Bulent Aliev wrote:

I also understand the blow-off valves are useful in car applications, where
the driver will shut the throttle suddenly after a period of high boost. The
turbo can not shut down so fast and the compressed air has no place to go.
The valve than releases the air out to prevent damage. I don't see why we
need blow off valve in aircraft?
Bulent
  

Because if you do something stupid (for whatever reason), like chop the throttle, you could cause all sorts of problems. Think of it as engineering away the "human factors". A good example would be an aborted takeoff.. i would hate to think that an abort would set you up for a potentially adverse turbocharger event.
 
BUT.. the context that was being discussed was the use of blow off valves on supercharged (not turbocharged) applications, and using the BOV to relieve excess boost generated by the supercharger to develop what I would call a "super-normalized" setup.
 
My point was that BOV's.. (and most automotive turbocharger wastegates as well) are referenced to ambient pressure for their desired effects. So, in either instance (BOV or wastegate), they will not give the desired performance at altitude,because ambient pressure isn't the "Sea level pressure" that we are desiring to refer to.
 
Dave