AI Overview

For a standard AE86 4A-GE ignition system using the stock distributor and igniter, the ECU does not control dwell time directly. Instead, the Toyota igniter receives a trigger signal (IGT) from the ECU and manages the dwell time internally.

However, if you are configuring an aftermarket ECU (like Haltech, Link, or Speeduino) to control a 4A-GE with modified or direct-fire ignition, here are the typical settings:
Stock/OEM Igniter Dwell (Base)

Dwell Time: ~3ms is the typical base setting for the standard Toyota igniter/coil setup.
Behavior: The Toyota igniter controls the spark duration based on the input signal, but it has a minimum hard-wired dwell time.

Aftermarket/Direct Fire (COP) Dwell
If converting to Coil-on-Plug (COP) using Toyota coils (e.g., 1ZZ/1NZ or Yaris coils) without the stock igniter:

Recommended Dwell: 3.0ms to 4.5ms is commonly used for 1ZZ or 1NZ coils.
Cranking Dwell: Around 4.5ms at 10v is used to ensure starting, as reported in Speeduino setups.
Max Dwell: Setting dwell too high (e.g., >4ms) on COP can cause coils to overheat and fail.

Important Notes

1. Igniter Type: Ensure your aftermarket ECU is set to the correct ignition mode (e.g., "Constant Charge" or "Duty Cycle").
2. Stock Igniter: If using the factory igniter with an aftermarket ECU, the ECU should only send the IGT signal, not direct dwell timing, as the igniter handles it.
3. Dwell Scatter: If you notice significant dwell fluctuation (scatter) at high RPM, it is often due to poor trigger accuracy from the cam-driven distributor