It was brought to my attention that we don't have anything on the forum describing boost control, how wastegates work, and how to "turn up" the boost. If someone searches for this info, they shouldn't come up blank anymore. Also, there are tons and tons of myths when it comes to boost control especially comparing manual to electronic. I want to shed some light on those as well. I wrote a lot of this during a debate on a BMW forum and copied it over so it might not flow as well as I'm used to. I'll work on it.
How common wastegate actuators work.
Here is an external wastegate for reference.
Internal wastegates have external actuators, so there's nothing fundamentally different about them that makes one different from the other when it comes down to the actuator. Internal wastgate actuators have the same diaphragm/piston/spring layout that external wastegates do.
The major difference being, very few internal wastegate actuators you're going to run into will be serviceable, meaning the spring can not be changed. External wastegates by-and-large have a serviceable design. This means you can open it with common tools, and change the spring inside if you'd like. This spring is what holds the wastegate closed during operation. You have to fight against this spring with positive air pressure from the "compressor reference port" to get the wastegate to open. When the wastegate opens, exhaust gases bypass the turbine and boost is limited. Simple enough.
Typical internal wastegates do not even have a "boost controller port" and this port is usually left open (to atmospheric) on external wastegates. It is commonly used with electronic boost controller setups, but I'll get to that later.
The wastegate spring.
This spring comes in different flavors. Most factory wastegates are internal, with their springs set anywhere from 6-12 psi. Some, like the newer Mitsubishi Evolutions have 14.5 psi or higher springs (very high for a factory car). In turbo Porsche cars, they tend to use external wastegates, again with the springs ranging from 6-12 psi depending on year and model.
Simplifying things a little bit, the lowest amount of psi you can run depends on the wastegate spring. If it takes 6 psi to compress the spring, you can't make less boost than that because you're going to be using the pressure from the manifold to work against the spring. As you start to make 6 psi to the manifold (and the engine) your wastegate actuator will see 6 psi, and start to open the wastegate (it happens earlier than this, but don't worry about that). Now that the wastegate is open, some exhaust gases are "wasted" and not used to spin the turbo. This limits boost pressure to the amount dictated by the spring (provided you have enough flow in your wastegate system).
Controlling boost with the wastegate spring.
One, expensive, complicated, limiting way of raising the amount of pressure the turbo outputs involves swapping the low pressure spring in an external wastegate to a higher pressure spring. This gets expensive and time consuming if you're looking to move from low pressure to high pressure in stages as most tuners like to tune in stages. Once the high pressure spring is in, there's no simple/quick way to run lower boost pressure either (valet mode, etc).
All in all, it's a pain in my opinion, and most people don't do it this way. They get the lowest pressure spring they can (to allow for low boost) or they just work with what ever comes with the wastegate.
Traditional boost control theory.
Here's where the other methods of boost control come in. You can fool the wastegate actuator into opening at higher air pressures than the spring is designed for by controlling the pressure signal the actuator sees (connected to the "compressor reference port"). This can be done many different ways regardless of internal/external design. There are three main ways of doing this.
Firstly, you can "bleed" off some of the signal to the wastegate with a controlled leak in the pressure line going to the actuator. This method is simple and marginally effective and usually overlooked as the next example is better and no harder to implement. The bleed can even be an adjustable valve that is controlled inside the cabin. "Dial-a-boost" they like to call it. This is one method of manual boost control. Many say manual boost controllers cannot be controlled from inside the cabin, which is untrue. This method can be employed typically for $15.
Traditional manual boost controller.
The next method involves a one-way ball-and-spring check valve. This is what is typically referred to as a "manual boost controller". This is placed in the signal line. The device has an adjustable spring inside which takes over the job of the original actuator spring. Set the pre-load on the spring to what ever psi you'd like and the wastegate actuator won't see any pressure until that level is reached. This method is simple, cheap, and marginally configurable (adjustment can also be routed into the cabin for control from inside the car). Even set at the same psi as the original wastegate spring, this method gives quicker spool than normal, as it doesn't allow the wastegate to "creep" open before the desired boost level is met, causing full boost to come on quite a bit sooner.
Many people believe this method is flawed or unreliable, causing dreaded boost spikes and the like. While I don't doubt others have experienced this, I know with a properly set-up turbo system, there should be no problems. I've never experienced boost spikes on the 6+ turbo systems I've helped with or friend's of mine have, all using this method. My experience ranges from tiny, internally wastegated Mitsubishi turbos to large gt35 turbos, all on 2.7 liter or smaller engines.
This method can be employed typically for $25 if you build your own, buy an inexpensive unit, or around $80 if you buy a fancy (but no better functioning) one.
Traditional electronic boost control.
The third main method involves controlling the pressure signal to the wastegate with an electronic valve, controlled by an electronic controller of some type. This typically gives complete control over the operation of the valve and thus the wastegate actuation, but requires a bit of set-up time and quite higher start-up cost as the electronically activated valve runs about $50 and the controller itself can range from $150?-$600 or so. Engine control units typically have this control functionality built in, making this cost a non-issue for some.
With this method, you can control how quickly, or how slowly boost comes on. Allowing for boost to build progressively through the rev range if desired, and allows for the control over the maximum boost depending on what gear you're in (if your controller has that functionality). There is no limit to what you can do with this set-up, paired with a good controller (except you can't run less boost than what your wastegate actuator spring will allow). However, most find this method overkill, or unneeded for their goals.
Hybrid boost control.
Finally, you can mix and match any and all of the manual/eletric control methods to create your own home-brew system that provides a startlingly high amount of control.
If you'd like to see great, configurable examples of what can be done with manual boost control, I'd recommend these terrific articles by the late Gus Mahon: