Blow Off Valves - How does it work ?
01-04-2006, 09:19 AM
#1
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Blow Off Valves - How does it work ?
from GFB , a good writeup and very informative for me.
All you ever needed to know about…
Blow off valves:
What is a blow off valve and how does it work?
A blow off valve goes under many different names such as dump, bypass, recirc (short for recirculation), diverter, and pop off valves. No one term is correct, the lingo just tends to change in different regions and countries. Basically these valves are all designed to the same job, and we will refer to all types as blow-off valves to save confusion.
What it does: The only job a blow-off valve is supposed to perform is to relieve excess turbo pressure that results from shutting the throttle rapidly under boost conditions. This then answers one of the questions we get asked a lot – yes you need a turbo to fit a blow-off valve! Otherwise you will have a rather expensive paper-weight. It is necessary to release this pressure because it will try to find a way out of the system, and the only way is back out through the turbo. This produces a fluttering noise as the air passes backwards through the turbo, which is often unwanted. There is also a school of thought that a blow-off valve can improve turbocharger longevity or, in extreme cases, prevent damage to the turbo.
There are two ways in which the majority of blow-off valves work. Most aftermarket and many factory types use two pressure signals to determine when the valve should open. The other type uses only one pressure signal to open. We will discuss the former type first.
How it works: The usual arrangement is to have the pressure side of the valve attached to the pipe between the turbo and the throttle, and a vacuum hose on the top of the valve hooked up to the inlet manifold after the throttle body. A spring holds the valve shut. When you are on the throttle, the pressure in the turbo piping and the inlet manifold is equal, meaning that the pressure on each side of the valve is the same and therefore cancels itself out, leaving the spring holding the valve shut. When you lift off the throttle, you have high pressure in the turbo piping, and a vacuum in the inlet manifold. The pressure on the bottom of the valve and the vacuum on the top combine to lift the valve open and release the pressure in the turbo piping, since it can no longer go into the engine.
Blow-off valve myth #1: One of the biggest misconceptions about blow-off valves is that you need to tighten the spring to run higher boost. This is totally incorrect (at least, for a GFB valve anyway), as you can see from the last paragraph, at full throttle, the pressure on the top and bottom of the valve is equal, therefore cancelling itself out. So no matter what boost pressure you run, the valve will stay shut.
Leaking factory valves: Having said that, some factory valves have systems designed into them to crack open at high boost, in order to protect the engine from boost spikes. They do this by designing the pressure to be unequal on the top an bottom of the valve, thereby overcoming the spring at a certain pressure.
If you are intentionally raising the boost level, this is bad news as the valve will begin to leak pressure. Examples of factory valves which exhibit this behaviour are the Subaru WRX MY95-98, and MY01-04, Mitsubishi Lancer GSR, and Nissan 200SX to name a few. Replacing these valves when increasing the boost is a good idea.
Blow-off valve myth #2: The fluttering sound is usually believed to be the blow-off valve. In reality, it is caused by a blow-off valve, but does not come from the blow-off valve. If the spring pre-load is adjusted too tight, this will cause compressor surge, which as described above is the sound of air exiting the turbo.
Compressor surge: You can think or surge as the point at which the compressor blades begin to “slip” in the air, losing their pumping ability, much like an aircraft wing loses its lift when it stalls. In a turbo, this happens in a series of bursts, as the blades slip, then bite, slip then bite. This sets up a pulsing wave in the turbo piping and explains why the sound has that characteristic “flutter”.
The interesting thing about compressor surge is that it occurs much more readily at low turbo shaft speeds. At these low shaft speeds, on road cars this is generally between 2000 and 3000 RPM, compressor surge is not much of a problem, as the loads generated by the surge are miniscule compared to what the turbo encounters at high boost. However, if surge occurs at high RPM and boost, it is possible to reduce the turbo life and/or damage the compressor.[/b]
All you ever needed to know about…
Blow off valves:
What is a blow off valve and how does it work?
A blow off valve goes under many different names such as dump, bypass, recirc (short for recirculation), diverter, and pop off valves. No one term is correct, the lingo just tends to change in different regions and countries. Basically these valves are all designed to the same job, and we will refer to all types as blow-off valves to save confusion.
What it does: The only job a blow-off valve is supposed to perform is to relieve excess turbo pressure that results from shutting the throttle rapidly under boost conditions. This then answers one of the questions we get asked a lot – yes you need a turbo to fit a blow-off valve! Otherwise you will have a rather expensive paper-weight. It is necessary to release this pressure because it will try to find a way out of the system, and the only way is back out through the turbo. This produces a fluttering noise as the air passes backwards through the turbo, which is often unwanted. There is also a school of thought that a blow-off valve can improve turbocharger longevity or, in extreme cases, prevent damage to the turbo.
There are two ways in which the majority of blow-off valves work. Most aftermarket and many factory types use two pressure signals to determine when the valve should open. The other type uses only one pressure signal to open. We will discuss the former type first.
