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Thread: The basics of boost control, wastegates and how to turn up the boost.

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2009-03-24 11:19:48
The basics of boost control, wastegates and how to turn up the boost.
It was brought to my attention that we don't have anything on the forum describing boost control, how wastegates work, and how to "turn up" the boost. If someone searches for this info, they shouldn't come up blank anymore. Also, there are tons and tons of myths when it comes to boost control especially comparing manual to electronic. I want to shed some light on those as well. I wrote a lot of this during a debate on a BMW forum and copied it over so it might not flow as well as I'm used to. I'll work on it.

How common wastegate actuators work.

Here is an external wastegate for reference.

Internal wastegates have external actuators, so there's nothing fundamentally different about them that makes one different from the other when it comes down to the actuator. Internal wastgate actuators have the same diaphragm/piston/spring layout that external wastegates do.
The major difference being, very few internal wastegate actuators you're going to run into will be serviceable, meaning the spring can not be changed. External wastegates by-and-large have a serviceable design. This means you can open it with common tools, and change the spring inside if you'd like. This spring is what holds the wastegate closed during operation. You have to fight against this spring with positive air pressure from the "compressor reference port" to get the wastegate to open. When the wastegate opens, exhaust gases bypass the turbine and boost is limited. Simple enough.
Typical internal wastegates do not even have a "boost controller port" and this port is usually left open (to atmospheric) on external wastegates. It is commonly used with electronic boost controller setups, but I'll get to that later.

The wastegate spring.

This spring comes in different flavors. Most factory wastegates are internal, with their springs set anywhere from 6-12 psi. Some, like the newer Mitsubishi Evolutions have 14.5 psi or higher springs (very high for a factory car). In turbo Porsche cars, they tend to use external wastegates, again with the springs ranging from 6-12 psi depending on year and model.

Simplifying things a little bit, the lowest amount of psi you can run depends on the wastegate spring. If it takes 6 psi to compress the spring, you can't make less boost than that because you're going to be using the pressure from the manifold to work against the spring. As you start to make 6 psi to the manifold (and the engine) your wastegate actuator will see 6 psi, and start to open the wastegate (it happens earlier than this, but don't worry about that). Now that the wastegate is open, some exhaust gases are "wasted" and not used to spin the turbo. This limits boost pressure to the amount dictated by the spring (provided you have enough flow in your wastegate system).

Controlling boost with the wastegate spring.

One, expensive, complicated, limiting way of raising the amount of pressure the turbo outputs involves swapping the low pressure spring in an external wastegate to a higher pressure spring. This gets expensive and time consuming if you're looking to move from low pressure to high pressure in stages as most tuners like to tune in stages. Once the high pressure spring is in, there's no simple/quick way to run lower boost pressure either (valet mode, etc).
All in all, it's a pain in my opinion, and most people don't do it this way. They get the lowest pressure spring they can (to allow for low boost) or they just work with what ever comes with the wastegate.

Traditional boost control theory.

Here's where the other methods of boost control come in. You can fool the wastegate actuator into opening at higher air pressures than the spring is designed for by controlling the pressure signal the actuator sees (connected to the "compressor reference port"). This can be done many different ways regardless of internal/external design. There are three main ways of doing this.

Bleed method.

Firstly, you can "bleed" off some of the signal to the wastegate with a controlled leak in the pressure line going to the actuator. This method is simple and marginally effective and usually overlooked as the next example is better and no harder to implement. The bleed can even be an adjustable valve that is controlled inside the cabin. "Dial-a-boost" they like to call it. This is one method of manual boost control. Many say manual boost controllers cannot be controlled from inside the cabin, which is untrue. This method can be employed typically for $15.

Traditional manual boost controller.

The next method involves a one-way ball-and-spring check valve. This is what is typically referred to as a "manual boost controller". This is placed in the signal line. The device has an adjustable spring inside which takes over the job of the original actuator spring. Set the pre-load on the spring to what ever psi you'd like and the wastegate actuator won't see any pressure until that level is reached. This method is simple, cheap, and marginally configurable (adjustment can also be routed into the cabin for control from inside the car). Even set at the same psi as the original wastegate spring, this method gives quicker spool than normal, as it doesn't allow the wastegate to "creep" open before the desired boost level is met, causing full boost to come on quite a bit sooner.
Many people believe this method is flawed or unreliable, causing dreaded boost spikes and the like. While I don't doubt others have experienced this, I know with a properly set-up turbo system, there should be no problems. I've never experienced boost spikes on the 6+ turbo systems I've helped with or friend's of mine have, all using this method. My experience ranges from tiny, internally wastegated Mitsubishi turbos to large gt35 turbos, all on 2.7 liter or smaller engines.
This method can be employed typically for $25 if you build your own, buy an inexpensive unit, or around $80 if you buy a fancy (but no better functioning) one.

