You understand what compressor stall is... right?
http://en.wikipedia.org/wiki/Compressor_stall
Quote:
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Axi-symmetric stall, more commonly known as compressor surge; or pressure surge, is a complete breakdown in compression resulting in a reversal of flow and the violent expulsion of previously compressed air out through the engine intake, due to the compressor's inability to continue working against the already-compressed air behind it.
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Given the above...
This is valid -
http://www.turbobygarrett.com/turbob...o_tech101.html
Quote:
Blow-Off (Bypass) Valves
The Blow-Off valve (BOV) is a pressure relief device on the intake tract to prevent the turbo’s compressor from going into surge. The BOV should be installed between the compressor discharge and the throttle body, preferably downstream of the charge air cooler (if equipped). When the throttle is closed rapidly, the airflow is quickly reduced, causing flow instability and pressure fluctuations. These rapidly cycling pressure fluctuations are the audible evidence of surge. Surge can eventually lead to thrust bearing failure due to the high loads associated with it.
Blow-Off valves use a combination of manifold pressure signal and spring force to detect when the throttle is closed. When the throttle is closed rapidly, the BOV vents boost in the intake tract to atmosphere to relieve the pressure; helping to eliminate the phenomenon of surge.
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I really can not explain it any clearer than that. Now thats cleared up, lets address lag (although it should be self explanatory at this stage).
From here -
http://www.turbosmartonline.com/index.php?id=497
Quote:
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The secondary function of a BOV is to reduce the “turbo lag” effect between gear changes. Without a BOV, the compressor surge slows the turbine down, which then takes longer to spool up again when the throttle is opened. With a BOV, the excess boost pressure is released, keeping the turbine spinning and thus reducing the turbo lag effect.
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TurboSmart even have a nice picture to explain:
Whilst they are singing their own praises (obviously) the principle they're talking about (the peaks and troughs of the boost cycle) are valid
