This is also in there
Wastegates:
A turbo is a positive-feedback device. The more boost you make, the more exhaust you make, which makes more exhaust, which makes more boost . . .in a vicious circle. So we have to have some way of limiting boost.
What we really want is a way of keeping the turbine operating at a constant speed (see previous post - "The Compressor") so that we can maximize the compressor efficiency - remember that turbines like to run at a single speed. However, as measuring turbo RPM is not practical, and as boost level is directly related to turbo speed, keeping boost constant is the wastegate's job.
The wastegate is just a valve that opens when we have exceeded our desired boost level, and allows exhaust to flow around the turbine, instead of through it. This lowers the pressure differential across the turbine, less work is done, and the turbo slows down.
The only "gotcha" with the wastegate is that it must be able to flow enough gas to let the turbo slow down. If it can't, the you get "boost creep" where boost levels slowly grow as the car remains under boost. Bad.
Blow-Off-Valve (BOV):
Everybody like's BOV's because of the nifty sneeze sound they make. However, a BOV is an evil device. It's taking your precious boost and venting it someplace else. Bad!
Unfortunately, it's a nessecary evil, and we have to live with it. Here's why:
You're under boost, the turbo is fully spooled, and life is good - then you shift. That means that you foot comes off the gas - and the throttle plate slams shut. Suddenly, instead of flowing in a continous stream through the engine, the intake air smacks into a closed throttle plate. The turbo, which is still spinning and producing boost because if it's rotational inertia keeps producing pressure, and the intake stream, caught between a rock and a hard place, jumps in pressure. In fact, you get a high-pressure shockwave that travels from the throttle plate back to the compressor vanes, that once it gets there, is a little like poking a stick into the spokes of a bicycle wheel.
The repeated shock is hard on the compressor vanes and the shaft bearings, and in any case acts like a brake, slowing the turbo, and requiring it to be spooled up again.
The BOV sits in between the turbo and the throttle plate, and if it detects the shockwave created by a shift, vents it elsewhere - either to the atmosphere, or back into the inlet side of the turbo.
So, we loost boost pressure, but we kept the turbo spooled . . .tough to say without a dyno if that was a fair trade on a race vehicle. On a street vehicle, it was definately a good idea, because we spared our expensive turbo a mechanical shock.
