is an integral part of any turbocharger
system. It is a device that allows the boost control
system to regulate the amount of boost pressure
developed by the turbocharger at any particular time.
Most turbochargers utilize separate compressor and turbine housings. The hot exhaust gases from the motor are diverted into the turbine housing and pass through the turbine blades, causing the compressor blades to spin. This in turn creates boost, creating more exhaust gas, and so on. This is a system that must be regulated to provide a constant amount of boost versus engine speed.
To control the amount of boost, a device called a wastegate is employed. The concept is to simply divert or bleed off a certain amount of exhaust gas around the turbine blades and through a secondary path into the exhaust pipe. A typical wastegate comprises a spring-loaded valve connected to manifold pressure. When the boost pressure reaches that matched by the spring, the valve opens slightly allowing exhaust gas to be diverted. As engine speed grows, the valve is opened further as the boost pressure attempts to rise, keeping boost at a constant level.
Many turbo systems utilize what is called a internal wastegate. This system employs a secondary channel inside the turbine housing just before the turbine chamber. A hinged flapper (literally, a door) sits inside the channel and is operated by a lever on the outside of the housing. Opening the flapper allows some of the gas to escape through this channel into the exhaust before it reaches the turbine chamber. The flapper is opened and closed by spring-activated actuator, usually mounted to the compressor side of the turbo and connected to the lever by a thin metal arm. This actuator is connected to manifold pressure.
Internal wastegate systems tend to be inefficient due to space constraints inside the turbine housing, mechanical advantage of the actuator arm, or just because too much exhaust gas is trying to go through a small wastegate passage. In many high-performance applications an external wastegate is used. These devices comprise a valve and actuator in an enclosure similar to a blow off valve. The inlet side of the gate is often attached directly to the collector at the exhaust manifold where it feeds the turbine chamber, or to another point in the turbo system before the turbine itself. External gates usually have far bigger openings allowing for better exhaust gas diversion, and don't rely on an actuator arm to open the valve. Most external gate setups require custom manifold fabrication or a little modifying of stock parts, and the gates themselves are more expensive.
Most turbocharged production cars used internally-gated turbos. Enthusiasts that go to larger turbos on their cars sometimes go with external gates due to what is called boost creep, or the inability of the wastegate to keep the boost pressure constant with engine speed as it fails to divert sufficient exhaust gas at higher RPMs.