How to MegaSquirt Your TPI or TBI Chevy Small‑Block V8 (and even‑fire V6)

Overview

This guide covers pre‑OptiSpark Chevrolet/GM small‑block engines with Tuned Port Injection (TPI) or Throttle Body Injection (TBI) using HEI ignition with electronic spark timing (7‑pin or 8‑pin modules). Much of this also applies to 1980s GM big blocks and even‑fire V6s. We focus on proven MegaSquirt setups (MS3 w/ MS3X and MS2), HEI wiring, and TunerStudio configuration.

Why MegaSquirt for GM TPI/TBI

  • MegaSquirt‑II is a natural fit for these GM platforms—pre‑calibrated GM sensor curves, native HEI control, and stepper IAC support
  • MegaSquirt‑I can be used for a budget build but does not drive the stepper IAC; plan on setting the idle screw for cold start speed
  • Have noise on the HEI tach signal? Our HEI RPM‑signal mod kit improves crank reference integrity

Using MegaSquirt‑3 with MS3X (TPI/TBI)

Note: HEI provides no cylinder identification; this is a batch‑fire solution (not sequential). No internal ECU mods required.

External ignition wiring (HEI)

  • HEI “R” pin (tach out) → MegaSquirt mainboard pin 24 (or Relay Board “Tach”). Use only the inner conductor on the shielded wire in our harness
  • HEI “E” pin → MS3X pin 14 (timing signal back to the HEI module)
  • HEI “B” pin → MS3X pin 33 (bypass control)
  • IAC → connect the four stepper IAC leads (Relay Board terminals S1–S4 if used)

TunerStudio ignition configuration (MS3X)

  • Spark mode: Basic trigger
  • Trigger angle/offset: 10° (verify with a timing light; see “Trigger Offset” below)
  • Ignition input capture: Rising Edge
  • GM HEI/DIS: GM Bypass on SpkB
  • Spark hardware in use: MS3X spark
  • Dwell type: Standard dwell
  • Spark Output: Going High
  • Dwell: start 2.5–3.0 ms (adjust per coil)

Using MegaSquirt‑II PCB V3.0 or V3.57 — Original setup

Can be run on B&G code or MS2/Extra. This configuration does not use ECU‑controlled bypass.

If assembling from kit (MS230‑K): key departures from standard build

  • Step 22: install all IAC jumpers
  • HEI input inversion (module grounds pin 24 to signal):
    • Remove D1/D2 and replace with jumpers
    • Remove XG1–XG2 jumper and any jumper to TachSelect
    • Jumper TachSelect → XG1
    • Jumper OptoIn → +5 V in proto area
    • Remove C30
    • Replace R12 with ~1.3 kΩ (1–2 kΩ acceptable)
    • Jumper OPTOOUT → TSEL
    • Output: JS10 → IGN (pin 36)

If using our assembled MS2357‑C

  • Install the HEI input mod kit (drop‑in on current V3.57—see our video)
  • Remove JP1 and XG1–XG2 jumpers
  • Red jumper: 5 V near processor → JP1 pin 3
  • Green jumper: XG1 → JP1 pin 2.

External ignition wiring (original setup)

  • HEI “R” → ECU pin 24 (or Relay Board “Tach”; inner conductor only)
  • HEI “E” → ECU pin 36 (Relay Board S5)
  • HEI “B” → use a relay per the MSExtra HEI notes. Coil gets “Run”‑only 12 V; route VREF from Relay Board to relay switch, then out to HEI “B”
  • IAC → four stepper leads (or Relay Board S1–S4)

TunerStudio — B&G code (original setup)

  • Trigger offset: 10° (verify with light)
  • Ignition Input Capture: Falling Edge
  • Cranking Trigger: Trigger Rise
  • Coil Charging: Standard
  • Spark Output: Going High (Inverted)

TunerStudio — MS2/Extra (original setup)

  • Spark mode: Basic trigger
  • Trigger angle/offset: 10° (verify)
  • Ignition Input Capture: Falling Edge
  • GM HEI/DIS: Off (no ECU bypass control)
  • Spark A output pin: JS10
  • Dwell type: Standard dwell
  • Spark Output: Going High (Inverted)
  • Dwell: start 2.5–3.0 ms

Using MegaSquirt‑II PCB V3.0 or V3.57

If assembling from kit (MS230‑K)

  • Install all IAC jumpers
  • VR input: TachSelect → VRIN, TSEL → VROUT
  • Main spark (D14): 1 kΩ from +5 V (proto or R24 right side) → D14 negative; jumper D14 negative → IGN
  • Bypass output (D16): 1 kΩ from +5 V (proto or R28 right side) → D16 negative; jumper D16 negative → SPR4 (DB37 pin 6).

If using our assembled MS2357‑C (new setup)

  • JP1 = 1–2, J1 = 3–4 (VR input)
  • Remove JS10 → Q16 center wire; remove Q16 if fitted
  • PAD1 → Q16 center (spark out)
  • PAD3 → SPR4 (bypass out).

External ignition wiring (new setup)

  • HEI “R” → ECU pin 24 (or Relay Board “Tach”)
  • HEI “E” → ECU pin 36 (Relay Board S5)
  • HEI “B” → ECU pin 6 (SPR4)
  • IAC → four stepper leads (or Relay Board S1–S4).

TunerStudio (new MS2/Extra setup)

  • Spark mode: Basic trigger
  • Trigger angle/offset: 10° (verify)
  • Ignition Input Capture: Rising Edge
  • GM HEI/DIS: GM Bypass on D16
  • Spark A output pin: D14
  • Dwell type: Standard dwell
  • Spark Output: Going Low
  • Dwell: start 2.5–3.0 ms

Trigger Offset — how to set and verify

  1. Warm the engine fully; hold a steady idle.

  2. In TunerStudio → Tools → Trigger Wizard, set “Fixed” timing and adjust Trigger Offset (and, if needed, rotate the distributor) until the timing‑light reading matches the large value shown in the wizard.

  3. Positive numbers are BTDC; negative are ATDC. Re‑check after any distributor movement.
    See also: MSExtra forum threads on GM HEI modules and stepper IAC control.
    [Link: MSExtra HEI Notes] [Link: Stepper IAC Notes]

Swap‑friendly notes

  • The DIYAutoTune harness uses shielded tach wiring; ground shields at ECU end only.
  • Many TBI/TPI swaps benefit from fresh grounds, clean engine‑to‑chassis bonding, and proper alternator noise suppression.

Suggested parts (Chevrolet/GM TPI & TBI)

MS3 + MS3X path

MS2 path