A blow-off valve releases boost pressure from the intake when you close the throttle on a turbocharged gas engine. Without it, the pressurized air has nowhere to go and pushes back against the still-spinning turbo compressor, a condition called compressor surge that makes a fluttering sound and can stress the turbo. A blow-off valve opens when vacuum appears behind the throttle and vents the pressure, either to the atmosphere with the familiar whoosh or back into the intake.
Blow-off valve vs bypass (diverter) valve
| Vent to atmosphere (blow-off valve) | Recirculating (bypass or diverter valve) | |
|---|---|---|
| Where air goes | Out to the air | Back into the intake before the turbo |
| Sound | Loud whoosh | Quiet |
| Common on | Aftermarket setups, some factory cars | Most factory turbo cars |
Why MAF engines often use recirculating valves
On engines with a mass airflow sensor, the air being vented has already been measured. Venting it to the atmosphere means the engine adds fuel for air it no longer has, which can cause a brief rich condition, stumbles or stalling when you lift off the throttle. Recirculating the air avoids that; see MAF sensor.
What does a blow-off valve do for a turbo?
It protects the compressor from surge and helps the turbo keep spinning between shifts, so boost returns faster when you get back on the throttle; see what is turbo lag?
What about diesels?
Most diesels have no throttle plate in the intake, so there is no sudden pressure spike to relieve when you lift off, and they generally do not need a blow-off valve.
Compressor surge explained
When the throttle snaps shut under boost, the compressed air in the intake pipes stops abruptly and reverses against the compressor wheel, which is still spinning fast. The result is a fluttering sound and a shock load on the turbo’s bearings. Occasional mild surge is common on some cars, but repeated heavy surge is better avoided, which is the job of the blow-off or bypass valve.
Blow-off valve problems
- A leaking valve or torn diaphragm causes boost leaks and low boost; see P0299.
- A valve that is too stiff causes surge flutter.
- A valve that is too soft can open under boost.
Blow-off valve vs wastegate
The blow-off valve is on the intake side and relieves pressure when the throttle closes. The wastegate is on the exhaust side and limits boost; see what does a wastegate do? Superchargers have their own bypass valves for similar reasons; see supercharger vs turbo.
More guides: how long an intercooler lasts.
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