A turbocharger increases its airflow rate much faster than an internal combustion engine can adapt to the increase in airflow. If left unchecked, the turbo will almost instantaneously produce extremely high boost pressures with catastrophic results on the engine. To prevent this catastrophe, we need to regulate the boost pressure that the turbo can produce so as to prevent overboosting. This is where the wastegate and Blow-Off valve (BOV) comes in.
A wastegate is a mechanical device that controls boost pressure by regulating the exhaust gas energy that flows around the turbine-wheel by bleeding off excess exhaust gas energy. In so doing the wastegate controls the speed at which the turbine-wheel can spins. The turbine-wheel drives the compressor-wheel, which in turn produces boost pressure. The wastegate is held shut by a spring. As boost builds, the wastegate actuator diaphragm pushes the wastegate open against the spring. The size of the diaphragm and strength of the spring determine how much boost is required to open the wastegate.
There are two types of wastegates on the market: integral wastegates that are built into the turbo; and remote wastegates that are integrated into the exhaust system ahead of the turbo. The remote wastegate allows for a bigger valve and a smoother flow path which provides better boost control and produces better performance. A remote wastegate can also be controlled by a manual boost controller or by an electronic boost controller. The manual boost controller begins opening gradually as the boost pressure builds up. This affects the efficiency of the turbo system and increases turbo lag. An electronic boost controller, on the other hand, can be programmed to open at a preset boost pressure and can even be programmed for increased boost under certain circumstances. Needless to say, the electronic boost controller is the way to go!
The exhaust feed for the remote wastegate should be integrated into the exhaust header so that it is exposed to as much of the pressure in the exhaust system as possible. This means that the pipe for the wastegate should be connected at or after the collector where all the primary exhaust pipes join together, or after the last exhaust port on a log-type header. Also, the wastegate should be located at an angle that does not restrict exhaust gas flow to the wastegate. The exhaust gas must be able to flow to the wastegate so that the wastegate can detect the correct exhaust pressure in the system. A Y-pipe that provides symmetry and easy flow paths would be ideal.
The exhaust gas flow from the wastegate into the tailpipe should also not interfere with the gas flow from the turbine. Any interference will increase back pressure! For best performance, the pipe that feeds exhaust gas from the wastegate to the tailpipe should be at least 18 inches long. A completely separate tailpipe for the wastegate exhaust gas would be even better as this provides the best wastegate response and the lowest back pressure. When going this route remember to allow for tailpipe expansion as the tailpipe from the wastegate will experience fluctuations in temperature. However, in most road cars legislation requires that your wastegate feed back into the main exhaust system ahead of the catalytic converter.
Whether you use an electronic boost controller or not, you should have an emergency boost control device in case the wastegate fails. This device can be a simple vent valve or a boost-sensitive electronic switch that cuts the fuel supply. A boost-sensitive electronic switch that cuts the fuel supply can be set to 1 or 2 psi above the wastegate setting. Should the wastegate fail, the cut-out switch will stop the fuel supply, which will cause the boost pressure to drop.
The Blow-Off Valve (BOV)
A GReddy Blow-Off Valve
The Blow-Off valve (BOV), or compressor bypass valve, performs a related function on the air inlet side of the turbocharger. The purpose of the BOV is to relieve intake pressure and to reduce compressor surge that occurs when the throttle is opened and closed suddenly. When the throttle is closed suddenly, the airflow in the intake is greatly reduced, which causes airflow and pressure fluctuations. These fluctuations cause compressor surge which places high loads on the compressor wheel and the thrust bearing. Left unchecked, this will lead to thrust bearing failure and is where the BOV comes in.
The Blow-Off valve (BOV) should located between the compressor outlet and the throttle body, or between the intercooler and throttle body if and intercooler is fitted. It uses the manifold pressure signal and spring force to detect boost pressure fluctuations in the air intake that is caused by the sudden closure if the throttle. When it detects pressure fluctuations, the BOV vents the boost pressure to atmosphere to reduce compressor surge.