FAQ: Boost Control

[KevinD]

This FAQ hopes to address the most common questions surround boost control and boost control systems.

What is the function of a boost controller/boost controller device?
All BCS (Boost Control Systems) are designed to limit boost pressure. Yes, you read that correctly. They all limit boost. As long as exhaust gases are passing through the turbocharger, the compressor wheel will continue to gain speed, thus increasing boost. Without a BCS, that boost will continue to build to the units mechanical maximum, which can be in upwards of 50psi. Most stock BCS systems are managed electronicially, opening and closing the wastegate, utilizing a solenoid. This process of managing the wastegate is known as the Wastegate Duty Cycle (WDC).

What is the Wastegate Duty Cycle (WDC)?
The WDC is essentially the name of the process most electronic boost controllers and stock BCS use to describe the opening and closing of a wastegate. A WDC of 100% means the wastegate is always closed, allowing the turbo charger run up to maximum boost pressure, which can be very high, (>50psi). A WDC of 0% means the wastegate is always open, allowing the turbo charger to run up to the maximum mechanical boost. Which is usually the minium boost pressure required to open the wastegate. Typically around 7-11psi. So by adjusting the WDC, these BCS can easily regulate boost.

How does a BCS manage boost?
To answer this question, first, we must discuss one of the main BCS components, the wastegate. A Wastegate is a vacuum acuated device that regulates boost pressure by opening up a valve, diverting exhaust gas flow away from the exhaust wheel, thereby slowing down the turbo to a consistent speed. BCS devices limit boost by adjusting the amount of vacuum pressure applied to the wastegate. There at 2 main types of wastegates, internal and external.

What is the difference between an internal and external wastegate?
An internal wastegate is "internal" to the turbocharger, meaning the wastegate spring and valve are part of the turbocharger exhaust housing. An external wastegate is a standalone unit, sharing the same functions as the internal unit.

What are the advantages/disadvantages of an internal wastegate?
The main benefit is ease of installation. With an internally gated turbocharger, there is no extra fabrication or exhaust routing needed. Just bolt up and go.

The main disadvantages lie in adjustability. The amount of boost an internal wastegate can manage is directly related to the size of the opening in the exhaust housing. Also, the range of boost is limited to the length of the wastegate actuator arm, as this is connected directly to the wastegate valve and spring. If your desired boost levels are outside of this range, then the wastegate will need to be enlarged, actuator arm will need to be replaced or adjusted, or an external wastegate will be needed.

What are the advantages/disadvantages of an external wastegate?
The main advantage of an external wastegate is adjustability. These units, typically found on very large turbochargers, provide a much larger range of adjustments when compared to an internal wastegate. There is not actuator arm and the size of the valve is very large. Typically, adjustments are performed by altering the spring pressure via small screw on the top of the unit.

The main disadvantage is installation. External wastegates are very large and need to be installed correctly on the exhaust system to function properly. Usually a custom manifold and downpipe assembly are needed. Exhaust routing must be thought out and planned to acheive maximum efficiency.

Which wastegate is right for me?
For most, an internally gated unit will be more than enough. For an external wastegate, your goals should fall in the range below:

a. To Increase Air Flow at High PSI (>25psi)
b. Use of a Larger Turbocharger that lacks an internal wastegate.

I hear external wastegates come in different sizes. Is that true? If so, what are the sizes?
Yes, external wastegates do come in different output sizes. The main player here is TiAL. The most common sizes are 38mm and 44mm.

What size is right for me?
For those running lower boost, the 44mm is a good choice. For higher boost, 38mm.

Are you sure? Why would I want a larger wastegate when running lower boost?
The key here is the function of the wastegate. It's there to limit boost. The less boost you run, the less you need to limit it. So, by using a larger wastgate, you don't have to run a high Wastegate Duty Cycle (WDC). If you want to run higher boost, you'll need to limit the system more. By using a smaller wastegate, you can run a higher WDC and have more control over boost pressure. Basically, lower boost = larger wastegate and higher boost = smaller wastegate.

What types of aftermarket boost controllers are there?
There are 2 main types:

1. Electronic - Uses an electronic solenoid to control whether the wastegate is attached to the intake or atmosphere.
2. Mechanical - Uses a diaphragm/spring to control how much intake pressure is send to the wastegate actuator.

Electronic units are easier to tune and can be more reliable. Mechanical units are typically cheaper and significantly easier to install than electronic boost controllers.

Which type of boost controller is better?
This all depends on your goals. Most people prefer electronic units for their ease of use, ability to store multiple boost maps, and their real time results. Others prefer the simple installation and configuration of a mechanical unit. My personal opinion on this is to use electronic units over mechanical units. It is very easy to adjust a manual boost controller to high, resulting in engine and turbo damage. Electronic units provide a more fine grained range of adjustments and tend to be more consistent in their control.

How hard is it to install a Boost Controller?
Mechanical units simple. Bolt in under the hood, run vacuum lines, tune.

Electronic units typically require routing wires from the cabin to the engine bay to the solenoids. This is not that difficult and can be done in about an hour by a somewhat experienced mechanic. If you are the least bit squeamish about this install, have it done professionally. Costs typically run about $90.

Where can I buy aftermarket boost controllers and wastegates?
Check with any of the NAHGOG Vendors.