Installing a Greddy Profec B Spec II boost controller on your first-generation (NA1/NA2) NSX is a smart upgrade if you are running a forced induction system—whether a supercharger or a turbocharger. Proper boost control not only protects your C-series engine but also allows you to dial in power delivery for street or track use. This expanded guide walks you through every step, from gathering tools to final calibration. Work methodically, and you will achieve a clean, reliable installation.

Understanding Boost Control and the NSX Platform

The Greddy Profec B Spec II is an electronic boost controller that regulates wastegate duty cycle to manage boost pressure. Unlike a manual bleed valve, it offers features like gain control, start boost, and high/low boost settings. On a first-gen NSX (1991–2005), the factory ECU is not designed to handle forced induction signals directly, so a standalone boost controller is essential for safe operation. This guide assumes you already have a turbocharger or supercharger kit installed—the boost controller connects to the existing wastegate and vacuum system.

For background reading on the NSX's engine management, refer to NSXprime’s engine section and Greddy’s official product page for wiring diagrams and specifications.

Tools and Materials Needed

Before starting, gather everything in one place. Missing a tool mid-installation can lead to mistakes.

  • Greddy Profec B Spec II boost controller kit (includes main unit, harness, solenoid, and vacuum hoses)
  • Hand tools: Phillips and flathead screwdrivers, 8mm/10mm wrenches, socket set, trim removal tools
  • Electrical: Wire strippers, crimpers, butt connectors or solder kit, heat shrink tubing, electrical tape
  • Multimeter (digital preferred for continuity and voltage checks)
  • Vacuum hose: 4mm inner diameter (if extra needed for longer runs)
  • Mounting hardware: Zip ties, double-sided tape or small screws, a relay (if tapping a circuit that requires one)
  • Safety: Mechanic’s gloves, safety glasses
  • Reference materials: Your forced induction kit’s manual, NSX factory service manual, or a wiring diagram specific to your NSX model year

Optional: A soldering iron for permanent connections, a test light, and a boost leak tester for final validation.

Preparation Steps

Park Safely and Disconnect the Battery

Park the NSX on level ground, engage the parking brake, and let the engine cool completely. Disconnect the negative battery terminal using a 10mm wrench. This prevents short circuits and accidental airbag deployment while working near the dashboard.

Remove the Lower Dash Panels

Start on the passenger side. Use a trim tool to gently pry off the kick panel (the plastic cover below the glovebox). Next, remove the glovebox itself—typically four to six screws hold it in place. You will see a metal brace and the ECU mounting bracket. On the driver’s side, you may also need to pop off the under-dash cover to route wiring later. Store fasteners in labeled bags.

Locate the ECU

The NSX Engine Control Unit (ECU) sits under the passenger-side dashboard, mounted horizontally on a metal bracket. It is a silver or black box with multiple connectors. To access it fully, remove the two 10mm bolts holding the bracket, then slide the ECU out slightly. Do not unplug it yet—just get it within reach for wire tapping.

Wiring the Boost Controller

This is the most critical section. Every connection must be secure and insulated to avoid a boost spike or controller failure. Always refer to the wiring diagram included with your Profec B Spec II; the description below applies to typical installations.

Identify and Prepare the Controller Harness

The Profec B Spec II main unit comes with a harness containing several wires:

  • Red (+12V constant) optional; not usually needed unless wiring for memory backup
  • Orange (+12V switched) – connects to a circuit that turns on with the ignition
  • Black (Ground)
  • Yellow (Solenoid Output) – goes to the solenoid’s power wire
  • White (Boost Sensor / Input) – connects to the solenoid’s signal? Check diagram
  • Blue (Duty Cycle Signal?) – varies; some kits have a separate wire for the solenoid ground

Important: Verify each wire’s function with the manual. The modern Spec II kits often have a 4-pin connector to the solenoid, so the controller harness may only carry power, ground, and two wires to the solenoid. Consult the latest documentation from Greddy.

Connect the Switched 12V Power

Find a switched 12V source under the dash. The cigarette lighter or radio fuse are common picks. Better: tap the ignition harness wire behind the steering column (white/black wire for 1991–1994 NSX). Use a multimeter set to DC volts: with ignition off, probe the wire to ground—should read 0V. Turn ignition to ON (without starting)—should read battery voltage. Connect the orange wire from the boost controller harness to this wire using a T-tap or soldered splice. Add a 1A inline fuse if not already present.

Connect the Ground Wire

Attach the black wire to a clean chassis ground. Under the passenger side, there is often a bolt near the ECU bracket. Scrape any paint away and use a ring terminal for a solid connection. Confirm continuity with the negative battery terminal.

Connect the Solenoid Wiring

Mount the solenoid first (see next section). Then route the solenoid harness back into the cabin. The solenoid has two wires (typically blue and yellow). Connect them to the corresponding outputs from the controller. Polarity may not matter for a two-wire solenoid, but check the manual. Use waterproof connectors if routing through the firewall.

Tap the Boost Sensor (If Required)

Some installations require a boost reference wire from the ECU’s MAP sensor or a separate pressure sensor. For most NSX forced induction kits, the boost controller only needs the solenoid and vacuum hoses—the internal sensor in the solenoid does the measuring. However, if your Profec model includes an external map sensor, wire it according to the separate instructions. When in doubt, keep it simple: use the standard solenoid kit without extra sensor inputs.

