This discussion is nothing new, but something I think is missing from this forum. Many who come on here understand what a turbo is and what a turbo does but not everyone may know how it works, the different parts, and what these parts look like. While turbo models vary and where their parts are located are different, they all have these same parts to do the same jobs. So realize that the way these parts look on different setups may not be the same, but they will operate the same way and have the same job.
On that note however, keep in mind when we discuss valves in the turbo, there are two different types, recirculating and atmospheric. Recirculating valves will take whatever airflow they are controlling and recirculate it back into the system. Atmospheric will take the airflow and vent it out to the atmosphere. As a matter of fact, a Blow Off Valve and a Bypass Valve are the same thing with the only difference being a Blow Off Valve vents to atmosphere while a Bypass Valve recirculates. In the K04, all the valves are recirculating systems and all OEM turbos (at least all the ones I've seen in cars since 1992) are this way for smog reasons.
Let's start with a picture of the K04 assembled and the different components of it.
Thanks to Jgasser, I was able to document a disassembled K04 and take these pictures. This turbo wore out and needs to be rebuilt but if you just looked at these pictures you may not realize it. This first picture shows the center section of the turbo. This is the heart of the turbo. This includes the compressor fan, turbine fan, turbo shaft, and the oil and water passages used to lubricate and cool the turbo shaft and it's bearings. Hence on this part you will find all the oil and water line ports.
In this picture the turbine fan is on the right and the compressor fan on the left. The turbo shaft connects these two through the center of this part.
The turbine fan's blades are curved to meet the incoming flow of exhaust and use it's energy to spin up the fan. Here is a picture of the turbine fan and the housing that it goes into on the turbo body. (note the housing is in the distance, hence why it looks like the fan is too big to fit in the housing)
This spins the turbo shaft and, with the compressor fan attached to the same shaft, turns the compressor fan. This fan also has curved blades to catch the incoming air, compress it, and force it out the turbo's outlet. Below is a picture of the compressor fan and housing.
The concept behind these components is pretty easy to understand and is the part most people know about when it comes to turbos. The more exhaust gases move into the turbine housing, the faster the fan spins the shaft. The faster the shaft spins, the faster the compressor fan spins. The faster the compressor fan spins, the more boost is produced in the compressor housing and delivered to the engine.
What is less understood is the amount of boost a turbo can make, the amount it should make, and controlling the amount you want it to make. To understand how this works and why it needs to work you have to understand a couple of general ideas about compressing air.
When you compress air, you also are heating the air. As air compresses it gets hotter and while the pressure of the air will go up, it's density will go down. This presents means the two are a bit counter productive. We're compressing air to get more air in the engine per revolution but hot air is less dense and thus the hotter air charge has less air in it at a given pressure than a colder air charge at the same pressure. This is what we have intercoolers for. Because of this fact, a turbo has a certain range of boost it can generate effectively without the heat that boost range generates overcoming the amount of air the boost level it is generating delivers.
For instance, say a turbo flows 30,000 cfm of air at 23 psi of boost, the upper limit of it's boost map. If you push that turbo to 24 psi, the cfm generated drops to 29,000. 25 psi and it drops to 27,000...and so on. That's called running a turbo off it's map.
The K04 turbo can generate more than 25 psi of boost but on the LNF, running more boost than that is running it off it's map. If left uncontrolled, the turbo would regularly exceed 24 psi of boost generating unnecessary heat and cause detonation in the engine. To prevent this from happening, turbos are equipped with a wastegate. Many times people confuse the wastegate with the blow-off or bypass valve...the valve that makes that signature PSSST sound on a turbo car when you let off the gas after accelerating really hard. The wastegate controls the amount of exhaust that is allowed to move through the turbine and drive the turbo.
When the boost level of the turbo reaches where the system designer wants it, the wastegate begins to open (which is done mechanically with the wastegate actuator) allowing some of the exhaust gases to bypass the turbine and go directly to the exhaust outlet. This limits how much energy is driving the turbine to a level where the turbo will only produce a certain level of boost. Here is the integrated recirculating wastegate valve in the K04 in the closed position. The studs you see in this picture are the studs that connect the catalytic converter to the turbo housing.
And here it is in the open position. As you can see, when the wastegate opens the output is directly into the exhaust stream heading into the cat.
Below is a picture of the turbine housing to give you a better idea of how the wastegate is oriented. To the right is the part of the housing that connects to the exhaust manifold. The hole next to the wastegate to the right of the picture is where the exhaust gas exits the turbine after driving the turbine fan. Exhaust gases flow from the manifold, through the housing, to the fan, and then out that hole. The wastegate is open to this exhaust flow just before the turbine housing so that when it opens, part of this flow goes right out the exhaust bypassing the turbine fan.
In an atmospheric wastegate system the air diverted from the turbine just goes to atmosphere. These systems can be very loud and obviously this exhaust gas will bypass any catalytic converter. Thus for noise and smog reasons, manufacturers use a recirculating system. With the LNF, the wastegate is controlled by the ECM. The ECM dictates when the wastegate opens via the Wastegate Control Solenoid.
The bypass (or blow off) valve system is on the compressor side of the turbo. When you are accelerating hard, the throttle plate is mostly open and allowing large amounts of air to pass into the intake manifold. The intake track is pressurized by the boosted air charge from the turbo, through the charge pipes, through the intercooler, the throttle body, the intake manifold, into the heads and into the engine.
When the driver lets off the gas the throttle plate goes mostly closed causing the intake manifold, heads, and engine see vacuum again rather than boost but the rest of the intake system from the throttle body back to the turbo is still pressurized and the turbo is still trying to push boost. If this pressure isn't relieved, it can flow back into the turbo (which will stop producing boost since the engine is now seeing vacuum and thus the pressure in the compressor side of the turbo is lower than that in the charge pipes and intercooler) causing what is called compressor stall. This condition can cause damage to the turbo.
To prevent this from happening, the bypass valve vents this excess pressure and recirculates it back into the intake track (or a blow off valve vents it to atmosphere). Below is a picture of the port in the compressor housing that will see the pressure left in the intake track when the throttle plate is closed.
The two holes you see in that fitting we're looking through is where the output elbow bolts to the turbo housing on the K04. This is the elbow the first charge pipe going to the intercooler connects too. In the following picture, we see another port that is entering the area where fresh air enters the turbo to be compressed (I think it is the square port...the round port feeds a tube that gets connected to the wastegate control solenoid on the LNF):
Between these two ports is the bypass valve itself. You can see it here in the photo below. The fitting we were looking through in the first picture can be seen just above this valve. The red cap below the beige valve is the intake to the turbo and it is in this intake track where we saw the other port.
The nipple on the valve you see here is connected to the intake manifold. When the intake manifold is under pressure, this valve stays closed and the two ports we've seen are not connected. When the manifold sees vacuum, the boost pressure in the intake port pushes the valve open and allows the pressure to be vented through the port going into the intake of the turbo thus preventing compressor stall.
This is how our turbos work and the parts we talk about when we deal with boost and controlling boost. Hope this helps out those that may be unfamiliar with how turbos work and if any of our experienced members find problems in my post, please feel free to point them out so I can make corrections to this.