Newbie guide to wrx boost control, written by a newbie. Below is just what I've learned, and as always I'm no expert....
^diagram of boost control system. I didn't make the diagram though and it's a bit incorrect. Replace the words intake manifold with turbo inlet pipe and electric solenoid with boost control solenoid.
In an effort to completely explain how boost is controlled, certain things must be understood first. In this section, we will try to explain how boost is produced, concerns about boost pressure, and how to control it.
Everything starts with the turbo and the normal combustion of the engine. We depress the throttle pedal, air and fuel go into the engine and get sparked. This creates exhaust gasses and what not. The exhaust gasses flow through the exhaust side of the turbo and spin the exhaust turbine. Since the exhaust turbine is connected to the intake compressor wheel, the compressor wheel on the intake side pushes more air into the engine. When combined with more fuel and sparked, more exhaust gasses are made and so on. Depending on how much exhaust gasses there are and how hot they are at a given load (gear/weather what not), they can make the turbo spin at a certain rate. This means that at any given point, you can only get a certain amount of boost with a certain setup in a given gear at a given rpm on a certain day in a certain place.
With the exhaust gasses continuing to increase, the turbo speeds up and pushes more air into the intake side creating more and more pressure. If the pressure continues to increase and the wheel doesn’t slow down, many different parts of the engine/turbo can be broken (from over boosting/detonating and whatever). The most logical way(and the way it actually works) to stop the wheel from pushing air into the engine is to divert some of the exhaust gasses around the exhaust wheel making the pressure going through the wheel lower and letting it slow down. This is what a wastegate does. It opens a path around the exhaust wheel for the gasses to flow (instead of through it).
On WRX's we have internally-wastegated turbochargers. These turbos have wastegate actuators which are mounted on the turbo and open the wastegate. A wastegate actuator is basically an air operated strut that activates (opens the wastegate a certain amount) when it detects that the pressure in a tube connected to it reaches a certain level. The stock wrx turbo wastegate actuator starts to open when it sees about 7psi.
There's a little nipple on the intake side of the turbo which lets air that the turbo compresses into a rubber tube (with a brass restrictor in it to slow down the flow so the Boost Control Solenoid doesn’t have to vent as much air to lower the pressure in the tubing) which sends the air towards the wastegate actuator. As the turbo builds pressure, the pressure in the vacuum tube increases. There's a T fitting between the tube coming from the turbo and the tube going to the wastegate actuator. The last side of the fitting has a hose which goes from the fitting to the BCS (boost control solenoid). The boost control solenoid is an electric valve controlled by the ecu which bleeds a air from the tubing (to decrease pressure in the tubing and raise boost) or holds it in (to raise the pressure, open the wastegate and decrease boost). The less pressure the bcs bleeds, the more the wastegate opens at a given pressure, and the more the boost gets reduced. The minimum boost that can be run is whatever the wastegate actuator’s spring is. No matter what boost control is used, you can never set a lower max boost than that.
Setup for Boost
Depending on the setup of a car, the boost can do many different things. People always say that a free flowing exhaust is best for turbocharged cars, but what do they mean by that? IMO, they mean that with a better flowing exhaust, the engine has more ability to put out power because it can flow more and has less load on it. This is all very true from what I hear, but what else does making the exhaust free-flowing do? Well, the less restriction there is on the down pipe side of the turbo, the less pressure needs to be in the uppipe side of the turbo to spin the wheel at a certain rate (good for performance). At the same time, the wastegate needs to flow more gasses to keep the pressure in the up pipe side of the turbo lower then before (to get the same boost as before), and it’s harder to do it because the gasses go through the turbine wheel easier then before. This basically means that the less restrictive the exhaust we use, the less boost control we have (on every turbo car). We can’t hold the boost as low, or whatever with better flowing exhausts.
If we can’t hold the boost at the boost pressure we want for a setup, it’s called boost creep. Since the main thing we need to do with the boost is keep it below a certain level, this can be a problem. When thinking about boost control, the least restrictive exhaust isn’t always the best for every setup, even though it might flow the most air(giving the most power). This isn’t really a concern with the stock wrx turbo (unless you’re trying to hold the boost really low), but for people who are having troubles keeping the boost at normal levels while accelerating at high rpms, the exhaust restriction can be very important. Porting the wastegate of the turbo, or getting a larger wastegate will help keep the boost lower while flowing a given amount of air at a given load. Adding a little more restriction (then when there was creep) will do a much better job of helping control the boost since the exhaust restriction is the problem in the first place(this is obviously just my opinion, do what you will ).
Now that everything’s setup, how much boost should I run? The long and short answer is this. The closer to 18psi(on 93 octane pump gas), or the higher above 18psi, the more the ignition timing will need to be retarded and richer the a/f ratio will have to be to avoid things like detonation. The lower the boost, the safer the setup (given all else is equal). As a general rule, 18psi(on 93 octane) is the boost at which the compression ratio of a stock wrx engine is pushed close to its limits. After this or even at 18psi on 93 octane, the timing will have to be retarded a bunch to avoid detonation. Setting the boost at 17psi would be a good safe setting for most wrx’s running aftermarket turbos (with 93 octane). With lower octane gas, or different load/environmental conditions, things can change. This could make it much harder to run such high boost (low octane), or make it more possible (high elevation) depending on the situation. This is all given that the turbo is not stock since the stock turbo probably shouldn’t be run near 18psi in the first place.
One more thing comes to mind about boost control and car setup. People talk about pressure drop a lot with larger front mount intercoolers. Well, we aren't looking to up the pressure when we turbocharge a car. We are, but the real idea is to put more air into the engine. Since this is the case, if the pressure drops a lot, the turbo just pushes more air to make the boost what it should be(then with a smaller intercooler). This makes me think that if the turbo can support it, the more pressure drop there is the more power a car should be able to make at a given boost.