On modern turbocharged gasoline engines a wastegate is almost always required, unless you are building a drag car with a built motor that isn’t too worried about regulating boost, and has a turbo that is properly sized to reach peak boost at redline.
The reason behind this is because of how a turbocharger functions. A turbo is driven by the engine exhaust gases via a turbine. This turbine housing is also connected to the turbo compressor housing on the intake side, which means that the turbo’s compressor wheel speed (boost) is directly related to exhaust gas volume and velocity. Without a wastegate a turbo can easily end up in a runaway boost situation at higher loads and over spin the turbine wheel, destroying the turbo and possibly your engine.
A wastegate is a pneumatic actuator that is mounted as close to the turbine housing inlet as possible and is connected to a vacuum line that can reference boost pressure. Once boost pressure is greater than the preset spring pressure inside the wastegate, the diaphragm will begin to open. This will allow a controlled amount of exhaust gasses to bypass the turbine wheel, regulating boost pressure and turbine speed.
An internal wastegate is most common on factory equipped turbochargers and stock frame aftermarket turbos because it improves emissions by forcing all of the exhaust gases through the catalytic converter. This design is also much more compact and has the wastegate port located directly inside the turbine housing. Due to the limited space within the turbine housing, the wastegate port diameter is restricted and the overall efficiency of the turbo is reduced. Although, it should be noted that a respectable amount of power can still be made when using this wastegate style in combination with the latest turbo technology.
An external wastegate design is common on racing applications that have high boost targets (35+ PSI) and need to move a lot of air. An external wastegate is mounted independent of the turbocharger, usually being welded into the exhaust pipe as close to the turbo as possible or even directly on the turbine housing inlet. Using this external design is great for running high levels of boost because it gives the exhaust fabricator essentially unlimited options for wastegate port diameter. This means that the external wastegate is custom designed for the motor and its specific flow requirements, providing much better performance than its internal wastegate counterpart.
Another relevant wategate topic that we feel is important to cover is boost controllers. A boost controller is common on almost all turbocharged engines today. The controller is spliced into the vacuum line that the wastegate uses for a boost reference source and acts as a choke to control how much boost the wastegate actually receives, allowing the tuner to reach boost targets that are above the preset wastegate spring pressure. The two main types of boost controllers are mechanical boost controllers and electronic boost controllers.
Mechanical Boost Controller
Mechanical boost controllers are the older version of boost control. It is a gated air valve with a twistable knob that controls the amount of pressure being allowed to the wastegate, making these controllers very easy to use and a “set and forget” kind of tool. It is manually set by the owner or tuner, which means that it can easily be adjusted by an inexperienced user and increase the odds of engine or turbo damage.
Electronic Boost Controller
An electronic boost controller is the latest form available to most builders. It is an electronic air control solenoid or stepper motor that is usually controlled by pulse width modulation and uses a signal from the ECU or an external controller to adjust the wastegate duty cycle. This style of boost controller can be setup to adjust duty cycle using the ECU and a manifold pressure sensor or solely based on RPM or gear. This style allows for much more precise control over the boost curve and turbo spool with the added benefit of boost error correction on most ECU’s.
A wastegate is one of the most important components of a turbo build. If sized incorrectly or forgotten during fabrication, most turbochargers will very quickly destroy themselves and possibly your motor without a way to regulate turbine speed. With the addition of a boost controller a wastegate can become one of the most important components to properly tune; providing great boost response and power control.