This article is a follow up to my posts on how compound turbochargers work and how anti-lag systems work. If you're reading this article I'm going to assume you know the basics of how turbochargers work and just jump right into it.
See How A Wastegate Works
Wastegates & Basics
Since a turbo is spun by the flow of exhaust gases out of an engine, the only limit to how fast the turbocharger can spin is the amount of exhaust you can pump through the turbine. The amount of boost you can make, then, is also only limited by how much exhaust you can push out. Luckily though, turbochargers usually see the maximum boost they're intended to make for a given engine before the exhaust flow peaks. This means that if a turbo was left unregulated it would continue to accelerate and create more boost than your engine can handle. To keep the level of boost under control turbochargers use wastegates. A waste gate is a spring controlled valve attached to the turbo that opens when it sees a certain amount of pressure. This means that a turbo which is designed to provide a maximum of 10psi will have a wastegate valve spring that compresses under the pressure at that boost level. This allows exhaust to flow around the turbine and therefore limits the level of boost to 10psi.
Purpose of Boost Controllers
Wastegates are the limit to the amount of boost a turbo can provide, and as a result, people who are looking to create more boost with the same turbocharger must somehow address the function of a wastegate. In addition, due to the spring controlled valves, wastegates are prone to be partially open before the desired maximum boost level. If the spring sees pressure it will compress at least a little and allow the wastegate to crack open. This issue forces turbos to spool more slowly than ideally possible and this keeps engines from making maximum power throughout the entire RPM band. What a boost controller does is bypass the wastegate in one way or another. By lowering the amount of pressure the wastegate sees you can "fool" the turbocharger into spooling faster and creating more boost. By disconnecting the wastegate before the maximum boost is reached with certain types of boost controllers you can also increase the spool rate of a turbo because no exhaust is being leaked out. There are two primary ways that boost controllers work and these are separated into manual boost controllers and electronic boost controllers.
Manual Boost Controllers
A manual boost controller is a manually controlled valve that diverts air away from the wastegate. The valve on a manual boost controller is much like that on a hose. If you want more or less water (boost) you turn a dial and either open or close the valve by hand. By controlling how much this valve is open you can control how much pressure the wastegate "sees". This means that the turbo will continue to spin until it creates enough pressure to open the wastegate even after some of this pressure is leaked out.
Electronic Boost Controllers
Electronic boost controllers differ from manual boost controllers in that the valve they operate is controlled by an electronic solenoid and computer. Electronic boost controllers completely interrupt the flow of air into the wastegate. This means that the electronic boost controller can make the wastegate see 0 pressure (atmospheric pressure, really) and stay completely closed. This eliminates the issue of wastegates creeping open below the desired maximum boost level. When an electronic boost controller sees the maximum boost level desired it will then reconnect the flow of air to the wastegate and the wastegate valve will open to quickly limit the amount of exhaust flowing through the turbine. As soon as the amount of boost drops back to just below the max level the electronic boost controller will again disconnect the flow to the wastegate. Another advantage to electronic boost controllers is that they can variably limit the amount of boost developed according to other parameters. Since boost controllers are ran by computers that often have other data inputs you can control boost at different RPM levels, temperature levels, air-fuel ratios, gear, or whatever you have a sensor for really. The computer control of electronic boost controllers is far superior to manual boost controllers because it gives full control over when you limit the amount boost created and how much you limit boost levels by. You can limit boost to 5psi in 2nd gear but let it make 10psi in 3rd gear. There are also some applications that will lower the amount of boost allowed relative to temperature. If the engine is running too hot the boost controller will drop the amount of boost being created and allow the engine to cool down, avoiding damage to engine components. Certain boost controllers, like those offered by Gizzmo, also allow overboost or multi-scramble modes to temporarily run higher boost than normal. This means that if your engine safely runs 10 psi, but can handle 15 for short periods of the time, that the boost controller will limit boost to 10 psi until you hit a button and it will then push up to 15 psi for a few seconds. This can easily make the difference between winning and losing in a drag race.
Tuning Electronic Boost Controllers
If you get into using electronic boost controllers there are a few things you might need to know to get everything tuned in perfectly. The two variables that you often change in an electronic boost controller to achieve the desired boost level are the controller's duty cycle and gain. The duty cycle is the percentage of time which the valve is open and gain is used to control how long the valve is left closed. A proper gain setting allows the valve to open just after maximum boost is created. The proper duty cycle is just long enough to let boost drop right below the maximum level once the valve is opened. If you have boost spikes you probably have too high of a gain setting. You really want to set the gain as high as you can WITHOUT seeing sudden spikes in boost. This will let you achieve the fastest spool up as possible. Most electronic boost controllers have intelligent settings that automatically determine duty cycle and gain for you, but it's always best to know how these systems work if you want to make the most power possible.
The steps for actually tuning the duty cycle and gain parameters for an electronic boost controller are listed below. Make sure you ALWAYS start with low duty cycle and gain settings.
- Drive the car in third or fourth gear and turn up the duty cycle until you reach the desired maximum boost level (as seen on a boost gauge or a friend with a laptop in the passenger seat). If you followed the advice above you have a low gain setting and you won't have any boost spikes (sudden jumps in boost). If you do see spikes, make sure your gain is turned down.
- Now drive the car in first or second gear and turn up the gain has high as you can before seeing boost spikes.
Now that you have the ideal duty cycle and gain settings you should be making the most of your electronic boost controller and see considerable gains in performance from your turbocharged engine.