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Volvo does not use a blow-off valve, but a compressor bypass valve which is integrated in the Mitsubishi turbo's used. (this is *NOT* the wastegate! Very different device.. That's on the exhaust/impeller side of the turbo and not the compressor side)
This device works a lot like a blowoff and it's operated by the vacuum in the intake manifold when you close the throttle, but feeds the compressed air from the exit of the turbo back into the intake. It's a silent device and you won't hear it operate.
This 'loopback' of air is done so that no air is lost to the outside that has already passed (and has been measured by) the MAF sensor.
If you were to blow off to the open on a Volvo it may run rich as the ECU still thinks that all the air that has been measured is in the intake system and injects the proper amount of fuel. But if some of this air has been vented off then there will be too much fuel.
You could fit a blow-off, which feeds back to the turbo intake side to prevent problems.
You will need to disable (eg. weld shut) the original bypass valve though. This is often required if you run very high boost pressures because the bypass valve is spring-loaded and the (increased) boost can cause it to open prematurely and bleed off pressurised air to the turbo intake.
Bye, Arno.
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