What does an Anti-Surge Compressor housing do?

An anti-surge housing is designed to prevent compressor surge which can occur during Wide Open Throttle (WOT) conditions, not to a replace a Blow Off Valve or Bypass Valve which are designed to relieve charge air when the throttle closes. Its purpose is to relieve pressure on the throttle and is designed to move the Surge Line over on the compressor map. Thus ANTI SURGE. Surge under WOT is much more destructive to a compressor wheel and bearing assembly.

The Surge Line is the left hand boundary of the compressor map and represents a region of flow instability. This region is characterized by mild flutter to wildly fluctuating boost from the compressor. Continued operation within this region can lead to premature turbo failure due to heavy thrust loading. Surge will decay once the turbo speed finally slows enough to reduce the boost and move the operating point back into the stable region.