You guys are pretty close concerning the open 3" exhaust being the culprit. It is indeed part of the problem here.
"The best exhaust for a turbo is no exhaust" (well, almost)...we've probably all heard this. It is mostly true. A diverging nozzle, basically a long shallow cone, would ideally be the best exhaust straight out of the turbine.
But your tiny turbine with tiny wastegate port and large open exhaust system are effectively acting like "no exhaust" (system) is present. What this does is reduce the turbine pressure ratio needed to produce the shaft power required to drive the compressor. Which is great, as long as you can control boost. By reducing post-turbine restriction we are actually asking the turbine wheel to flow more. If you look at a turbine map, we are above the choke flow curve the whole time here since the wastegate is open. Power required by the compressor hasn't changed, and lines of constant power are diagonally arranged on the turbine map, going up as you move left.
The turbine operating point went from a high pressure ratio, above the choke curve on the flow axis, to a lower pressure ratio (lower on x-axis) but to a higher flow condition, even higher above the choke curve on the y-axis.
Increasing your corrected turbine flow means that a small wastegate port might reach choke, and therefore even if the wastegate valve is wide open, turbine power won't be reigned in enough to control turbo speed (hence boost). This is known as boost creep.
At low engine speeds & lower loads, before you hit peak boost, you would likely notice a difference in power & response if you drove the car with the wastegate valve wired shut vs. flapping in the breeze. If you removed the actuator and left the valve crank arm free, the valve would open way early and you'd reduce your available turbine energy and feel a difference in both power and response.
The key is that once you reach full boost in this case,
even with the wastegate valve in the turbine housing completely open, that opening can't flow enough.
Potential solutions:
- Upgrade to a larger turbocharger or bigger turbine housing that flows more. Either way, more turbine flow capacity.
- Port the ever-loving crap out of the wastegate, opening the port hole diameter, adding a radius at any sharp corners. This may not ever be enough however.
- Add restriction to your exhaust system. Smaller tubing diameter, or add a cat or a muffler. That will increase turbine pressure ratio and bring overall flow back down, to a point.