Most stock engines are only meant to take about 10 PSI if not less. In order to regulate the amount of boost that comes into the engine, a wastegate acts as a door only allowing a given amount of exhaust to hit the turbocharger's exhaust turbine. Once the engine starts producing more exhaust pressure then the wastegate system will allow, a flap is opened to redirect excess exhaust away from the turbine blades. In turn, this is where a wastegate gets it's name. It's a gate to carry away waste. In order to regulate when a wastegate opens, a boost conroller can be used.
There are two types of wastegates. The first one is an internal wastegate. An internal wastegate is a component on the turbo unit itself. The gate is opened via an actuator which is a diaphram type system (see left picture). Excess exhaust is then fed directly into the exhaust system. We also have what is called an external wastegate, unlike an internal wastegate, it is seperate from the turbo unit and does not require an actuator. Excess exhaust can either be fed into the exhaust system or it can be vented straight out and into the atmosphere. High performance set-ups typically follow the latter alternative.
Most stock systems come with an internal wastegate as this set-up is better suited for low boost applications. However most aftermarket systems perform better with an seperate external wastegate assembly making it an ideal choice for those generating boost in the range of 20-30 PSI.
40mm External Wastegate
On left, an example external wastegate. Partially visible is the shaft of the poppet valve built into the wastegate. The port on top of this wastegate is a secondary control port. Not visible is the primary control port.
Internal Wastegate Assembly and Actuator
An internally gated turbocharger. The internal gate is located to the right of the turbine wheel, but built into the turbine housing. Partially seen at the top is the wastegate actuator arm.
Atmospheric/Divorced Wastegates
A divorced wastegate dumps the gases directly into the atmosphere, instead of returning it with the rest of an engine's exhaust. This is done to prevent turbulence to the exhaust flow and reduce total back pressure in the exhaust system. 'Divorced' wastegate dumper pipes are commonly referred to as Screamer Pipes due to the unmuffled waste exhaust gasses and the associated loud noises they produce.
Internal gates cannot generally be vented to the atmosphere because the internal wastegate port and turbine exducer are built into the same housing. Occasionally an exhaust downpipe for an internally gated turbocharger will be called 'divorced' because it integrates two tubes where one is intended for the wastegate only. It should be noted that it is difficult to truly separate the exhaust flow of the turbine and wastegate. Thus, internal wastegate turbos will rarely if ever feature an external dump.
Wastegate Chatter or "flutter" myth
There is a common myth in the automotive world about so called "wastegate chatter", a noise created on lifting off the throttle in a turbocharged car. The sound is commonly described as a chipmunk or a rattlesnake. This sound is in fact caused by a component called a blowoff valve (BOV) malfunctioning or being incorrectly setup. The BOV releases the excess pressure (from turbocharging/supercharging) between the throttle flap and the turbo when the throttle closes. However when this volume of air is too large the BOV cannot release it all and the pressure is vented back through the compressor turbine. As the air passes back through the turbine the pressure wave is "chopped" by the still spinning turbine creating the fluttering sound that is commonly termed wastegate chatter.
Source websites:
http://www.streetrac...o/wastegate.php
http://www.madabout-.../kb.php?aid=259
