Boost Control Plumbing – Get it right, save money.

I’m not sure what the deal is with boost control plumbing/wiring, but it is by far the most common issue I have when tuning turbocharged cars. I get that it’s not something you can test on an un-tuned car, as you’re likely not going to be blasting off into boost to see where (and if) the gate opens. But re-plumbing wastegates isn’t something that you want to be doing while you’re paying for dyno time. So let’s get it right. And to get it right, we’ll make sure you understand exactly how the boost control solenoid works, and how the wastegate you’re using works.

First of all – let’s look at how a boost control solenoid works. As the name implies, the valve is a solenoid. Meaning it mechanically actuates when it receives 12 volts on one side, and ground on the other. Like most solenoids, polarity does not matter. One wire will typically get 12V switched power, shared with other ECU functions, and the other wire will go to an output on the ECU.

In Motec land that would be called an AUX out, or HB out, in Haltech land that would be called a DPO. Most ECU manufacturers call their outputs something different, but you need to ensure you connect the ground side of the solenoid to an output that can be pulsed at a medium frequency (typically ~30hz), that is designed to work for boost control. Some ECUs have mandated pinouts, and some ECUs can only “switch” some outputs versus pulse width modulation, so be sure the pin you’re connecting to is acceptable.

A boost solenoid typically has 3 ports. One port is always connected to one of the other two ports – this is called the “Common” port. This is important – I’ll touch on it later.

The other port will be “NC” or Normally Closed. And the final port will be “NO” or Normally Open. The common port will always pass air through to the solenoid’s NO port when no power is applied, and the NC port will be a dead end. When the valve is energized, the “NC” and common port will have a pathway for air to flow, while the “NO” port will be a dead end.

So that’s all well and good. The next thing you need to figure out, is which ports are which on the boost control solenoid you have. Usually a quick google search will reveal this. On the common blue “MAC” solenoids, which are what the majority of cars use, the port layout is most likely as follows:

Port 1 – Normally Closed
Port 2 – Common
Port 3 – Normally Open

Now that you understand how the valve works, let’s look at how a wastegate works in both an internal and external setup, and how to properly plumb our solenoid. Firstly the simple, internal wastegate setup where the actuator has only one port.

Internal Wastegate:

When pressure is applied to the actuator, it begins to push against a preloaded diaphragm, and at a certain pressure it will overcome the spring preload and crack the wastegate flapper open. At a higher pressure above that, the wastegate actuator arm will be fully extended and the wastegate flap will be fully open. Any pressure applied to the actuator beyond this point will have no effect in reducing boost pressure.

When the wastegate door opens, obviously boost goes down (google how a wastegate works if you need more information on that). When it stays shut, boost will continue to rise (assuming the engine is producing the energy to spool the turbo). The highest boost pressure possible would occur when no signal is given to the wastegate actuator, at which point the wastegate would only open if the back-pressure in the exhaust manifold exceeded the force that the actuator spring preload was applying to the door. Usually at low to medium RPM, this is engine destroying levels of boost even with a factory turbocharger.

Plumbing The Boost Solenoid:

This is the simplest of cases. We want to pass all of the pressure through to the wastegate normally, so boost will be the same as without a boost controller. When we pulse the solenoid, we are reducing the pressure that the wastegate actuator is receiving, so we are increasing the pressure at which the wastegate actuator will open the door. If we give the solenoid permanent power, we have the same situation as if we removed the hose to the actuator completely. In either case – it gets no signal, and boost is as high as possible.

Note: we cannot LOWER boost below the wastegate spring pressure. 

So, we connect the actuator to Common, the intake manifold to “NO” and the “NC” port is either left open to atmosphere, or connected back into the intake post MAF for emissions / correct MAF metering. If the NC port is left open to atmosphere, be sure to mount the solenoid so that excessive moisture cannot enter the port, as this can fail the solenoid over time. A drain spout, or pointing the exhaust port downwards can help ensure the solenoid will drain if any moisture happens to enter.

