Boost Control FAQ

The genesis behind this FAQ is mainly due to the rampant “monkey see-monkey do” approach due to the fairly recent introduction of aftermarket boost control solenoids. While they do play a role in the overall boost control world, the press they have received has gotten way out of hand, so this FAQ will discuss your boost control options.

What is the function of a boost control device? To control boost. While this seems to be an easy definition the reason behind boost control is lost to many people. A boost controller of any type is designed to limit boost. Yes, you read that right….limit boost. Without a boost controller of any kind, your turbo would spin up until it explodes and produces upwards of probably 50PSI. This seems to be the most unknown fact of boost controllers, that they limit boost. Now that you have your mind wrapped around the proper function of a boost control device, you can explore the rest of the story.

What controls boost on my stock turbocharged car? The two main components are the boost control solenoid located on the passenger side strut tower and the internal wastegate on the turbo itself.

What is an internal wastegate?
A spring/diaphragm based mechanism which controls the movement of the wastegate valve. A turbo wastegate is normally closed, forced shut by a compressed spring inside the actuator canister. As pressure is applied to the nipple of the canister, the wastegate shaft moves away from the actuator, swinging open the wastegate valve. When the wastegate opens, this allows air to bypass the turbine inside the turbine housing. Less flow = slower spinning of the turbo, so you can grasp the concept of how this controls boost.

Can I adjust my internal wastegate? Pre-loading the wastegate actuator arm, also known as adjustment of the wastegate actuator rod (if the rod length is not fixed and adjustments can be made) will allow proper calibration and some additional tuning. All IHI turbochargers have a fixed wastegate actuator rod that cannot be adjusted, while all MHI turbochargers have an adjustable wastegate actuator rod. If the rod coming out of the wastegate actuator is shortened, it will pre-load the spring inside the wastegate actuator increasing the pressure level at which the actuator will allow the wastegate valve to open and the total boost pressure that the turbo can generate will increase (as long as the turbo is still within its efficiency range). This pre-load will also limit how far the wastegate valve can open. Pre-loading (shortening) the wastegate actuator rod too much can potentially create a mechanical boost creep issue that cannot be tuned out. If the wastegate actuator rod is lengthened, the actuator will decrease the load on the spring and decrease the pressure level at which the actuator will open and the total boost pressure the turbo can generate will decrease. If the wastegate actuator rod does not put enough pre-load on the wastegate valve, then you could see boost fluctuations of + or – 2psi even when the wastegate solenoid duty cycles are constant. Help to adjust it can be found HERE.

What about a helper spring? A helper spring is a clever, cheap, easy, and removable way of wastegate adjustment as well. Basically its a small spring that stretches from the wastegate door arm to the wastegate itself. This puts more tension on the wastegate flapper door so the actuator has to work harder to open the door. Installation and more helper spring advice can be found HERE.

What is an external wastegate? External wastegates are used for two purposes:
a. To increase the flow when using high PSI
b. Required for larger turbos that have no internal wastegate

Using an external wastegate on an internally gated model will show benefits as it effectively reduces choking of the turbo. Choking is where the flow gets backed up due to the more restrictive internal wastegate. By replacing the internal with an external, this generally allows one to run more boost safely. While every turbo can benefit from an external unit, it really shines on larger turbos.

How can I run an external wastegate? You will need some custom fabrication and parts. Generally speaking you will need an uppipe fitted with a flange to run an external wastegate. Then you need a wastegate suited to your application. You also need to consider plumbing of the wastegate gasses.

External wastegate tuning tips can be found HERE

Where do I plumb my wastegate?
Plumbing is the routing of wastegate gases. You can “vent to atmosphere” which is where the wastegate opens and air is directed straight to the outside air. Downsides to this are the need to route the venting so it won’t heat up anything in the engine bay and the increased noise. If noise is a factor, you might consider having a small motorcycle or 2 stroke muffler welded onto the end of your vent pipe, room permitting. The other, quieter option, is to plumb the external unit back into your downpipe. The key to successfully doing so is to ensure it is plumbed far away from the turbo (at least 18” past the downpipe to turbo flange, and hopefully beyond any catalytic converters in your system) and ensure the external piping is welded to the downpipe at a narrow angle to encourage aft flow vs. say a 90 degree entry that will encourage more back pressure into the exhaust. The downside to a “plumbed back” wastegate is even with a perfectly designed system, it is not as efficient as a vent to atmosphere system.

