June 2, 2016 Products

5 ‘Facts’ About External Wastegates That Are Wrong

The External Wastegate

External Wastegates are one of the most commonly misunderstood pieces of the turbocharging puzzle. These devices are responsible for regulating boost by allowing exhaust gasses to bypass the turbine of the turbocharger, making sure the compressor does not spin too quickly and produce too much boost.

However, there are a lot of myths and misconceptions floating around, so we picked five of our favourites that we have heard, and let our engineers bust the myths.

1. You don’t need a wastegate.

Well, if your aim is to destroy your turbo and/or blow up your engine, then yes, you don’t need a wastegate! While in theory, you could run a turbo system without a wastegate by carefully choosing a turbo that will only reach its maximum turbine speed and desired boost pressure at the engine’s max RPM, it’s really not practical in the real world.

2. The more powerful the engine, the bigger wastegate I need.

False. A wastegate is possibly the only component in your whole engine package that can actually be made smaller as you increase your boost/horsepower output – in certain circumstances. An external wastegate is used to drive exhaust gasses away from the turbocharger to regulate turbine speeds and therefore boost pressure. Selecting the correct wastegate size depends on the amount of exhaust to be bypassed, the desired boost pressure, and the amount of exhaust gas the turbo requires to reach that boost pressure. An engine that produces a lot of exhaust gas but running low levels of boost will require lots of exhaust to be diverted away from it by the wastegate, requiring a bigger wastegate. Higher boost levels will require less exhaust to be diverted away from the turbine, so the wastegate can be smaller.

3. External Wastegates don’t operate in high temperatures.

External wastegates are usually mounted at the hottest part of the exhaust. It’s the place where all the exhaust gases meet, creating extra heat.

A wastegate should always be mounted somewhere it can cool off, rather than being choked with hot air, surrounded by hot exhaust and engine with no airflow. Access to fresh air helps keep the wastegate at a safe operating temperature, extending its life and reducing the risk of failure.

Even so, Turbosmart’s wastegates are designed and tested to cope with extreme heat. Their carefully selected elastomer, materials and tolerances ensure that they are up to the job and can handle the heat of the most serious applications.

4. External Wastegates are responsible for turbo flutter.

The sound of turbo flutter, or compressor surge, is sometimes referred to as ‘wastegate chatter’ – but this noise has nothing to do with the wastegate. Compressor surge, as the name suggests, occurs on the compressor side of the turbocharger, where outside air is drawn in and compressed for the engine to consume. An external wastegate is used on the turbine side, where exhaust gasses are used to spin the turbine wheel to operate the compressor. That fluttering sound, which typically occurs when the throttle is suddenly closed, is the sound of the turbo trying to push air into the engine but failing, so the compressor blades ‘chop’ through the air. You can read more about this phenomenon here.

5. Wastegate spring pressure is the highest boost level you can reach.

False. Wastegate spring pressure is the lowest boost level you can reach. You can easily increase boost pressure over wastegate spring pressure by using a boost controller, but you can’t achieve a maximum boost level under the wastegate’s spring pressure. If an external wastegate is fitted with springs rated to 7PSI, it will open when it receives 7PSI of pressure to the actuator. Any pressure lower than that will not be able to overcome the spring force, and it will remain closed. * Note: this spring function is explained for a dynamic condition when installed on a vehicle. The spring is designed to include elements of backpressure/airflow – if bench testing a waste-gate you will expect a higher crack pressure.  

Further Reading