What does a blow off valve do

What does a blow off valve do?
You seem to answer your own question:
lose boost pressure
But to add & expand - the valve releases the built up air pressure to the atmosphere rather than recirculate it back into the engine. The idea behind this is to prevent a backdraft towards the turbo (this is bad as it will stall the turbo and cause damage). A "blow off" valve releases this air to the atmosphere and gives the satisfying (to some) whooosh noise. The other type of valve is a recirculating valve. This type pushes the air back into the system.

I can only speak from experience with Imprezas - you don't want one that vents to atmosphere. On a Scooby engine the ECU has measured the air coming in and takes this into account when forcing air in from the turbo. If the engine suddenly doesn't have that air (because it has been vented to the atmosphere by the blow off valve) it will run rich. This can cause many problems.

As to why one would want one on another engine? I can't answer that as I've never done any research into other turbo powered engines. I'm sure others with the Knowledge will be along :)
 
That moment you lift off the gas from high RPM. The turbo is still pulling in lots of air but the engine is not using it as the throttle is now closed. This causes a massive build up of pressurised air. That pressure has to go somewhere or you risk over pressurising the intake or damaging the turbo.

The blow off valve cuts into vent this surplus air into the atmosphere. In some cars it is dumped back into the air intake just before the turbo so you don't lose all the benefit of it and it helps the turbo spool up more quickly. The are known as recirculating dump valves and are what most manufacturers fit to their cars.
 
I reckon damage to the turbo blades could be a real issue, sort of like cavitation damage to a boat propeller. I dealt with a company who made Air Cycle Units (Cold Air Units) with turbines where they were experiencing the intakes being blocked up periodically with plastic bags causing massive RPM surges. They said this was one of the worst things that could happen to turbine blades because it caused fatigue fractures at the blade root. Any such failure of course leads to catastrophic failure because of the extremely high RPM's.
 

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