Traditional electronic boost control.

The third main method involves controlling the pressure signal to the wastegate with an electronic valve, controlled by an electronic controller of some type. This typically gives complete control over the operation of the valve and thus the wastegate actuation, but requires a bit of set-up time and quite higher start-up cost as the electronically activated valve runs about $50 and the controller itself can range from $150?-$600 or so. Engine control units typically have this control functionality built in, making this cost a non-issue for some.
With this method, you can control how quickly, or how slowly boost comes on. Allowing for boost to build progressively through the rev range if desired, and allows for the control over the maximum boost depending on what gear you're in (if your controller has that functionality). There is no limit to what you can do with this set-up, paired with a good controller (except you can't run less boost than what your wastegate actuator spring will allow). However, most find this method overkill, or unneeded for their goals.

Hybrid boost control.

Finally, you can mix and match any and all of the manual/eletric control methods to create your own home-brew system that provides a startlingly high amount of control.

If you'd like to see great, configurable examples of what can be done with manual boost control, I'd recommend these terrific articles by the late Gus Mahon:


boost control
Last edited by BenFenner on 2012-05-24 at 22-59-16.
2009-03-24 11:21:58
How the traditional manual boost controller allows for quicker boost onset.

Assume a system designed for 14 psi. Assume the wastegate creeps from 7 to 14 psi (typical for internal wastegates, as slack in the actuator exists). Assuming creep from 0 to 14 psi can be left for another day.

Wastegate system without any controller: The intake charge pressure (boost) works against the spring that is holding the wastegate shut. As the pressure rises the wastegate creeps open proportionally. There's a point where everything reaches equilibrium, but there is a relatively long amount of time that the wastegate is creeping open. This is bad for quick boost onset.

Wastegate system with typical ball and spring check valve (manual boost controller) installed inline: The intake charge pressure (boost) pressure works against the spring in the check valve this time creeping it open as it did before on the wastegate, however there are two things to note here. The amount of force the spring sees is based on the surface area of the ball in the check valve. Pounds per square inch, remember? So presumably the ball is much smaller than the entire diaphragm in the wastegate. This allows for a higher degree of control over how much pressure it takes to open it. Regardless, the real difference comes when you realize there will be a pressure differential between the intake charge pressure (boost) and the pressure seen after the partially open check valve. As the check valve is creeping open there may be 10 psi on the manifold side, and 3 psi on the wastegate side. This 3 psi is not even enough to begin to open the wastegate. Soon you're at 12 psi and the wastegate may only see 7 psi, still not enough. Next you're at 14 psi, the check valve is fully open, and now the wastegate sees 14 psi, and fully opens as well. This is my explanation from what I've seen in practice. For what ever reason manual boost controllers allow for just as quick a spool as perfectly tuned electronic boost controllers. Don't know why for sure, but that's how I see it.
Last edited by BenFenner on 2012-05-18 at 20-43-40.
2009-03-24 11:22:00
Traditional electronic boost control. (Expanded edition)

I barely scratched the surface of electronic boost control in the explanation in the first post so I'm going to expand on that here and provide some diagrams to help out.
There are three typical methods of connecting the electronic solenoid.

Example #1

In the first example the solenoid is plumbed in such a way that it acts as a valve that is open until you close it with an electrical signal. Close the solenoid with an electrical source and the wastegate doesn't see pressure and stays closed. Allow the solenoid to open and the wastegate sees pressure and opens. This control method does not absolutely require a "controller" and can be operated by a pressure switch.
This method is fail-safe. If the solenoid or control method fails the solenoid will not block flow to the wastegate and the system will not prevent uncontrolled boost levels.
If you connect the boost source to the other solenoid input, things will work that way as well, but it will not be fail-safe.
This method can allow boost onset to happen faster than normal as it prevents the wastegate from creeping open early.

Example #2

In the second example the solenoid acts as a pressure bleed in the system. The amount of pressure bled off will need to be controlled by a tunable "controller" to maintain the desired boost level. With this method, boost onset will happen at normal speeds. This is the common method used with wastegate actuators that have a single input (typical internal wastegate actuators).
This method is fail-safe. If the solenoid or control method fails the solenoid will close the bleed and the boost level will return to that dictated by the wastegate spring pressure.

