On the 1986 Thunderbird TurboCoupe, (as well as others,) boost is controlled by a solenoid known as a Boost Control Solenoid. A fitting on the turbo sends the boost level to the wastegate actuator and the boost control solenoid (BCS). If the BCS is closed, the wastegate actuator sees all of the boost. The actuator on the 1986 TurboCoupe is set to open at 10psi. If the BCS is open, part of the boost from the fitting on the turbo is bled out through the BCS, so the wastegate actuator only sees a portion of that signal, and doesn't open until the boost is higher. BCS activation is controlled by the car's EEC (engine computer), and is usually open in limited circumstances.
The actions below are a list of what I did and how I did them, and are not recommended for anyone. If you follow those procedures and bad things happen, don't blame me, I take no responsibility.
One way to have maximum boost available full time is to put a jumper line between the line in to and the line out of the BCS. With this line in place, boost is always bled, and the wastegate actuator would never see the full boost signal. This resulted in about 16.5 psi max available boost when I did this modification. Others may see different amounts.
Feel free to copy the images below, and/or reference them if useful for helping others.
Here is a view from the front driver's side showing the BCS and the jumper line, as well as some other parts for a location reference.
Here is the view looking down, leaning over the passenger side fender.
Here is a picture of the fitting on the turbo that sends boost to the wastegate and the BCS. Drilling the inside of the fitting on the side that goes to the BCS will increase the available boost. Different sizes result in different amounts of boost. Once drilled, the fitting cannot be undrilled. The drill bits I used and apparent maximum boost (to the nearest 0.5psi on the stock gauge) are:
If you have used different size bits and/or have different values, feel free to email me and I will include those values.
I hope this helps someone. If you have any questions, feel free to email me at firstname.lastname@example.org