Free Boost Modification for the
by Jeff Lucius
1991-1993 Mitsubishi 3000GT and Dodge Stealth
The TD04 turbocharger wastegate is opened by an actuator attached to it with a rod. Air pressure from the Y-pipe pushes against a spring inside the actuator. When air pressure reaches about 7 psi (48 kPa) the actuator opens the wastegate allowing exhaust to go around the turbine wheel, thus controlling the speed of the turbine wheel and boost pressure.
Boost pressure is raised above the 7 psi limit of the actuator by bleeding air out of the hose that goes from the Y-pipe to the actuator. This lowers the pressure that the actuator receives. Taken to the extreme, if no air pressure reaches the actuator, by intention or because of a leak or blockage in the air/vacuum hose, the actuator will not open the wastegate and boost pressure can reach the maximum level capable by the turbocharger. The factory TD04-09B turbos in a factory engine can reach about 20 psi boost up to about 4000 RPM. Boost level drops from there to about 13-14 psi near 7000 RPM. On the other hand, if all the pressure in the Y-pipe reaches the actuator then boost should not exceed about 7 psi at any RPM. The engine control unit (ECU) controls boost levels using a solenoid to bleed air out of the air/vacuum hose that goes to the actuators. When the solenoid opens, air is bled out of the hose from the Y-pipe and into the intake hose just before the rear turbocharger. The ECU uses this solenoid to limit maximum boost levels and to reduce boost when knock levels are high (usually above about 7-9 counts). Additional information concerning turbo pressure control can be found in the 1991 Stealth Technical Information Manual, pages 14-21 and 14-22.
The service manuals (see the Intake Chapter) indicate the factory boost levels (supercharging pressures) are set at 8.7 psi (60 kPa) for 1991 through 1993 models, and at 10 psi (69 kPa). Many owners report stock boost levels for 1991-1993 models in the 8-9 psi range, and for the 1994+ models in the 10-12 psi range. This difference in boost levels is achieved by removing a plug from the lower nipple of the wastegate solenoid. It is almost as if Mitsubishi intentionally lowered max boost for the early models with plans to increase boost for later models. With the restricting plug in place, the opening into the lower nipple is only 0.055 inch. This reduces the amount of air that can be bled off off and so keeps boost levels low. After the plug is removed, the opening is 0.140 inch, allowing more air to be bled off and higher boost levels.
Free Boost Mod
The solenoids are located on a bracket near the throttle body as shown in the picture below. Identify the boost control solenoid. Gently remove the two hoses that attach to it. Press on the release lever of the wiring harness connector and pull the connector downward and off the solenoid. Either by hand or with a tool like a slotted screwdriver, push the integral solenoid mounting tab back toward the firewall and then down. Slide the solenoid off the bracket. Be careful to not break a nipple. Use a "dental pick" or similar tool, possibly a small drill bit, or even a paper clip appropriately bent (thanks to astrodabu at 3si.org for this tip) to remove the plug. Again, be careful to not break a nipple. Re-install the solenoid, connecting the wiring harness and air/vacuum hoses. There you go. Free boost!
Page last updated May 29, 2003.