[sperry]

Okay, I'll take a whack at this:

As you know the turbo is made of two parts, the hot side and the cold side. The hot side is a gas turbine that spins up from the heat/pressure/flow of the exhaust gasses flowing through it. The turbine then spins up a compressor in the cold side that raises the pressure of the intake charge going into the motor.

Now, a turbo doesn't have to have a wastegate. If the turbo is big enough that it won't over-rev or generate too much boost when the engine is running at maximum power, then a wastegate is not needed. The problem with a turbo that large, especially on a small motor, is that it will have *tons* of lag. So, a much smaller turbo is used. The problem with a small turbo though is that if left unchecked it can spin so fast that it'll destroy it's bearings, or overboost the motor if there isn't some other means of limiting intake pressure. That's where the wastegate comes in.

The wastegate is a valve in the exhaust stream that variably bypasses exhaust around the turbo's turbine, limiting the top speed of the turbo. In it's simplest form, the wastegate is actuated directly by the boost level on the cold side. So there's a pressure actuator on the wastegate valve that at, let's say, 7psi the valve begins to open diverting away the pressure on the turbine and limiting the spool of the turbo. That's what's commonly known as "wastegate boost", since without further boost control the turbo won't make more than about 7-8psi of boost.

The problem with that of course is that straight wastegate boost control results in a fixed boost curve that tends to oscillate around the wastegate target pressure level. So to better control boost, a boost controller is used. The boost controller is a device that bleeds pressure off the line between the pressurized intake and the wastegate actuator. By bleeding off pressure, it can prevent the wastegate from opening, thus allowing not only higher boost levels, but also allowing control of the boost curve itself. A manual boost controller simply changes the target boost of the system and can react quicker than the wastegate itself, giving the user a means to set a higher target boost that the turbo can ramp up to very quickly. An electronic boost controller can actually map the bleed-off amount in reaction to things like engine rpm, gear, etc as well as react to the actual boost level seen in the intake to properly control the wastegate on the turbo.

With regards to the blow-off valve, as you mentioned it's there for protecting the turbo from surge. When the throttle body closes, the intertia of the turbo keeps it spinning and generating boost that no longer has anywhere to go. To prevent the turbo from spinning backwards, the blow-off valve vents the surplus pressure either to atmosphere or back to the pre-turbo side of the intake.

Anyway I just wrote all this in between conversations at work so I don't know how much sense it all makes. It would probably be much simpler to understand if I just had a picture. Maybe one later.