Introduction

The Greddy Profec B Spec II boost controller is a time-tested performance upgrade for the Mitsubishi 3000GT VR4, an icon known for its twin-turbocharged 3.0L V6. Factory boost control on these cars is managed by a simple solenoid that opens and closes in response to the ECU’s duty cycle, which often results in spikes, taper, and inconsistent boost across the rev range. Replacing that archaic system with the Profec B Spec II gives you precise, adjustable control over your wastegate actuator, allowing you to fine-tune boost pressure to match fuel, ignition timing, and your personal driving goals.

This guide walks through every step of the physical installation—removing the factory boost control solenoid, mounting the controller, wiring the harness, routing vacuum lines, and performing the initial setup. Follow these instructions carefully, and you will gain a reliable, cockpit-adjustable boost control system that transforms the driving experience of your VR4.

Tools and Materials Needed

Before starting, gather the following items. Having everything on hand prevents mid-job trips to the hardware store.

  • Greddy Profec B Spec II boost controller kit (includes the main unit, wiring harness, mounting bracket, and vacuum fittings)
  • Socket set (10 mm, 12 mm, 14 mm) and ratchet
  • Wire strippers / crimpers
  • Electrical tape or heat-shrink tubing
  • Heat gun or lighter (for heat-shrink)
  • Vacuum hose – 4 mm (5/32″) silicone recommended; the kit includes a short length but you may need more for routing
  • Zip ties, various sizes
  • Trim panel tool for interior plastic removal
  • Multimeter for verifying continuity and voltage
  • T-tap wire connectors (optional but convenient for tapping into power and ground)
  • Boost leak tester (for post-install leak checking)
  • Factory service manual (FSM) for the 3000GT VR4 – available at 3SI.org

Preparation and Safety

Park the car on a level surface, engage the parking brake, and allow the engine to cool completely. Disconnect the negative battery terminal using a 10 mm socket. This prevents accidental shorts and resets the ECU so it can relearn idle and trims after the install. Read through the entire Greddy Profec B Spec II manual before beginning – it contains wiring diagrams and solenoid orientation details that vary slightly between revisions.

Understand the Controller’s Basic Functions

The Profec B Spec II uses a single solenoid (the “fast acting solenoid” or FAS) that pulses the wastegate actuator. You control three main parameters: SET (target boost, kPa or psi), GAIN (how aggressively the solenoid reacts to reach target), and BOOST (a built-in trim to adjust wastegate spring baseline). Setting these incorrectly can cause boost spikes or overboost protection kicks. Have the manual nearby for reference during step 6.

Step 1: Remove the Factory Boost Control Solenoid (BCS)

On a 3000GT VR4, the factory boost control solenoid is mounted on a bracket near the intake manifold, just forward of the throttle body. It has two vacuum nipples and a two-wire electrical connector (typically a black and a yellow/red wire).

  1. Locate the BCS: Follow the small vacuum line from the wastegate actuator to the solenoid. It sits on the driver’s side intake runner, bolted to the upper plenum.
  2. Disconnect the electrical connector: Press the tab and pull straight off. Tuck the connector out of the way with a zip tie; you will no longer need it.
  3. Remove the vacuum lines: Factory hoses are brittle after years of heat cycles. Carefully pull them from the nipples. One line comes from the intake manifold pressure reference, the other goes to the wastegate actuator. You will reuse the line from the manifold reference as the source for the Profec solenoid.
  4. Unbolt the BCS: Using a 10 mm socket, remove the two bolts holding the bracket. Set the entire assembly aside. The bracket can be removed completely to clean up the engine bay.

Tip: If the factory vacuum hoses crack during removal, replace them with new silicone 4 mm hose. A single crack will cause a massive boost leak and erratic behavior.

Step 2: Mount the Greddy Profec B Spec II Controller

Choose a mounting location that is easily accessible while driving, but not in the way of your knees. Common placements include the driver’s side lower dashboard (near the fuse box) or the center console, below the radio. Avoid locations that block air vents or interfere with pedal movement.

  1. Remove the necessary trim panels to access the mounting surface. For the lower dash, pop off the fuse box cover and the knee bolster panel.
  2. Mount the included bracket using self-tapping screws or existing bolts behind the trim. The bracket can also be double-sided taped or zip-tied to a solid structure.
  3. Slide the controller into the bracket until it clicks. Ensure the rear wiring harness can reach the controller without pulling tight.
  4. Plan the wire pass-through: You will need to run the wiring harness from the engine bay (solenoid) to the cabin (controller). Look for an existing grommet on the driver’s side firewall, near the steering column. Use a coat hanger or fish tape to pull the wires through.

Step 3: Wiring the Harness

The Profec B Spec II harness has four wires typically: Red (12V constant), Black (ground), White (solenoid positive), and Black/White (solenoid ground). Some units have an additional wire for a warning light or RPM input; ignore those for basic operation unless you want an overboost warning.

  1. Route the harness from the controller location to the engine bay. If you are using the fuse box area, pull the harness through the firewall grommet. Use a multimeter to verify the wire colors match the diagram in your manual if you have a used unit.
  2. Connect the Red wire to a switched 12V source – the Profec should power on only when the ignition is in the ON position. Tap into the fuse box’s “ENGINE” or “ACC” slot using a mini add-a-circuit fuse. Alternatively, crimp a ring terminal and connect to the positive terminal of the fuse box’s interior relay. Do not connect directly to the battery constant.
  3. Ground the Black wire to a clean chassis ground under the dashboard. Sand any paint off the metal contact point near the steering column.
  4. Run the White and Black/White wires to the solenoid. The Profec solenoid has two wires (usually red and black). Crimp on bullet terminals or solder them; polarity does not matter for the solenoid. Connect the White wire to one solenoid terminal and the Black/White to the other. Use heat-shrink over the joints.
  5. Secure all wiring with zip ties along the existing harnesses. Avoid routing near hot exhaust components or moving parts like the throttle linkage.

