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Controlling Boost - Under Pressure

The many paths to boost control, and which one's right for you

Aaron Bonk
Oct 11, 2017
Photographer: GReddy

The whole idea behind why forced induction works has to do with one thing—pressure. The turbo generates pressure, the intake manifold contains that pressure and, as a result, you end up with a whole lot more cylinder pressure. And more cylinder pressure, people, means more horsepower.

But it turns out that that turbo of yours wants to make more pressure than your engine's able to put up with, which means controlling all of that pressure is just as important as generating it. Make too little and you'll hardly realize that turbo's even there. Make too much and the oil pan will become a storage container for parts it was never supposed to see. There's more than one way to tame that boost; read on and decide which one's right for you.

  |   Whatever turbo you've got is determined to make more pressure than your engine's able to handle, which means you somehow controlling all of that pressure is critical. And diverting exhaust gases away from that turbine wheel by way of some sort of wastegate is exactly how it's done.

But first, a word from our lawyers: While turning up the boost can be fun, it can also lead to things like connecting rods playing a Peeping Tom out of the front of a short-block. Adjustments should be made sensibly, with caution and proper tuning, and all under the watchful eye of you and a boost gauge.

EARLY BOOST CONTROL

In a perfect world, you wouldn't need any sort of boost control; you'd just pick a turbo that'd reach its maximum turbine speed and boost pressure by the time that B-series of yours hit its redline. But it's not a perfect world, which means understanding how to tame that boost is just as important as you think.

Restrictors: Early modes of boost control were as simple as they were ridiculous. Here, sticking some sort of restriction right in front of that turbo compressor's inlet or outlet or even the exhaust stream would tame that boost. It's as silly as it sounds—putting a turbo on to increase airflow and then closing it back up part way—and it resulted in all sorts of problems, like increased intake temperatures and exhaust gas temperatures hot enough to start melting valves.

  |   Early modes of boost control consisted of silly things like stuffing restrictors in front of intake or exhaust paths and integrating radiator cap-like pop-off valves onto the system's charge pipe. Early methods of increasing boost pressure were just as prehistoric, the crudest of which was poking a calculated pinhole into the wastegate's boost-reference hose.

Vent valves: Imagine a radiator cap stuck onto that charge pipe of yours and you've just imagined the complexity of a vent valve. Like a radiator cap, vent valves were designed to crack open once a predetermined amount of pressure's been reached. And like restrictors, they were designed to keep boost in check and not to increase it.

IT STARTS WITH THE WASTEGATE

Internal wastegates: As it turns out, those old-timers with their exhaust restrictors were on to something, but boost control didn't get serious until somebody smarter than them developed the wastegate. It's the wastegate's bypass valve that controls exhaust gas flow, which, in turn, determines boost pressure. Here, a small flapper valve located inside the turbine housing opens and closes, determining exhaust gas volume and, ultimately, managing boost. The whole thing's leveraged by a swing arm and an actuator diaphragm that's typically connected to the turbo's compressor housing. Once that diaphragm recognizes a certain amount of boost, it goes into action.

  |   Proper boost management doesn't get simpler than the internal wastegate. Here, a swing arm that's activated by a boost-referenced actuator opens and closes a small flapper valve located inside the turbine housing. When opened, exhaust gases are diverted away from the turbine wheel, slowing it down and limiting boost pressure.
  |   Here you can see the swing arm's lever on the right and the flapper valve just below it. Once opened, the flapper valve diverts exhaust gases away from the turbine wheel and instead toward the downpipe.

External wastegates: External wastegates have the same job as internal ones but are able to do it a whole lot more efficiently. Their external placement means there aren't as many valve-size constraints, which means boost can be controlled better. They also don't have to route those wasted exhaust gases back into the exhaust stream, disrupting the flow pattern. Placing an external wastegate in the right spot is key, though; if each cylinder's exhaust gas pulses don't reach the wastegate with the same amount of effort, things won't work all that well. Where the wastegate gets its boost reference signal from is also important since boost varies throughout the system. Generally speaking, hooking it up to the turbo's compressor is a good choice since, as opposed to the intake manifold or the charge piping, it offers optimum wastegate control. And bigger isn't always better, either, when it comes to wastegates. The more boost there is and the more power that's made, the less important it is to drive away those excess exhaust gases, which means something with a smaller valve might be exactly what you need.

  |   Some of the first external wastegates were used in Formula One and other realms of professional motorsports. The advantage here was more precise boost control and the ability to move larger volumes of exhaust gases away from the turbine wheel.
  |   Unlike internal wastegates and their flapper valves, external wastegates are made up of poppet-style valves, not unlike what's in your cylinder head. Here, the valves move up or down, creating an airtight seal or an opening for exhaust gases.
  |   Early versions of consumer-minded external wastegates weren't terribly complex. This early '90s Deltagate from Turbonetics featured a 7-psi spring, a reference port for a boost controller, and an adjustment screw on top that allowed for small changes in boost pressure.
  |   Where that wastegate actuator gets its boost pressure signal from is critical. Get it from the compressor housing, like you see here, and you'll maximize boost control while marginally sacrificing the torque curve since the wastegate opens immediately. Get it from the intake manifold and you'll maximize boost response but at the expense of slightly higher air temps.