Example #3

The third example is the most commonly used and recommended method for wastegate actuators with two inputs (almost all external wastegates and some internal wastegate actuators). The solenoid, when at rest (fully closed) in this configuration blocks flow to the top of the wastegate so no pressure reaches the top. This means the full boost pressure will work against the bottom of the diaphragm and the system will maintain a boost level based on the wastegate spring. If the solenoid is opened (fully) with an electrical signal then the full pressure acts against the top of the diaphragm keeping the wastegate closed no matter what. With this situation, the wastegate spring is easily able to hold the wastegate closed with the added pressure on top so the wastegate will never open. This would allow for uncontrolled boost.
What happens then, is that the solenoid is opened and closed very quickly (60-250 Hz or there about) and it is tuned to spend more time in one position versus another based on your needs. This creates a situation where you can vary the amount of pressure provided to the top of the wastegate to "help" the spring. If you have a 7 psi wastegate spring and you want to run 8 psi with this method, the controller needs to open and close the solenoid in such a way that only a little bit of pressure makes it through to the top of the wastegate. If you want to run 30 psi the controller needs to spend a lot more time "open" to allow more pressure to the top of the wastegate to "help" the spring keep the wastegate shut against the pressure.
This method is fail-safe. If the solenoid or control method fails the solenoid will close the pathway to the top of the wastegate and the boost level will return to that dictated by the wastegate spring pressure.

There are many other ways you can design an electronic boost control system with combinations of controllers, solenoids, switches and different plumbing techniques, but these are the most common from what I've seen.
Last edited by BenFenner on 2015-08-20 at 20-02-49.
2009-03-24 11:23:00
Where should I get my wastegate boost source from?

This is a common question so it is time to go over the options.

First off, you only want the wastegate to see positive pressure (boost). Under no circumstances should the wastegate ever see negative pressure or vacuum unless you're a mad scientist doing something crazy in which case you don't need my help.

There are two reasons for this.

1) Allowing a diaphragm-type wastegate to see boost and vacuum in quick succession or vacuum at all will wreak havoc on the diaphragm. The diaphragm is not designed to see vacuum or cycle between vacuum and boost. I'm not sure how much this shortens the life of the diaphragm, but the wear is significant.

2) Even if you don't have a diaphragm-type wastegate, there is a more important reason not to let the wastegate see a vacuum signal. If you use a combination boost/vacuum source then there will be situations where you're trying to limit boost to the engine with the throttle plate but the turbo will be forced to create more and more boost.

Think about what happens during a WOT shift.
You have boost to the wastegate and it is fully open as you approach the redline. Then you lift off of the throttle which instantly pressurizes the system from the compressor to the throttle body. Hopefully your BOV or recirculation valve is now opening to prevent compressor surge. It is at this time that you want the turbo to stop creating boost. As the pressure in the intake system drops from the spike it experienced at throttle closing the wastegate should be open to reduce the chance of adding additional pressure to the charge pipes. If you have the wastegate plumbed to the intake manifold it will see vacuum this entire time and stay securely closed, promoting more boost in the charge system at the very moment you're trying to get rid of it.
On some systems this can cause minor drivability issues and on other systems this can cause severe on/off throttle behavior and reduce your partial throttle control to almost nil.
The worst situations happen during partial throttle closing situations. You're closing the throttle trying to reduce power output and creating a vacuum in the intake manifold, meanwhile the wastegate forced closed by the vacuum signal it is getting from the manifold so it tries to force more boost into the system. Transient throttle response gets quite messed up.

Now that we've decided the boost source for the wastegate should never go to vacuum, the next question is this. Which is a better boost source for the wastegate, near the compressor housing or near the throttle body?

That's a complicated question with a complicated answer. Here are two things to read on that subject that cover mostly different things.

Originally Posted by BenFenner
I would not do it. Personally, I do not like boost sources at the compressor. They make sense if you're trying to make that vacuum hose as short as possible for aesthetics or weight (distance from compressor port to internal wastegate actuator is usually extremely short). They also make sense if you're trying to run the absolute lowest possible boost you can, or try to combat very minor boost creep. In those instances it can also make sense.

For me, I like my boost source from right before the throttle body plate (usually a port ON the throttle body itself). That means the wastegate and MAP sensor (if you have one) and boost gauge (if you have one) are all getting the same signal, and are all getting the most important pressure info in my opinion, which is what the intake manifold is seeing, not what anything else is seeing closer to the turbo.

