Blow off valves

[loominaz34]

What exactly do these things do? I know they have something to do with boost pressure in turbo applications but I haven't found a clear answer. (damn them howstuffworks.com guys)

[gp90se]

see If i can explain this:

the turbo makes boost, when the throttl body closes, the pressure has nowhere to go except back into the turbo which causes turbo surge ( the turbo wheels stops quickly which is bad for the turbo). A BOV allows the pressure in the hoses out saving the turbo from surge. The air escaping from the valve makes the pshh noise.

 

Hope that clears everything up.

 

-Jeff

[mihela816]

It also releases extra boost when the tranny is shifted.

[Guest TurboSedan]

It also releases extra boost when the tranny is shifted.

 

a BOV is actually completely independant of transmission shifts. it is completely dependant however on manifold pressure/vacuum.

 

a BOV helps lengthen the life of the turbo. when you let off the gas pedal, the throttle slams shut and the boost pressure has nowhere to go so the pressure wave will slow the turbo down quick. considering a turbo can spin at upwards of 100,000 RPM, this is very hard on turbos. with a BOV, the moment you let off the gas pedal, you no longer see boost in the intake - you see vacuum. the BOV gets the vacuum signal and opens up allowing the pressure wave to escape and not hurt the turbo (which makes that "pshhhttt" sound). it also helps performance since the turbo is not slowed down by the pressure wave - it's still spinning fast and does not have to spool up again after you get back on the throttle. a BOV is more important for cars with a manual tranny since the throttle shuts at every shift, but it is still important on cars with automatics.

[90lum31v6euro]

very well put.