Boost Controller Designs
For simplicity's sake, we'll say the wastegate opens at 20 psi and regulates the boost pressure to 20 psi.
The way any mechanical boost controller works is fool the wastegate into thinking you are are making less boost then you actually are. So if you want to build 40psi, you need to make the wastegate think that you are only building 20psi at your set pressure, so in this case, you want it to think the boost is half of what it actually is. There are a couple designs.
The Needle Valve
This is simple and cheap, and works OK for people that use there trucks for fun. How it works is it delays the pressure going to the wastegate, so when you floor it, the boost has time to build up to your set pressure before the wastegate realizes it. The problem is when you're towing, or at/above 20psi for extended periods, the wastegate will get the full boost pressure you are making and will open way up to try and get boost back down. And because of the restriction, when you let of the skinny pedal to shift (with a manual) and you boost goes down, the wastegate will take a while to close again, making it harder to re-spool the turbo once you're done shifting.
The boost elbow is basically like a needle valve, but it has a small orifice after the needle valve that bleeds pressure to the atmosphere, if anyone here is familiar with a "resistor devider" (electronic circuit) it works in exactly the same way. By bleeding pressure after the valve you lower the pressure that the wastegate sees, making it so it will always see lower pressure then what you're actually making. Using the valve, you can change the amount of pressure drop. This is a step above the plain needle valve, and works very well for many, however there is still one issue. In order to wastegate the boost to 20 psi, the wastegate has to start to open around 10 psi. So if you set your pressure to 40 psi with the boost elbow, then it will start to open around 20 psi. And because of the orifice, you will always have a small boost leak, nothing huge, but it is there.
Ball & Spring Boost Controller
Learn how to make one HERE.
This is another step above the boost elbow. The Spring pushes the ball into a seat, sealing off the passage to the wastegate. The spring is on one side of the ball, and on the other side, there is the boost from the engine pushing the ball open. Once the boost pressure is enough to overcome the pressure given from the spring, the ball unseats and lets air into the waste gate opening it. On paper it looks like it will keep the wastegate closed until you your set pressure, however that's not quite the case. Because once some air pressure goes around the ball, it is now applying pressure trying to close the ball again. So in reality this design makes the wastegate think your at X less pressure then you actually are. So if you have it open at 40 psi, it will think you are at 20, and at 30 psi, it will think you are at 10. This design works very well, and should be good for all but the biggest perfectionists out there. The only real downside to this design is it still has a pressure spread between starting to open, and open enough to regulate the boost to that pressure.
This design's only advantage over the "Ball & Spring" design is that it fixes the "problem" of the pressure difference between starting to open, and open enough to regulate the boost. This controller works by doing it's best to keep the pressure exactly where it's set to, so fully closed @ 38 psi, and fully open by 42 psi. There are some "over shooting" issues with ones I've played with though, due to the issue that the wastegate has only the time it takes to build ~2 psi to go from fully closed to open enough to regulate the pressure.
This is a much rarer design that only a few I know of have, this works extremely well for those who use there truck for towing, as they never really "punch" the throttle, giving the wastegate plenty of time to open.
Note: "Boost Governor" is a name I came up with, as I'm not sure what this type is actually called.