ALLTECH DIESEL & TURBOCHARGER LTD
A blow-off valve is a pressure-relief device on the engine intake tract to prevent a turbocharger’s compressor from going into what is known as surge. When the throttle is closed rapidly, the airflow is quickly reduced, causing air back-flow instability and pressure fluctuations. These rapidly cycling pressure fluctuations are the audible evidence of surge. Over time surge can eventually lead to turbocharger thrust bearing failure due to the high loads associated with it. In some instances there can be so much surge that it can stall the compressor wheel and unwind the compressor wheel nut. The BOV is installed between the turbocharger compressor discharge and the throttle body, preferably downstream of the intercooler (if equipped). BOVs use a combination of manifold pressure and spring force to detect when the throttle is closed. In modern vehicles, when the throttle is closed rapidly, the BOV vents boost pressure back into the intake tract before the turbocharger and after the air flow meter, as it is air that has already been measured. Some aftermarket BOVs vent to atmosphere to relieve the pressure. Both methods help to eliminate the phenomenon of surge.