What the hell is a Blow Off Valve

Posted by Magnum

Blow-Off Valve (BOV)
The function of the blow-off valve is to vent the "excess" compressed air entering the throttle-body when the throttle plate closes. Consider the case of the car running with high boost in second gear. In order to shift to third, the driver lets off the accelerator, momentarily closing the throttle plate. All of the sudden, the compressed air from the turbo (which was rushing into the engine) has no place to go. Instead the air "bounces" off of the throttle-body plate and begins travelling backward through the intercooler and into the turbo. This "backpressure" causes the turbo to slow down and produce less boost during a shift. This backpressure not only reduces the boost level, it is also potentially dangerous to the turbo, and is referred to as compressor surge.

To avert this problem, vacuum opens the BOV (mounted in the upper intercooler pipe) when the throttle plate closes. This diverts the compressed, intercooled air back into the intake stream, and allows the turbo to continue to run normally "across" shifts.

The blow-off valve is also known as the bypass or compressor bypass valve.

Some cars, like certain Starions, don't use a blow-off valve.

On second-generation DSMs, the blow-off valve is made of cheap plastic and tends to leak when the car's boost level is increased.

Someone care to define what "shim" is on a BOV?

shims are what you use to stiffen the internal spring. Most have an adjustable top that is threaded but if you need it to be stiffer and don't have a stiffer spring available you but shims in it to effectively smash the spring down harder and seal the bov tighter so it doesn't leak or open too soon.

However, you don't get the really cool PSSHH sound with a Recirculation Valve. LOL
hahahahaha, Yah without the PPSSHH sound, it isn't worth having a turbo...not...but the sound it makes is definately cooler...8)

[span style=\'color:red\']Posted by Magnum[/span]

Blow-Off Valve (BOV)
The function of the blow-off valve is to vent the \"excess\" compressed air entering the throttle-body when the throttle plate closes. Consider the case of the car running with high boost in second gear. In order to shift to third, the driver lets off the accelerator, momentarily closing the throttle plate. All of the sudden, the compressed air from the turbo (which was rushing into the engine) has no place to go. Instead the air \"bounces\" off of the throttle-body plate and begins travelling backward through the intercooler and into the turbo. This \"backpressure\" causes the turbo to slow down and produce less boost during a shift. This backpressure not only reduces the boost level, it is also potentially dangerous to the turbo, and is referred to as compressor surge.

To avert this problem, vacuum opens the BOV (mounted in the upper intercooler pipe) when the throttle plate closes. This diverts the compressed, intercooled air back into the intake stream, and allows the turbo to continue to run normally \"across\" shifts.

The blow-off valve is also known as the bypass or compressor bypass valve.

Some cars, like certain Starions, don\'t use a blow-off valve.

On second-generation DSMs, the blow-off valve is made of cheap plastic and tends to leak when the car\'s boost level is increased.

A couple notes to add onto this.  Proper placement of a BOV can also be something of interest.  The closer to the throttle body, the better, when it comes to mounting a BOV.  Also, remember that BOV\'s vent to the atmosphere, and since our cars run MAF sensors, the air vented by the BOV will already be metered.  Therefore, you will tend to run a bit rich when you get back on the throttle between shifts.  On the other hand, Recirculation Valves are just like a BOV, except instead of venting to the atmosphere, they vent back into the intake system, thereby eliminating the rich environment that a BOV would create.  However, you don\'t get the really cool PSSHH sound with a Recirculation Valve.  LOL

F the pss sound, if you\'re running enough fuel a straight enough exaust and pimp enough turbo, the \"rich\" enviroment will cause a oh no FIREBALL out the tail pipe and possible external waste gate. I see NO reason to complain.

cause a oh no FIREBALL out the tail pipe and possible external waste gate

you have a catalytic converter?

I have a question? Anyone knows why my vg30e keeps on dying whenever it wants to? Idil keeps on dyin when it wants to, then I would have to turn key off for 10 secs, otherwise if before 10 secs, the car would choke. It only does it sometimes. Please help, I'm a noob to the vg30e motor.

rofl, question why are you asking that question in this post. ?