vents out of if its a VTA valve), check to see if it is moving at all at idle, or worse, partially open. You want that piston rock solid not moving at idle.
Now give it a little gas. It shouldnt move when you step on it, and when you lift, it should pull up a bit. Depending how hard you hit the gas, it will either JUST move, or fully open.
If its doing anything but what it should be, adjust it a little tighter.
Now go drive the car
When you shift at light throttle, is it venting? If so, is it a pure straight vent, or do you get a little bit of a "chufchchchc" noise right before it vents? That noise is compressor surge. Which isnt as bad as some people like to think and say. Ideally, you are looking to hear a little surge then a vent. Or no sound at all. Either is ok. Now drive it hard, does it surge a little or just straight vent?
Get out of the car and tighten the valve and drive it again. Repeat this until you get a LOT of surge when you lift under boost. Now start loosening it until you JUST start to hear no almost surge.
What the surge means is that it fought opening just a little bit. If you hear a LITTLE surge just as it begins to vent, that means that the valve is holding itself shut as tightly as it is capable of, without being so tight that it doesnt vent properly.
10. But OMG, compressor surge is horrible for your turbo!!
--Unless its really bad, it really isnt going to kill it. I had a conversation with Corky Bell (Author of MAXIMUM BOOST) during a BBQ at his shop about this exact subject and he assured me that compressor surge was nothing more than a bad sound, and that his early miata turbo kits didnt even use one, and never had problems with losing the turbo. This goes against everything I had previously heard, but as one of the godfathers of turbocharging I am going to take his word for it. This guy knows more about the subject than 90% of nasioc all added together.
Bottom line is this.
If you are wanting to be 100% sure you arent losing any power from it, and you are running under about 22PSI, you are best off sticking with stock. If you REALLY want the sound, then stock with a GOOD valve. You would be suprised at how many big name and very popular valves are junk.
Generally you can trust
TurboXS, APS, Perrin, TIAL (best one out there), the HKS RACE valve (not the SSQV) and a couple others. There is one very popular brand that makes a lot of 50/50 style valves, I am not going to name names, but searching my name and BOVs should bring it up. Its 3 letters and they are known for 50/50 valves.
How do I know this? Like what am I using to base my opinions?
I spent a good long time several years back testing valves. As many as I could get my hands on. Buying, borrowing, trading, you name it. All in all I had easily 30+ valves on my car. Multiple examples of the same valve whenever I could, to help eliminate the chances of getting just an extra bad (or extra good!) valve.
I was shocked at what I found. Almost all leaked.
11. So if I am making the same boost pressure, why does it matter if its leaking a little?
--Because even if you are hitting target boost, your turbo is having to work harder to get there. A harder working turbo produces a hotter air charge, as its higher in its efficiency range (or worse, completely out of it). This means less dense, which will produce less energy when ignited. This also means higher chance of detonation, which even if its very light, will result in your ECU pulling timing. Also bad. Lastly, hotter air in=hotter air out. Higher EGTs. Even if only slightly.
Source NASIOC - http://forums.nasioc.com/forums/showthread.php?t=1602235
VTA=Vent to atmosphere
-this is the noisey style that is the most popular. 90% of BOVs out there are fully VTA (meaning they vent 100% into the air)
Recirc
A BOV that dumps the vent back into the intake stream. Before the turbo, after the MAF sensor.
50/50
Vents half into the intake, half to atmosphere. Makes some noise, doesnt swing as rich. Typically these are set up sequentially, so that on light vents, it dumps 100% back into the intake, this is so that at light boost/part throttle you get no rich condition, and no/less noise. At higher boost, the valve opens all the way, and dumps into the intake and atmosphere.
Now lets clear a couple things up. If it seems like I am pro-bov, just read everything. I get to the down sides further down.
1. BOVS make you run rich.
--No they do NOT! Here is why. When you are idling, driving around, and under boost, the BOV is CLOSED just like stock. The only way to be running rich is if the BOV was somehow letting air out post maf, WHILE RUNNING, constantly. The exception to this would be a valve that is leaking REALLY badly, and in a case like this, you will know there is a problem right away anyway, as the car will probably barely run. For it to leak this badly, it would have to be physically damaged or just REALLY f'd up
2. But what about when you shift? Arent you rich then?
--Yes. BUT, you need to understand a couple things.
First-The rich condition is not THAT rich. On most tunes the target A/F under boost is @11.1-1 or so. Say you are at 11.1-1, then you shift and it vents. It will swing rich, typically to around 9.5-1. That is NOT that rich.
Second- The rich condition does not last very long. On a typical datalogger, the rich condition takes up no more than 2 rows. You are rich for well under a second.
3. I heard its bad for my cats!
--Its not great for them, but its not really enough to shorten their lifespan in any meaningful way. Exactly how long are you planning to keep the stock cats or even keep the car anyway? Seriously, there are guys running around with 100k+ on their car, totally stock exh, with a bov. No cat problems. If you have a VTA blow off valve, and you lose a cat, you would have lost it anyway, because either the cat was bad to begin with, or your vehicle has some other issue
4. But people will think I am a ricer!
--Did they pay for your car? Are they making your loan payments? Will them thinking this stop you from getting laid? Exactly, then tell them to die in a fire and enjoy your BOV if that is what you want.
