Let’s about Blow off valves
How Blow Off Valves Work
The blow off valve is opened on lifted throttle as the flow under pressure in intake system through intercooler and hoses is abruptly stopped and the pressure wave will effectively bounce off throtle and go backwards through turbo. This causes stress on turbo and can go backwards through MAF sensor. MAF registers the flow either direction so the ECM could get a signal and meter additional fuel and create a momentary over-rich condition if throttle is not completely closed (completely closed cuts off injectors on closed throttle, no fuel delivered, not rich). The BOV lets air get dumped to inlet side of turbo and the turbo pulls that in again and it effectively creates a temporary recirculation to stop the backflow. This also allows turbo to keep its speed up so a re-application of throttle results in faster boost build again.
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