FOOLING THE WASTEGATE

Internal adjustments: Most external wastegates come with spring-loaded diaphragms that can be adjusted for small boost changes. Adding shims or a stiffer spring inside are easy ways to increase boost pressure even more.

  |   Many cars like Nissan's R32 GT-R feature a restrictor right from the factory that limits boost. It's OEM boost control, only not the sort of control you're looking for. Here, the restrictor can be removed for a small increase in boost pressure—a good example of how altering what the pressure that the wastegate's actuator sees can affect overall boost pressure.

What they cost:

  • Adjust the spring you've already got for free or swap in a stiffer one for around $40.

Why you want one:

  • We just told you adjusting that spring is free.
  • Shims and stiffer springs are almost free.

Why you don't:

  • Don't expect big changes in boost.
  • You won't be making changes on the fly.

Bleeder-valve controllers: Early methods of fooling the wastegate consisted of little more than creating a controlled leak in the wastegate signal line by way of a small hole or a bleeder valve that you picked up from Sears. Remember, that wastegate won't start doing its thing until it recognizes a certain amount of boost pressure; if you can divert some of the pressure away from it, you've just delayed its opening up. For example, if your wastegate's got a spring that limits boost to 7 psi and you're looking to double that, it's the boost controller's job to bleed off enough pressure from that signal line to delay that wastegate valve from opening up until 14 psi has been reached. Bleed off 7 psi worth of pressure and, despite the 14 psi you just hit, that wastegate still thinks you've only reached 7 psi. More sophisticated bleeder valves are made up of a restrictor that makes these sort of valves much more accurate than what you got from Sears.

What they cost:

  • Make one yourself for about $10 or get something legit for about the same price as a new wastegate spring.

Why you want one:

  • You can make one yourself.
  • It's one of the cheapest forms of boost management that you can trust.

Why you don't:

  • You just made a boost controller out of spare air compressor parts.
  • You still won't be making any on-the-fly boost adjustments.

Ball-and-spring controllers: Later, more sophisticated ways of fooling that wastegate were introduced, like manual boost controllers that, internally, were made up of a spring-loaded steel ball that created its own controlled leak between the boost source and the wastegate. Boost controllers like these still bleed off boost pressure but are known for their ability to increase lower-end performance and turbo spool-up since they're able to hold that wastegate valve shut until the last moment. Adjustments can be made by cranking on a knob that stiffens the spring, which means more boost will be required to move that ball out of the way.

  |   Manual boost controllers like these are known for their simplicity and ability to crack open the wastegate's control valve at the last possible moment. Their main disadvantage is they don't offer on-the-fly adjustability and that finding that target boost level is a process of trial and error.

What they cost:

  • Plan on spending a few bucks more than that homemade bleeder-valve controller you just made.

Why you want one:

  • Quick and precise boost adjustments.
  • Keeps that wastegate closed longer for better turbo spool-up.

Why you don't:

  • Can lead to boost spikes.
  • Setting the desired boost level's a game of trial and error.

Electronic controllers: More sophisticated electronic boost controllers, like GReddy's longstanding Profec, are based off of an electric solenoid that acts like that ball-and-spring controller, only this one's controlled by electrical current. Here, the solenoid's held shut until the controller and its software tells it to open up. Electronic boost controllers can trigger boost changes based on gear changes, engine speed, time, or the flick of a switch, and they can even yield impressive gains without even raising the boost.

  |   Electronic boost controllers like GReddy's Profec allow for boost changes at just about any time and include features that can improve power without changing boost. That's because of how they're able to better control the wastegate's control valve, stopping precious exhaust gases from being diverted away until they absolutely have to be.
  |   It's the electronic boost controller's high-speed, electric solenoids that allow them to work their magic. No manual boost controller can react as quickly as anything driven by one of these.

What they cost:

  • Spend as little as $100 on something simple or as much as several hundred dollars for something with all sorts of features like GReddy's Profec, which goes for about $430.

Why you want one:

  • Control boost from inside the car.
  • Manipulate boost response and spool-up time.

Why you don't:

  • It can cost more than your turbo.
  • It's harder to set up than that bleeder valve you rigged up.

WHY YOU MIGHT NOT NEED ANYTHING AT ALL

The supercharger exception: This whole time we've been going on about turbos and boost control and you just want to know how to hit 12 psi with your blower. Unlike turbochargers that are driven by exhaust fumes, superchargers are driven directly off of the crankshaft. Here, a belt spans from the engine's crankshaft pulley to the supercharger's drive pulley. Change the size of either of those pulleys and you've just changed how much boost that supercharger will make.

  |   Boost management by way of conventional wastegates and boost controllers don't apply to every form of forced induction. Superchargers, for instance, rely on pulley diameter to determine boost pressure, and newer engines with electronic wastegates don't need any sort of added controllers at all since it's all managed within the car's ECU.

Electronic wastegates: Get yourself something newfangled and already turbocharged from the factory and, chances are, it might have an electronic wastegate. Here, electrical current activates the wastegate's actuator arm instead of anything pneumatic, which means that, as technology continues to get smarter, boost control will happen right at the ECU. The whole thing allows for better wastegate control over a wider range of engine speeds and load conditions, resulting in improved performance and better emissions.

Aftermarket ECUs and boost control: In some cases, boost control is already happening at the ECU. Many aftermarket stand-alone systems, like those from AEM or even Hondata's systems, feature integrated boost control. The whole thing works similar to an electronic boost controller in which an electric solenoid is used but the interface takes place within the system's software.

Sources

GReddy Performance Products
Irvine, CA 92618
949-588-8300
By Aaron Bonk
383 Articles

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