Blockquote warning:

Do not connect the Profec solenoid directly to the factory BCS wiring. The factory harness supplies 12V only when the ECU commands boost; it is not a constant switched source. Tapping it can cause erratic solenoid behavior and potentially damage the ECU. Always use a dedicated switched ignition source.

Step 4: Attach the Vacuum Lines

Correct vacuum routing is critical. The Profec solenoid has three ports: IN (from pressure source), OUT (to wastegate actuator), and VENT (atmosphere). Some solenoids are labeled differently – check your manual. The standard configuration uses the solenoid in “normally closed” mode, meaning it blocks flow until power is applied.

  1. Connect the IN port of the solenoid to a boost source. The best source is the intake manifold pressure reference that originally went to the factory BCS. If that line is short, extend it with 4 mm silicone hose. Do not use T-fittings between the solenoid and the wastegate – the solenoid must see unrestricted pressure.
  2. Connect the OUT port directly to the wastegate actuator. Use a short line (less than 12 inches) to minimize lag. The actuator is located on the passenger side of the engine, near the rear turbo. Run the line along the firewall to keep it away from heat.
  3. Leave the VENT port open to atmosphere. Some installers add a small filter to prevent dirt ingress; this is optional. Do not cap the vent.
  4. Double-check all connections. Use zip ties at each vacuum nipple to prevent hoses from blowing off under high boost. Perform a boost leak test if you have a tester – pressurize the intake system to 15 psi and listen for hissing. Fix all leaks before moving on.

Important: The Profec solenoid is not designed for wet environments. Mount it in a relatively dry location inside the engine bay, such as on the strut tower or the firewall above the transmission, but away from direct water spray.

Step 5: Reconnect the Battery and Perform Initial Setup

Ensure all tools are removed from the engine bay. Reconnect the negative battery terminal. Turn the ignition to ON (engine off) – the Profec should light up. Do not start the engine yet.

  1. Set the controller to its default position by turning the SET knob to the minimum (about 50 kPa / 7 psi) and GAIN to minimum (0%). Turn the BOOST knob to the middle (50). These values are safe for first startup.
  2. Listen for a click from the solenoid when you move the GAIN knob from 0% to 100% – you should hear the valve actuating. If not, check wiring and solenoid connection.
  3. Start the engine and let it idle for two minutes. Check for vacuum leaks at the solenoid and wastegate line. The idle should be stable. If the idle hunts or rises, you may have a vacuum leak – re-inspect the lines.

Step 6: Test the Installation

Now it’s time to verify the system under load. Find a safe, straight stretch of road with low traffic. Have a passenger monitor the Profec display or log with an OBD2 scanner if you have one.

  1. Gradually apply throttle in third gear from 2000 rpm to redline. Watch the peak boost number. With GAIN at 0, you should see stock wastegate spring pressure (roughly 7–9 psi depending on your car).
  2. If you see boost above stock (e.g., 15 psi) suddenly, the solenoid may be stuck open. Stop immediately and verify the wiring orientation. The solenoid should allow boost to reach the wastegate only when the controller commands it. With GAIN at 0, the solenoid stays closed, directing all pressure to the wastegate to limit boost.
  3. Check for boost spikes. Spikes above your target (when you eventually raise it) indicate GAIN is too high or the duty cycle is too aggressive. Start with GAIN in the 10–20% range and SET to 12 psi for initial tuning.

For a detailed tuning walkthrough, consult Greddy’s official Profec B Spec II page for target duty tables.

Step 7: Final Adjustments and Tuning

After confirming the system works without overboost, you can dial in the performance. Every VR4 is different – fuel octane, intercooler efficiency, and turbo wear affect the optimal settings.

  1. Set your target boost using the SET knob. For pump gas (93 octane), 16–18 psi is a common safe limit on stock turbos. Never exceed 20 psi on the factory 9B turbos – they run out of efficiency and the engines are prone to knock.
  2. Adjust GAIN in small increments (2–3%) until boost reaches the target sharply without overshoot. A good starting point is 15% GAIN for 16 psi. Log boost using the Profec’s peak memory function.
  3. Ensure the BOOST knob is set to 50 – this calibrates the internal pressure sensor. Many installed units require no change here, but you can trim it if the displayed boost differs from a known-accurate gauge.
  4. Perform a final road test in third gear from 2000 to 7000 rpm. Watch for boost drop-off at high RPM – if it tapers more than 2 psi, increase GAIN slightly or check for exhaust restriction.

Blockquote:

Warning: Always monitor knock levels when raising boost. The 3000GT VR4’s 9:1 compression pistons are sensitive to detonation. Use a knock detection method (factory knock sensor logging or aftermarket gauge). If you hear pinging, reduce boost immediately and consider using higher-octane fuel.

Conclusion

Installing a Greddy Profec B Spec II boost controller on your 3000GT VR4 replaces a restrictive factory system with a precise, driver-adjustable unit. Proper wiring and vacuum routing are the keys to reliability – take your time with Step 3 and Step 4. Once tuned, the improvement in throttle response and boost consistency is dramatic. You can run higher boost with confidence, as long as you respect the limitations of your turbos and fuel. For further discussion and tuning logs, the 3SI.org boost controller section is an excellent resource filled with proven base maps from veteran owners.