Boost Control

A wastegate provides a means to control the boost pressure of the engine. There are two configurations of wastegates: internal or external. Both internal and external wastegates provide a means to bypass exhaust flow from the turbine wheel. Bypassing this energy (e.g. exhaust flow) reduces the power driving the turbine wheel to match the power required for a given boost level. Similar to the BOV, the wastegates uses boost pressure and spring force to regulate the flow bypassing the turbine.

Internal wastegates are built into the turbine housing and consist of a “flapper” valve, crank arm, rod end, and pneumatic actuator. It is important to connect this actuator only to boost pressure.

External wastegates are added to the exhaust plumbing on the exhaust manifold. The advantage of external wastegates is that the bypassed flow can be reintroduced into the exhaust stream further downstream of the turbine, reducing turbulence between the wastegated gasses and the normal exhaust flow, and thus flowing more freely. On racing applications, this wastegated exhaust flow can be vented directly to atmosphere.

Not only can you achieve higher boost levels, but by keeping the wastegate closed as long as possible, you will the turbo will spool faster. The wastegate arm (internal) holds a valve shut until it sees boost. Once boost enters the wastegate actuator it slowly opens the valve until the boost level peaks. Keeping the valve closed the longest amount of time possible will allow the exhaust to spin the turbine faster, instead of being bypassed (and thus, 'wasted') into the exhaust.

Manual boost controllers (MBCs) can be had for as little as $5 to as much as $200. They are simple a ball and spring device**. The boost enters the MBC and pushes up on the ball which pushes on the spring. The ball will rise and eventually allow boost to enter the wastegate actuator. They can be adjusted by simply adding or removing tension on the spring with a twist of the knob.

Make one yourself!

Entry level electronic boost controllers (EBCs) are available on the market. These have built in MAP sensors and control a sileneoid. Installation is straightforward; boost can be adjusted on the fly and you'll impress all your friends. Price can be anywhere from $250-600 depending on the unit.

Made by Greddy, Apexi, Etc.

Some aftermarket ECU's have electronic boost control integrated with the ECU. Megasquirt, Hydra, etc.

Problems with boost spikes?

Resident super mod Joe Perez has your fix!

This topic has been coming up every now and then. Specifically, a lot of folks seem to be having boost curves that look like this, and perceive it as justification for the fact that their manual boost controller just isn't cutting it, and they need to go out and spend money on a fancy electronic boost controller.


So, does that look familiar? If so, save your money. Fixing this problem is going to cost you about $3.

First, let's take a look at why this is happening. Here's a simplified diagram of your turbo system, where we have the turbocharger itself, then the intercooler, and then the throttle body.


Point "A" in this system is where a lot of folks have their boost controller connected. It's that nipple that came from the factory on the side of your compressor housing, probably with a hose already attached between it and the wastegate actuator.

Well, that's just stupid.

What's happening here is that your boost controller is in fact maintaining a constant level of boost, however it's doing it in the wrong place. Specifically, it's maintaining a constant level of boost at the compressor, but that's not what your engine is actually seeing.



Yes, behold the simple drinking straw. Solver of great mysteries.

Here's a quick experiment. Stick a drinking straw into your pie-hole, and blow through it. Not too hard, very gently in fact. Very, very gently. This ain't Hustler's mom we're dealing with.

Feel the resistance that the straw is offering? No? Of course not. At the rate at which you are blowing into it, the straw is not much of a restriction at all.

Now, blow harder. And now you start to feel the straw fighting you.

A funny thing happens when we try to flow a gas through a restrictive orifice. The more we try to flow through the restriction, the more restrictive it becomes. In practical terms, at low rates of flow, we get very little pressure loss across the restriction. As flow increases, so does the pressure loss. And it's not linear, either. The magnitude of pressure drop increases almost exponentially with flow rate.

Now, it may not look like one, but your intercooler is a drinking straw. A large, heavy, aluminum, multi-faceted drinking straw. Or, at least, it exhibits a lot of the same characteristics as one. So, back to the diagram:


Say that we have our boost controller set such that we see a peak of 12 PSI in the intake manifold. At 4,000 RPM, we reach that point. We are, incidentally, flowing about 130 CFM through the intercooler (our engine has a 100% VE at all speeds) and we're loosing about 1 PSI across the intercooler. So while the pressure at point B is 12 PSI, the pressure at point A (which is what the boost controller is seeing) is actually 13 PSI.

Now, we increase the speed to 7,000 RPM. At this point, we're moving about 220 CFM, and yet, what's this? The pressure drop across the intercooler has increased to 3 PSI! We didn't even double the flow, and yet we tripled the drop. (Well, I'm ignoring the fact that these are relative, rather than absolute pressure values, but you get the idea.) So now, even though the MBC is faithfully holding 13 PSI at the compressor, we're only seeing 10 PSI at the manifold.

The solution here should be pretty obvious by now. Move the boost controller from point A to point B.

By doing this, we are now telling the boost controller, in essence, "Hey, I want you to do whatever it takes to maintain a constant pressure at point B in the system, and to hell with what's going on over at the compressor." And it will comply. (Boost controllers are pretty simple-minded like that. They don't question orders.) Specifically, it now does not matter what the drop across the intercooler is, at least insofar as your actual manifold pressure is concerned. As drop across the IC increases, the boost controller will cause the compressor pressure to increase accordingly. So by the time you get to 7,000 RPM and are experiencing 3 PSI of drop across the IC, the pressure at point A will be up to 15 PSI, and you'll still be getting your 12 PSI at the manifold.


And here's everything you need to make it happen:


Yup. One 1/8" NPT hose-barb fitting. About $3 at your local ACE Hardware store. Drill ye' olde hole into the pipe which leads up into your throttle body, install this fitting into it, and plumb a hose from there to your MBC. Using all-silicone tube? (fag.) Well, just drill a hole in the cold-side end tank of the IC itself. Anywhere is fine so long as it's after the IC core, and before the throttle body.

You'll probably have to turn the MBC down just a tad in order to achieve the same peak boost you had before, as it's no longer having to factor in even the smallest IC drop.

Vaya con Dios, friends. May your manifold pressure be stable.

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