A turbocharger uses either an internal or external wastegate to control the exhaust gas so the turbocharger can maintain a set amount of boost pressure.
With an internal or integral wastegate, the turbine housing of a turbocharger has a built-in flapper that opens and closes through the use of an actuator. The internal is more commonly found on vehicles that came turbocharged from the factory. When the set boost level is reached the flapper vents or redirects the exhaust gasses so they exit through the downpipe and thus attempting to maintain a steady boost level.
The actuator is normally being controlled by an electrical boost solenoid which has vacuum lines that control it. GM part number #1997152 is a popular choice that has been adapted to work on many vehicles either foreign or domestic. Some OEM boost solenoids for Volvo, Audi, and others accomplish the same thing.
One vacuum line typically runs from the intake side of the engine or turbo compressor housing to provide the actual boost pressure which the engine is operating. This line from the intake to the first port on the solenoid is sending positive pressure which is then bled off at the solenoid to deliver this output pressure to the actuator. The actuator is basically a spring inside a canister with a diaphram that is controlled by the pressure sent to it from the solenoid.
With the solenoid disconnected the boost level is determined by the spring in the actuator canister. This is also known as base boost, or the lowest boost setting that the turbo can be ran. A modification to the actuator by placing a stiffer spring will raise the base boost and is usually necessary if the vehicle has been modified to run a much higher than stock boost level.
The external wastegate application is usually an aftermarket product which is not built into the turbocharger. The placement of the external can vary depending on the application and fitment issues. A complete twin turbo kit for a 2005-2008 Corvette C6 has the turbos/wastegates rear mounted because of lack of space under the hood.
External wastegates can be either vented to the atmosphere or redirected so they are sent out the exhaust system. When they are vented to the atmosphere the noise produced is considerably loud and may push the legal limits for noise so be sure that you are aware of potential risks.
When the external wastegate is tied into the exhaust the amount of noise is minimized and similar to the noise level from an internal wastegate. Some people have mistaken a wastegate for a blow off valve and these are two completely different components that serve different purposes. The blow off valve emits a sound that is more of a sharp swoosh, slightly high pitched. The external wastegate vented to the atmosphere is much deeper and not nearly as smooth. A small turn down pipe is typically attached to the wastegate to direct the discharge away from any place that may not take well to heat.
Manual Boost Controllers
When you want to raise the boost, there are many gadgets out there on the market that claim to offer solid boost control. Before you go and spend $500 or so on a fancy electronic boost controller, lets first take a look at what is happening.
As previously mentioned with the integral wastegate, the actuator is used to hold the flapper shut by the use of spring pressure. The preload must be set accurately or there will be a serious lag in building boost or if its set too tight can cause a boost spike and damage an engine. Its extremely important to set the preload properly, whether you are running stock or higher boost levels.
A simple but extremely effective $45 boost controller can provide excellent boost control. The boostvalve is a product that has been proven reliable for years and they offer additional springs to allow higher boost settings.
Integral wastegates can become sticky, where the wastegate flapper becomes stuck or just doesn’t swing freely. This is an important inspection that could be costing you a lot of performance. If the flapper doesn’t open and close smoothly then it may get hung up causing a lag in building boost or boost spikes to overcome the spot where it hangs. A product from GM called heat riser lube can be injected to help loosen up the flapper.
The vacuum lines that run to the boost solenoid may need to be inspected depending on the condition or age of the vehicle. Replacement silicone vacuum lines are usually a good choice.
The last suggestion is to take a close look at your datalog files to see just how stable your boost control is, whether it spikes or drops off. A properly setup wastegate can make all the difference in terms of overall performance from a turbo engine, and you may just need to make a few small adjustments to see dramatic improvements in performance.