ASV = Anti Surge Valve, a collective term for various types of valves designed to relieve excess boost pressure. Blow-Off-Valve (BOV) a.k.a., Vent-To-Atmosphere (VTA) valve, a.k.a, Diverter valve releases to atmosphere and makes the whishhh noise. Bypass Valve (BPV) recirculates. There are also hybrids.
Need to say that M157 turbos and air ducts are located before the pressure sensor, and so the BOV system above does not impact the air fuel ratio like in some other designs. Engages when the throttle body rapidly closes (drops of RPM) and can help turbo impellers wear, and during gear shifts theoretically. However, the M157 turbos have diverter flaps in the hot side to combat same issues. In general, the negative impact is more pronounced with higher boost, larger engines, and manual transmissions where the throttle body shuts nearly 100% (foot off the accelerator) as opposed to ECU/TCU-controlled smart/partial throttle closures. Continuous pressure in the manifold available on throttle reopen may actually offset a potential lag needed to build the pressure back if vented. All that happens in a split second though.
I think to achieve optimal operation, it takes fine-tuning these diverters and ECU/TCU to work together under various conditions. IMHO boost management in the M157 is adequate out of the box. Having said that, big name tuners like RENNtech and Weistec make/sell these ASV/BOV/Diverter solutions so there must be use cases when they do make some difference in addition to noise.
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