Quick Tech: Wastegates vs. Blow-Off Valves…What’s the Difference?

Turbocharging continues to gain popularity.

But there are still a few areas of a typical turbocharger setup that remain fuzzy to turbo newbies. One of the most common misunderstandings involves the basic difference between a wastegate and a blow-off valve.

So here goes…

What is a Wastegate?

A wastegate prevents the turbo from creating too much boost.

Once the exhaust pressure from the engine spins the turbo’s turbine wheel fast enough to produce the desired level of boost, the wastegate will open to redirect the excess exhaust pressure around the turbine wheel. This keeps power levels at the desired level and prevents too much boost, which could cause detonation and engine damage.

Most wastegates operate off of pressure, or pneumatically, using an actuator spring. As pressure reaches a determined level of PSI, the spring is pressed open, actuating a valve which allows the exhaust gases to flow past the turbine and through the wastegate. These exhaust gases are essentially “wasted” because they no do not help spin the turbine wheel. The wasted exhaust gases will travel through the wastegate so long as peak boost is held.

Computer-controlled wastegates operate a bit differently. They use a sensor, which signals the ECM when boost pressure goes too high. When the pressure becomes too high, the ECM signals a solenoid valve that opens the wastegate.

There are a couple different configurations for wastegates. External wastegates are located on the exhaust side of the turbo unit, between the exhaust turbine wheel and the exhaust manifold. At this location, the wastegate can get the exhaust flow upstream of the turbine and then route it back into the exhaust on the other side. Internal wastegates are located within the turbocharger housing itself and offer a more compact option with less piping.

What is a Blow-Off Valve?

A blow-off valve (BOV) is similar to a wastegate, but works on the intake side of the turbocharger.

The BOV prevents pressure from building up in the intake tract. Excess pressure in the intake (ahead of the turbo) can back up and then cause compressor surge when the engine speed changes suddenly. The BOV is normally closed, but when a certain pressure level is achieved, the inlet air opens a spring to relieve the backed-up pressure. This reduces stress on the compressor and its bearings.

If you’ve ever heard a turbo car make a hissing sound, that’s the blow-off valve allowing excess pressure to escape when the driver suddenly lets off the throttle.