Info on BOV's

Tedium is the expert on fans, having written a thesis on axial fans, but I will give it a crack in lay terms having been involved in the design application of fans.

The wheel doesn't actually stall as in stop, it is a term used to describe the loss of flow or pumping due to a shutoff or high head condition. It occurs when the flow on the low pressure side of the blade separates from following the contour shape of the blade. You may find the compressor wheel actually speeds up. Fans and pumps tend to require less input power as they do less work. On axials the blades actually start to cut the air. On prop fans the slightest upset and they stall (stop pumping). Turbo compressors are more a mixed flow style impellor that combine the advantages of aerofoil axial and centrifugal fans.

If you have ever listened to an air compressor, you will note that when it turns off the relief valve gives a short vent due to a surge spike created when the velocity pressure is suddenly converted to static pressure (amongst other things).

The surge hammers the compressor blades and in turn its bearings. The air is now stalled and building a high pressure up against the throttle body. At this point a phenomenom called stall flutter starts where impellor tips are buffetted causing stress on bearings, the blades, etc. A large heat build up also occurs around the blades as the mechanical energy is converted. It is not uncommon in our industry to see a pump that has been left shutoff boiling the water in and around the volute.

Now if you have one of those cheapo desk or pedestal fans you may have noticed if you have it too close to a wall, the noise pitch rises and the air flow reduces significantly. Even when you pull it away it takes some time for the air to start pumping again, akin to a standing start as opposed to a rolling start. While the turbo compressor wheel is not a prop it still exibits similar behaviour and results in a lag of re-establishing force draft flow. As I understand it, this is the primary reason Porsche hit upon using a recirculating bypass valve in the first place.

Obviously its pointless putting a mismatched bypass valve on. And I would hazzard a guess a lot of the negative ideas about them is due to the upping of the standard boost without considering or knowing about the setpoint and range of the bypass setup