The wastegate bypasses exhaust gas around the turbine using a valve in the turbine inlet controlled by compressor outlet pressure. This serves to limit turbocharger speed at high engine speeds and loads. In doing this, it reduces the boost pressure attained at full speed full load.
Wastegate turbochargers are matched to give good performance at low engine speed with the valve closed. This improves transient response and reduces exhaust temperatures and emissions. As engine speed increases, the wastegate valve begins to open at a pre-set boost pressure. This has the effect of increasing the swallowing capacity of the turbine, reducing shaft power and avoiding excess air delivery and rotor overspeed.
Taken from holsets website.
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The Compressor Bypass Valve (CBV), also known as a blow-off valve, helps to keep the turbo spinning when the driver lets off the throttle.
Without a CBV, the throttle plate would close, creating a wall to the pressurized air wooshing out of the turbo. The air would crash into the closed plate, then air behind it would pile up, and eventually a wave of pressure would travel back to the turbo. Besides being a shock to the turbo, it would basically cause the turbo to come to a stop.
Then when you wanted to get back on the gas, you'd have to wait for the turbo so spin up all over again. With a CBV, the turbo can free-wheel for a while (like when you're shifting), because the intake pressure is relieved.
Definition note: "Blow-Off Valve" normally refers to a valve that releases its air to atmposphere, and does not return air back to the intake. "Compressor Bypass Valve" is often used to describe a valve that returns the released air back to the intake systems. Most cars come with a CBV from the factory.
The WRX comes with a decent compressor bypass valve straight from the factory, so for the most part it doesn't need upgrading. But a BOV makes a really loud "PSSSHHT!" noise at throttle lift, so some owners install the BOV for the sake of the sound.
Taken off ask.com
