The wastegate is what controls boost. It's a flapper valve that, when closed, allows for all exhaust gas to go through the turbo to build boost. When fully open, exhaust gasses will bypass the turbine and boost will not be built. The wastegate (the flapper valve) is attached to a rod that runs to a diaphragm actuator. On the side of the actuator opposite the rod is a vacuum line. As vacuum is ported to that side, it "sucks" on the diaphragm and the rod, causing the wastegate valve to remain shut. Without this vacuum on the actuator, the only thing holding the wastegate shut is a spring.
Boost will build to approximately 5 psig (boost pressure, not exhaust differential pressure, mind you) before enough differential pressure across the turbo allows for exhaust gas to over come the spring pressure that holds the wastegate shut. When the wastegate is open (stock setup,) the turbo will not be able to build sufficient boost, if any. The more it tries, the further the wastegate will open to bypass around the turbine.
To maintain the wastegate shut, a vacuum solenoid (boost control solenoid) will energize (via control signal from the car's computer, the ECM) to port vacuum to the wastegate to maintain it shut (sucks on one side of the diaphragm in the actuator, which pulls on the rod that attaches to the wastegate to keep it shut.) Fully shut corresponds to wastegate duty cycle (WGDC) of 100%. To control boost, the ECM will send fewer signal pulses to the vacuum solenoid valve to allow the wastegate to bypass some exhaust gas in order to maintain a target boost (< 100% WGDC.)
Let's put it all together now. Let's say you're at WOT. To maintain the wastegate shut in order to build boost, the car's computer, the ECM, will look at what target boost for the requested torque is and try to match it. The ECM will maintain the vacuum solenoid valve fully energized (100% WGDC) to port vacuum to the wastegate to maintain it shut, at first, but will shorten the pulse frequency to allow the wastegate to open to control boost at the target value.
How does the ECM know what frequency to pulse the vacuum solenoid valve? Easy, it looks at a table to know exactly where it should be. This table is generated based on the stock exhaust, with the stock back pressure. The ECM is smart enough to compensate for changes (no two cars are EXACTLY the same, and things change as cars age) to control boost, just as it can compensate for a cold-air intake, to some extent.
If you change the back pressure by swapping out the down-pipe, the aforementioned table will no longer control boost so easily. Instead of having a nice little table for the ECM to go from, and only needing to make minor adjustments, it's constantly playing catchup, and will overboost when it can't compensate quickly enough. Where the WGDC table for a stock down-pipe required the wastegate to remain shut longer or a higher WGDC to build boost, an aftermarket down-pipe will require lower WGDC's to control boost effectively.
Furthermore, if a catless downpipe is used, the the problem is confounded. SOME tuners have issues with boost creep after "properly" tuning the WGDC table. Boost creep is where the wastegate is fully open (0% WGDC) and boost continues to build due to too little backpressure from the exhaust downstream of the turbo. Image how this will work if the WGDC isn't even tuned; if it's trying to keep the wastegate shut because it thinks it needs to to build boost. :tdown:
If any of this is incorrect or if I reversed any of my concepts, please feel free to correct me and I'll edit my post. This is a very general description of how it all works, but it should illustrate the reason why running a higher-flowing down-pipe without a tune is bad news.