Which manufacturer of external wastegates is best?
This topic is highly debated, but the general consensus is TiAL is the king of the hill.

Are there different sizes of external wastegates? Yes. 38 mm and 44 mm are the two most talked about and utilized sizes.

Which size is right for me? That is a question for your turbo vendor and tuner as sometimes their advice counters the external wastegate sizing theory. The theory is that a larger wastegate (44 mm) should be utilized on turbos using low boost vs. turbos that are running high boost which should run a smaller wastegate (38 mm). Where did this theory come from? Remember…a wastegate limits boost. If you want to run more boost, you don’t have to limit it as much. Running less boost means you have to limit it more. Think this premise through to get a good mental picture of it as this concept is terribly misunderstood.

Some also erroneously put credibility on the TiAL’s 44 mm model due to its superior V-Band clamp vs. the 38 mm models flat flange design as the decision maker. While the V-Band is an overall better design, the problem with TiAL’s 38 mm model is that it apparently ships with what many consider being a sub-standard gasket. Replacing this gasket to start with or after leaks occur generally solves the issue. Funny how a $3 gasket will convince some to spend $200 more on a 44 mm unit as well as encourage others to do so.

Useful thread that discusses TiAL pros/cons

What about external or internal wastegate springs? Think back to the wastegate sizing question above. Once again, you have a theory that needs to be buttressed by the opinions of your turbo vendor and tuner. The theory is that if you want to run say 15PSI on your turbo, you want to run a spring that is 50% of that value or as close as you can get it for proper boost control.

How do I swap out wastegate springs
External wastegates are serviceable by the end user, so refer to your external wastegate documentation for details as well as ordering information for new size springs. Generally speaking, most internal wastegates need to have the whole assembly swapped out as removing spring, finding the new correct spring, and replacing the spring is not able to be performed. There are a few sources of upgraded internal wastegate solenoids as Deadbolt, AVO, and others sell them.

What is a restrictor pill? This component limits the amount of pressurized air flowing from the turbo compressor housing. The restrictor pill restricts the air flow so the wastegate solenoid valve and wastegate actuator are not overdriven, which would force the wastegate valve to open prematurely.

Can I tune with this restrictor pill?
A smaller diameter hole in the center of the brass restrictor pill will have a higher tendency to create boost spike in the system and require less wastegate duty cycle to run higher boost. The larger the diameter hole in the center of the restrictor pill, the less chance the boost control system will boost spike and greater wastegate duty cycle will need to run in order to produce higher boost. If you have installed a new turbocharger and you are using the stock boost control system to tune boost, please verify that the vacuum line coming off the turbo compressor housing (prior to the T-fitting) contains a restrictor pill with a hole machined in the center of the pill.

The stock boost control system most commonly uses a restrictor pill with a center hole size of 0.040”-0.048” +/- 0.001”

For larger-than-stock turbochargers or turbochargers with a stronger mechanical spring in the wastegate actuator you will need to use a restrictor with a larger center hole, something along 0.050”-0.055” +/- 0.001”

For similar-to-stock-sized turbochargers with a weaker mechanical spring in the wastegate actuator you will need to use a restrictor with a smaller center hole, something along 0.030”-0.040” +/- 0.001”. Be very careful when using a restrictor with a center hole of this size, there is a higher tendency for the system to boost spike and you will need less wastegate duty cycle to run higher boost.

Where do I get new restrictor pills? You can modify your OEM one via obtaining special small drill bits and a special small hand drill to drill it out. One can also obtain suitable sized nitrous jets and then file down the outside housing to fit inside the vacuum line. As well, many tuners have a collection of various sized restrictor pills.

What are aftermarket boost control solenoids?
Perrin and Prodrive make purpose built Subaru unit, though the GM unit may be retrofitted as well. These units mimic the purpose and function of the OEM boost control solenoid.