Originally Posted by Chriscar
A good thread on the subject - Boost sag? No, you don't need EBC. - Miata Turbo Forum - Home to the Turbo Miata

Last edited by BenFenner on 2015-08-20 at 20-03-30.
2009-03-24 11:24:00
Pitfalls of some boost control methods.

Boost control methods that work by limiting the pressure to the bottom of the wastegate diaphragm and don't add any pressure to the top of the wastegate diaphragm are limited in the maximum boost they can achieve. Examples of this method are the bleed method, and the tradition manual boost control method using only a ball-and-spring check valve.

Often you'll hear people say things like "manual boost controllers can't make more boost than twice the wastegate spring pressure". This isn't exactly true. There are manual boost control methods that do not have this limitation. There are many variables to take into account, but the claim does have some basis in fact.

There is a boost pressure amount that correlates to high enough pressure in the turbo exhaust manifold that the pressure in the manifold will push the wastegate open on it's own. This is the crux of the problem. Usually this happens on external wastegates somewhere around twice the boost pressure that the wastegate spring can hold on its own (because the diaphragm has usually about twice the surface area of the wastegate valve). So if the wastegate spring on its own would produce 6 psi, the max boost obtainable by some methods will be around 12-14 psi. This is because the pressure in the exhaust manifold overcomes the spring pressure and forces the wastegate open. This isn't true for all setups, but it's a general rule of thumb to go by.
Internal wastegates are a little different because their diaphragms are usually 3 or 4 times the surface area of the valve seat. And they have a lever that makes it easier for the wastegate actuator to open the wastegate flap, but it makes it hard for the exhaust manifold pressure to force open the wastegate so usually you can make over four times as much boost on an internal wastegate instead of only two times.
Last edited by BenFenner on 2015-08-20 at 20-05-31.
2009-03-24 11:25:00
Vadim clued me into this type of manual boost control for those with dual reference (two input) wastegates.

The intake charge pressure works against the wastegate spring like normal, but some of that pressure is redirected to the top of the diaphragm to help the spring. This makes the pressure on the top side of the diaphragm higher, so it will take more boost pressure below it to open. If you set the pressure regulator for 1 psi, it will add 1 psi of helping pressure to the spring so it should take 1 psi more to open the wastegate than normal. Set the pressure regulator higher to obtain higher boost levels.

It's a really elegant solution that helps retain a stock-like wastegate opening speed (slow) if that is something you desire.

Edit: I've been told this is the way some older turbo Porsches are set up, even having the regulator dial driver-accessible for quick boost level changes on the fly.
Last edited by BenFenner on 2015-08-20 at 20-06-00.
2009-03-24 14:26:22
Really good information, sticking. I liked the boost control links, giving me ideas on how I should hook mine up for
2009-03-30 23:19:09
Bleed method.

Firstly, you can "bleed" off some of the signal to the wastegate with a controlled leak in the pressure line going to the actuator. This method is simple and marginally effective and usually overlooked as the next example is better and no harder to implement. The bleed can even be an adjustable valve that is controlled inside the cabin. "Dial-a-boost" they like to call it. This is one method of manual boost control. Many say manual boost controllers cannot be controlled from inside the cabin, which is untrue. This method can be employed typically for $15.

I would like to add that this method of boost control, while it works, is a very poor way of increasing boost. You are making a boost leak in order to lower the pressure that the wastegate sees. This gives the turbo a very pronounced "laggy" and unresponsive feeling. Not to mention the loss of compressor effeciency and other complications you have by utilizing this boost contol.

These types of boost controllers are also very inconsistent as temperature changes. You may set your boost to 14psi when it is 40* outside, only to find out that you are boosting 19psi when the temp reaches 100*! This is not a good combination! This will require constant adjustments to regulate boost, and it is a really big PITA when you set the boost pressure at the track, only to experience pressure changes when temps change outside.
2009-10-08 19:02:55
Very nice thread.
2010-01-13 05:32:26
Figured I'd add this as it is a little different than manual and electronic, it incorporates the best of both worlds. Works like a manual bc but has a solenoid to raise boost from a switch. Hass autoworks. Switch off, stock boost (w/e your spring is), switch on whatever your manual bc is set at. Very simple and very effective.

The side of the solenoid that has one fitting needs to go to a vacuum source. The side with both fittings will go to your wastegate. Both wires (1 power, 1 ground), interrupt the ground wire with the switch. That will be a lot easier for mounting positions so you can run the ground wire inside the vehicle and mount the switch wherever you decide.
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