[span style=\'color:red\']Posted by Magnum[/span]
The wastegate is part of the turbocharger. On the hot side of the turbo, exhaust gasses can pass through two different openings. The normal opening routes the exhaust past the turbine wheel. The other opening bypasses the turbine wheel and sends the exhaust gasses directly on to the exhaust system via the downpipe. This second opening, normally sealed by a small vacuum-controlled flapper door, is called the wastegate.
When a boost controller senses that the turbocharger is producing too much boost, it opens the wastegate. With the wastegate open, some exhaust gasses take the \"path of least resistance\" through the wastegate, thus slowing or maintaining the turbine wheel speed.
In very high performance applications, the O2 sensor housing cannot be made efficient enough to pass the wastegated exhaust gasses out of the engine. When this happens, boost creep occurs. To solve this problem, an external wastegate is used, which diverts exhaust gasses around the turbo. Some external wastegates vent the exhaust directly to the atmosphere, which, as you might imagine, produces a very loud sound.