Why the actuator MUST be connected to common:

If you’re clever, you might be thinking it doesn’t matter which way the boost solenoid is oriented, as long as the C and NO ports are connected to the boost line. When the solenoid is not powered, the wastegate gets full signal, and when the solenoid is powered, the wastegate gets no signal, right? Well… yes but there’s more to it. The reason the actuator MUST be connected to the common, is so that it can vent to atmosphere when the power is applied to the solenoid, dropping pressure in the wastegate and causing it to raise the boost. If we don’t have the actuator plumbed to common this is what happens (THIS IS BAD, DON’T PLUMB IT UP THIS WAY):

1. Boost from the intake manifold (8psi) is applied to the wastegate actuator passing through the “C” port and then through the “NO” port, finally into the wastegate actuator.
2. We now want to raise the boost, so give the actuator less signal. The actuator currently has 8psi of pressure in it.
3. We energize the solenoid. What should happen is the actuator pressure should drop down, but since it is now connected to a dead end (NC and C are connected straight through, NO is a dead end), the pressure in the actuator stays at 8psi.
4. We continue adding duty cycle, and notice that the boost controller is having no effect.
5. At some point, the duty cycle is so high that the boost skyrockets out of control to what is effectively a disconnected actuator situation. Hope your tuner has a boost cut setup!

External Wastegate (simple):

The external wastegate plumbing is a little bit more complicated, because there are two ways you can plumb an external gate. You can do it the exact same way as the internal wastegate, as shown in the diagram below. When pressure is applied to the bottom of the wastegate the system works exactly the same as pressure applied to the top of an internal wastegate actuator. More pressure pushes the valve open, reducing boost. The NC port can be either left open to atmosphere, or connected back to the intake post MAF. The top port of the wastegate should be left open to atmosphere.

External Wastegate (tee method):

Let’s call this method the “tee” method, as there must be a tee installed from the boost pressure source to ALWAYS give boost pressure to the bottom of the wastegate. The way this setup works is quite simple when you think about it, despite maybe being confusing at first glance:

• The bottom of the wastegate ALWAYS sees boost pressure. If the boost pressure exceeds the wastegate spring pressure, the gate valve will begin to open and lower boost.
• The boost control solenoid’s “NC” port always sees boost pressure. The “C” port of the solenoid connects to the top port of the Wastegate. Since it is “Normally Closed” nothing happens unless the boost solenoid is energized.
• When the boost solenoid is energized, boost pressure is applied, through the solenoid, to the top of the wastegate. This pressure offsets pressure that is being applied to the bottom port, resulting in a reduction of differential pressure acting on the diaphragm. (The diaphram now has force trying to close the valve from the top, that is countering the force to open it that comes in from the bottom) – more pressure in the top, more boost.
• I.E. if there is a 14psi spring in the wastegate, normally 14psi would crack open the gate. If you add 2 psi to the top of the wastegate, then you now need 16psi at the bottom to crack open the wastegate.
• 100% duty on the solenoid will result in zero differential pressure, and the wastegate will act the same as if there were no hoses connected to it top or bottom – in other words the top and bottom of the wastegate will see the same pressure.

The tee method is the preferred method to plumb external wastegates, and while I haven’t tested the response and characteristics of the two boost control methods back to back on the same setup, that is certainly something that I should do to show the differences, if any. In my experience, the simple method works just fine in most cases, and may be preferred if you value simplicity (I know I do).

Common Mistakes:

• Getting the ports wrong on the solenoid is the number one mistake.
• Not removing a “restrictor pill” that is often used with and OEM 2 port boost control system
• Leaving in a tee or plumbing the boost solenoid through a tee like a 2 port boost control system
• Having a wastegate spring that is WAY too high that has no hope in hell of opening at a reasonable boost pressure.
• The tee method is often done entirely incorrectly, with any number of combinations of mistakes.

One last thing before I finish this post. If you’re installing an external wastegate, for the love of god, KNOW WHAT SPRINGS YOU PUT IN THE WASTEGATE. Tuner’s don’t like rolling the dice and having to just guess at when to let off. This is esspecially scary for applications that don’t have ECU’s capable of boost cut (some OEM naturally aspirated based ECUs). We like to be able to start at a safe boost pressure, and move up from there.

Thankfully I haven’t been caught out, but I’ve had my fair share of times when either a plumbing or wastegate problem was trying to set me up for disaster. So please, be prepared and let your tuner know what to expect. Have your boost control system plumbed correctly so that we can save you money and spend our time tuning, not fixing!