This guy has it right. The 'vane' discussion above is specific to a certain 'vaned' Garrett VNT turbo (there may be other models) that eliminated the wastegate by altering airflow inside the turbine housing (that's the hot-side to you).
Originally posted by Lee8995
Yall have it all wrong, A wastegate on a turbo is operated when boost reaches a predetermined amount it actuates a valve in the hot housing ...
I currently own and modify a Mitsu Eclipse turbo and I've pretty much seen all the setups you can run.
Most turbos have an internal wastegate, which is built into the turbine housing. You then have a canister looking device (called the acuator) bolted to the outside of the compressor housing and a bar protruding over and connecting to a lever on the turbine housing. This lever controls the door that covers the by-pass opening. The wastegate actuator contains a strong spring that holds the lever in the closed position. The way this works is, there is a boost source nipple on the actuator. You connect a vacuum hose from the compressor housing (or somewhere close to it) to the nipple on the actuator. When boosting, the actuator will get pressure exerted onto the spring. When the boost is strong enough to overcome the spring, it pushes the rod, and in turn, the lever closer to the open position. When the door opens, now exhaust gasses can skip the turbine altogether and start leaking out through the wastegate opening (which typically goes to the same exhaust pipes). Now the turbine is beginning to slow down and thus, creating less boost.
The other setup is an external wastegate. Some turbos are too big to have an internal gate because the gate would not flow enough to slow down the turbine. Externals are pretty damn expensive but they offer even better boost control over internal gates on the same turbo. They usually are welded into the exhaust manifold and the valve releases gasses durectly from there before it reaches the turbo.
The end effect is the boost gauge quickly rises and abruptly halts at the set boost limit. This is what you want, because obviously you don't want to overboost as this will both strain the turbo and more importantly blow up your engine in more ways than one.
You can adjust the boost limit by using a valve on the vacuum hose. Some people say to use a fish-tank bleeder valve, but there's a whole book on why that's wrong. Use a Grainger ball-spring type valve. There are a lot of companies selling manual and electronic boost controllers at various price ranges. I own an electronic and it's main advantage is I can adjust boost right in my drivers seat.
*whew* That's the best I can explain it for now.