It should be noted however, that if you are the type of person that doesnt like people trying to race them, you may want to stay away from a BOV simply because so many ricers think that everyone with a BOV wants to race.
5. I heard that if I want one, I should do a 50/50 venting BOV.
--These are somewhat pointless. You still get the rich condition, just not quite as badly. However, it isnt that bad anyway to begin with. These are however good for people that want a little noise when they are on it, and little to no noise when they arent on it hard. Dont fool yourself into thinking that you wont run rich (because you wont "run rich" with a fully VTA valve either), you still dump post MAF air when it vents, so you still get a rich spot when it vents.
6. But I need one because I run more boost than stock.
--In most cases this is not true. GENERALLY the stock valve can be trusted till @20PSI. Some leak a little bit after about 18PSI, but 90% hold rock solid till about 22PSI, and 99.99999% hold solid at 20PSI. If you are planning to run more than 24PSI, you may want to consider going aftermarket. Over 26PSI, you need to either mod your BOV, or go aftermarket.
7. So BOVs are fine? I can run one with no real downsides?
--Not always. MOST aftermarket valves leak. A leaking BOV, DOES slow you down, and can over time, damage things. To make matters even worse, 99% of people who buy BOVs, dont know how to, or bother to, take the time to set it up properly. So even if they bought a quality valve, it may be leaking.
8. I have had XXXX brand valve for years, it has never leaked.
--How do you know? By looking at your boost gauge? Looking at the boost levels in your datalog? Niether of those prove that the valve isnt leaking.
9. Ok so how do I set my BOV up, so that it doesnt leak/stands less chance of leaking?
--Read the manual first off. Learn how to adjust it. If it doesnt have a manual, just look at the valve. You can almost aways figure out how to adjust it. Some have screws and such, some use washers under the spring. Others use replacement springs.
Now that you know how to adjust it, here is what you do.
First off, start the car, get out and open the hood. If you can see the piston of the valve (look in the hole it vents out of if its a VTA valve), check to see if it is moving at all at idle, or worse, partially open. You want that piston rock solid not moving at idle.
Now give it a little gas. It shouldnt move when you step on it, and when you lift, it should pull up a bit. Depending how hard you hit the gas, it will either JUST move, or fully open.
If its doing anything but what it should be, adjust it a little tighter.
Now go drive the car
When you shift at light throttle, is it venting? If so, is it a pure straight vent, or do you get a little bit of a "chufchchchc" noise right before it vents? That noise is compressor surge. Which isnt as bad as some people like to think and say. Ideally, you are looking to hear a little surge then a vent. Or no sound at all. Either is ok. Now drive it hard, does it surge a little or just straight vent?
Get out of the car and tighten the valve and drive it again. Repeat this until you get a LOT of surge when you lift under boost. Now start loosening it until you JUST start to hear no almost surge.
What the surge means is that it fought opening just a little bit. If you hear a LITTLE surge just as it begins to vent, that means that the valve is holding itself shut as tightly as it is capable of, without being so tight that it doesnt vent properly.
10. But OMG, compressor surge is horrible for your turbo!!
--Unless its really bad, it really isnt going to kill it. I had a conversation with Corky Bell (Author of MAXIMUM BOOST) during a BBQ at his shop about this exact subject and he assured me that compressor surge was nothing more than a bad sound, and that his early miata turbo kits didnt even use one, and never had problems with losing the turbo. This goes against everything I had previously heard, but as one of the godfathers of turbocharging I am going to take his word for it. This guy knows more about the subject than 90% of nasioc all added together.
Bottom line is this.
If you are wanting to be 100% sure you arent losing any power from it, and you are running under about 22PSI, you are best off sticking with stock. If you REALLY want the sound, then stock with a GOOD valve. You would be suprised at how many big name and very popular valves are junk.
Generally you can trust
TurboXS, APS, Perrin, TIAL (best one out there), the HKS RACE valve (not the SSQV) and a couple others. There is one very popular brand that makes a lot of 50/50 style valves, I am not going to name names, but searching my name and BOVs should bring it up. Its 3 letters and they are known for 50/50 valves.
How do I know this? Like what am I using to base my opinions?
I spent a good long time several years back testing valves. As many as I could get my hands on. Buying, borrowing, trading, you name it. All in all I had easily 30+ valves on my car. Multiple examples of the same valve whenever I could, to help eliminate the chances of getting just an extra bad (or extra good!) valve.
I was shocked at what I found. Almost all leaked.
11. So if I am making the same boost pressure, why does it matter if its leaking a little?
--Because even if you are hitting target boost, your turbo is having to work harder to get there. A harder working turbo produces a hotter air charge, as its higher in its efficiency range (or worse, completely out of it). This means less dense, which will produce less energy when ignited. This also means higher chance of detonation, which even if its very light, will result in your ECU pulling timing. Also bad. Lastly, hotter air in=hotter air out. Higher EGTs. Even if only slightly.
Source NASIOC - http://forums.nasioc.com/forums/showthread.php?t=1602235