Installing the Boost Solenoid

Place the solenoid in the engine bay, away from direct heat sources (exhaust manifold, turbo housing) and rotating parts (belts, fans). Ideal locations: near the passenger strut tower or on the firewall, using the bracket provided. Secure with bolts or zip ties through the bracket holes.

Plumbing the Vacuum Hoses

The Profec B Spec II solenoid has three ports: COM (common), NO (normally open), and NC (normally closed). The standard hookup is:

  • COM → connect to the wastegate actuator
  • NO → connect to the compressor outlet (boost source) or a dedicated boost reference line from the intake manifold
  • NC → leave open or cap (it vents to atmosphere)

Use 4mm vacuum hose for all connections. Keep hose length as short as possible. Secure with zip ties at each nipple. Ensure no kinks or leaks by performing a boost leak test later.

Mounting the Controller Display Unit

Choose a location where the display is visible while driving but not obstructive. Common spots: on top of the steering column, in the empty DIN slot beneath the radio, or on a custom bracket near the gauge cluster. Use the double-sided tape or small screws provided. Route the wiring harness behind the dash, avoiding pinch points. Leave slack for any future adjustment.

Reconnecting the Battery and Initial Power Check

With all wiring and hoses in place, reconnect the negative battery terminal. Turn the ignition to the ON position (do not start the engine). The Profec B Spec II display should light up and show the default pressure reading (likely 0 psi or atmospheric). If the display does not power on, check the switched 12V connection and ground with a multimeter.

Configuring the Boost Controller

Set the Boost Level

Using the controller’s buttons, navigate to the BOOST setting. Enter your desired peak boost pressure (e.g., 8 psi for a safe street tune). The spec II has separate HI and LO settings—useful for switching between pump gas and race gas or street/track modes.

Adjust Gain and Start Boost

Gain (GAIN) controls how aggressively the solenoid responds. Start at a low value (20–30%) to avoid boost spikes. START BOOST (S/B) sets the boost at which the solenoid begins active control. Typically set this 2–3 psi below your target boost. Too low, and boost builds too slowly; too high, and you may overshoot. Consult tuning resources like Science of Speed’s tuning guides for NSX-specific recommendations.

Set Low and High Maps

Program a low boost map (e.g., 5 psi) and a high boost map (e.g., 9 psi). You can toggle between them via a wire on the controller (or a switch you install). This allows safe boost on daily driving and maximum performance when desired.

Finalizing and Reassembly

Double-Check All Connections

Review every electrical splice, ground, and vacuum hose. Use a multimeter to verify there is no continuity between power and ground. Start the engine and let it idle. Check for vacuum leaks at the solenoid hoses—listen for hissing and watch the boost controller reading (should show vacuum at idle, around -8 to -10 inHg).

Reinstall Dashboard Panels

Once everything is verified, reinstall the glovebox, kick panel, and lower dash covers. Route any excess wire securely and zip-tie it away from moving pedals. Ensure no trim clips are left loose.

Testing and Fine-Tuning

Take the NSX for a gentle test drive. Monitor the boost gauge (on the controller or a separate instrument) and watch for spikes. Gradually increase the gain in small increments until you achieve a steady boost curve. Watch the duty cycle percentage; keep it below 90% to leave headroom. If boost creeps or falls off at high RPM, adjust the START BOOST and GAIN accordingly.

Perform a final data log if you have a wideband O2 sensor—safe air/fuel ratios are critical. For in-depth tuning parameters, refer to the official Profec B Spec II manual (PDF).

Troubleshooting Common Issues

Controller Does Not Power On

Check the switched 12V source with a test light. Verify ground connection. Ensure the inline fuse isn’t blown.

Boost Spikes or Oscillates

Lower the GAIN setting. Check for vacuum leaks or a stuck wastegate. Ensure the solenoid hoses are routed correctly with no kinks.

No Boost Control (Full Wastegate Spring Pressure)

The solenoid may not be receiving duty cycle—check wiring between controller and solenoid. Also verify that the solenoid COM and NO ports are not reversed.

Display Reads “Err” or “---“

This usually indicates a sensor fault. Recheck the boost sensor wiring (if external) or reset the controller by disconnecting power for 10 seconds. If persistent, contact Greddy support.

Expected Performance Gains

A properly installed Profec B Spec II allows you to safely run higher boost than a spring-only setup, typically adding 20–50 wheel horsepower depending on your forced induction kit and fuel system. More importantly, it protects your engine from overboost with its fail-safe mode. If the controller loses power, the solenoid defaults to the wastegate spring pressure (low boost), preventing engine damage.

Additional Resources

For community support and wiring schematics specific to the NSX, visit NSXprime Forum and the Honda-Tech forced induction section. Always cross-reference with your specific boost controller kit version—Greddy has revised the Profec B Spec II several times, so verify wire colors and solenoid pinouts.

By following this detailed guide, you will gain precise, reliable boost control on your first-generation NSX. Take your time, double-check each step, and enjoy the newfound adjustability of your forced induction system.