How it works: The usual arrangement is to have the pressure side of the valve attached to the pipe between the turbo and the throttle, and a vacuum hose on the top of the valve hooked up to the inlet manifold after the throttle body. A spring holds the valve shut. When you are on the throttle, the pressure in the turbo piping and the inlet manifold is equal, meaning that the pressure on each side of the valve is the same and therefore cancels itself out, leaving the spring holding the valve shut. When you lift off the throttle, you have high pressure in the turbo piping, and a vacuum in the inlet manifold. The pressure on the bottom of the valve and the vacuum on the top combine to lift the valve open and release the pressure in the turbo piping, since it can no longer go into the engine.
Blow-off valve myth #1: One of the biggest misconceptions about blow-off valves is that you need to tighten the spring to run higher boost. This is totally incorrect (at least, for a GFB valve anyway), as you can see from the last paragraph, at full throttle, the pressure on the top and bottom of the valve is equal, therefore cancelling itself out. So no matter what boost pressure you run, the valve will stay shut.
Leaking factory valves: Having said that, some factory valves have systems designed into them to crack open at high boost, in order to protect the engine from boost spikes. They do this by designing the pressure to be unequal on the top an bottom of the valve, thereby overcoming the spring at a certain pressure.
If you are intentionally raising the boost level, this is bad news as the valve will begin to leak pressure. Examples of factory valves which exhibit this behaviour are the Subaru WRX MY95-98, and MY01-04, Mitsubishi Lancer GSR, and Nissan 200SX to name a few. Replacing these valves when increasing the boost is a good idea.
Blow-off valve myth #2: The fluttering sound is usually believed to be the blow-off valve. In reality, it is caused by a blow-off valve, but does not come from the blow-off valve. If the spring pre-load is adjusted too tight, this will cause compressor surge, which as described above is the sound of air exiting the turbo.
Compressor surge: You can think or surge as the point at which the compressor blades begin to “slip” in the air, losing their pumping ability, much like an aircraft wing loses its lift when it stalls. In a turbo, this happens in a series of bursts, as the blades slip, then bite, slip then bite. This sets up a pulsing wave in the turbo piping and explains why the sound has that characteristic “flutter”.
The interesting thing about compressor surge is that it occurs much more readily at low turbo shaft speeds. At these low shaft speeds, on road cars this is generally between 2000 and 3000 RPM, compressor surge is not much of a problem, as the loads generated by the surge are miniscule compared to what the turbo encounters at high boost. However, if surge occurs at high RPM and boost, it is possible to reduce the turbo life and/or damage the compressor.[/b]
01-04-2006, 11:52 PM
#5
Originally Posted by partypillow69
I have seen spring swaps increase power, they just might be of a diff design.
01-05-2006, 12:24 AM
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Originally Posted by TheQuietThings
Originally Posted by partypillow69
I have seen spring swaps increase power, they just might be of a diff design.
01-06-2006, 03:28 AM
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Originally Posted by partypillow69
Yes, but how else would you increase boost easily and cheaply without changing magor parts?
using any type of BOV will not increase power. using a boost controller will.
Originally Posted by timmyt
The author said it himself "You don't need to get stiffer springs to increase boost". You saw more power from increasing the boost. not from switching springs.
Originally Posted by tikbhoy
from GFB , a good writeup and very informative for me.
check it
01-06-2006, 12:30 PM
#13
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The only bov (actually bpv) that benefits from a stiffer spring is one that has an additionaly pop off valve function like the hks racing bpv. This unit has an additional port that you can connect to the compressor housing of the turbo. By connecting this port, the bypass valve will automatically open when reaching a predetermined boost level when at WOT. The pressure that the valve opens, in this case, depends on the stiffness of the spring used, and hks sells several different spring rates. This is really just a safety feature to prevent overboosting the engine from boost creep or an incorrectly set boost controller. Most regular bov's do not have this function and as stated earlier, a stiffer spring is not necessary for running higher boost.
01-13-2006, 06:56 PM
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Bosch Bypass Valve - knowledge is power
Read On 
Dont buy anything and slap it on without knowing how it works
Bosch BPV problems
http://www.audiworld.com/tech/eng21.shtml
Bosch VS Forge bypass
http://www.audiworld.com/tech/eng16.shtml
Dont buy anything and slap it on without knowing how it works
Bosch BPV problems
http://www.audiworld.com/tech/eng21.shtml
Bosch VS Forge bypass
http://www.audiworld.com/tech/eng16.shtml
02-06-2006, 12:55 AM
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Originally Posted by lovemysciontc
you ened to get a supercharger for a blow off valve, that noise it makes releases air pressure, go to mechanie to def install
02-07-2006, 04:05 AM
#19
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Originally Posted by raccastle2
can a sueprcharger have a blowoff valve?
A adjustable bypass valve/Diverter Valve would do the job with no reprecussions!
02-07-2006, 04:43 AM
#20
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you will need a recirculating BOV. Like the Greddy type S. I had tried just a BOV and plugging the recirculating hose, but I had problems after awhile. My gears would drag going into high gears like if i was pulling something extremely heavy. So i switched back. It sounded freakin awesome though. I miss the BOV.