What makes them so special? Nothing really. They serve the exact same purpose as the factory unit. Since they are slightly different in actuation values, they do require a custom tune. So what has appeared to happen is people with an off the shelf tune or no tune have purchased these. Then they get tuned with them. Then they attribute much of their gains, not on tuning, but on the boost control solenoid. While they can help, this is something to discuss with your tuner as one option for better boost control as 95% of the good/bad of this modification lies in the tuner’s skill vs. the boost control solenoid. The biggest difference in aftermarket units is that they are intercept style vs the OEM bleed style boost control. Intercept style allows for better boost response when compared to the OEM bleed style.

So how does a boost control solenoid work?
It is opened via a pulsed duty cycle based on inputs from the ECU. While you are building boost, the BCS is not operating. This allows the wastegate to remain closed to built boost quickly. As target boost approaches, it ramps up quickly to control/limit the boost. At this point is where the BCS is working the hardest.

More BCS interaction help : Article 1, Article 2

Why is tuning needed on aftermarket ones?
Port sizing and duty cycles can change with them which can lead to wild results. Even tiny changes in the turbo/wastegate associated hose length and diameter can cause drastic changes. Do not use a GM, Perrin, or Prodrive boost control solenoid until you are ready to be tuned.

What about a manual boost controller or electronic boost controller? With proper use and tuning they are other possibilities for controlling boost. These can be utilized as standalones for 02/03 WRX or STI models with judicious use and the proper knowledge. They may also be effective in conjunction with most engine management systems with proper tuning. These need to definately be discussed with your tuner prior to installation though.

Is there a simple, general guide to all these items?
There is now:

Small boost change that require no tuning:

Pressure check your system for leaks. How to do so can be found HERE.

Small boost change that may require tuning:

Adjustment of internal wastegate. There are no hard and fast rules, but if you are only adjusting the wastegate for 1 PSI or so, you can probably safely get away with this. Anything over that and you want to strongly consider a tune.
Helper spring.

Big boost changes that require tuning:

Aftermarket boost control solenoids
Manual/electronic boost controllers
Upgraded internal wastegate actuator
Going to external wastegate
Changing springs inside the external wastegate
Restrictor pill swaps

So what do I need to assist controlling my boost?
More than likely, nothing. Tuning is what ultimately controls your boost using the wastegate and BCS. If tuning doesn’t cure your ills, listen to your tuner’s advice as to what steps to proceed with. Do not surprise your tuner on the day of the tune with an aftermarket boost control solenoid or other fix because Tim in Kansas had “great results” with his. Tuning the boost control system is the hardest and most time consuming thing a tuner can do. You might have the latest “fad” boost control solution on the market, but will it be worth it when you put on a $175 wastegate actuator and it takes you tuner 1 extra hour of his and dyno time to the tune of $300? Consider he might be able to do the same thing in no extra time with a 10 minute swap out of a $2 restrictor pill.

And remember…if you are currently have boost control issues, you should fix them first rather than trying one of the methods described above to band aid fix the issue. For example, if you are only seeing 7 PSI, adding a spring to your wastegate to bring it back up to the normal 14.7 PSI is not the correct way to fix the problem. Troubeshoot and fix the main issue vs. performing a work around.

Where do I buy aftermarket boost control devices? Many NASIOC Vendors offer them.

How hard is it to install aftermarket boost control devices? Most are fairly simple to install and many come with directions that should be referred to and followed.

GM install instructions are HERE, though they would help with Perrin and Prodrive as well.
Another useful BCS install thread can be found via this link.

Editors Note

My thanks to Christian of for his assistance that provided a lot of the information contained within this FAQ. Another wonderfully informative boost control read that is authored by him can be found HERE.

This post was created because I wasn’t able to find a good boost control FAQ. I came up with the text based on LOTS of searching here. Upon reading this you should have an idea of whether a boost control solution best suits your needs. The type/manufacturer is up to you.

If you find an error in this FAQ, please PM me with factual details and I will update this post. Responses such as, “I have XXX’s BCS and it’s great!” or “XXX’s wastegate broke after 1 month” are not appreciated here, that is what the Car Parts Review Forum is for.

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