[urq] Wastegate "Chirp" on Rally, SCCA and IMSA Race Cars

[Stephen Kraus]

For more info:
http://en.wikipedia.org/wiki/Audi_Quattro#Sport_Quattro_S1_E2

The boost pressure was recirculated into the turbo when the driver comes
off the throttle, the throttle plate closes, the extra boost pressure
recirculated back into the turbo, keeping it spooling for when the throttle
plate re-opens.

No loss of boost pressure when the driver shifts and engine resume high
speed instantly, but the brief blockage of the pressure and recirculating
air makes the turbo chatter.