The 16V ECU unfortunately can’t* be reprogrammed, so enthusiasts often resort to custom piggyback boards, modified ROMs, or small add-on modules to adjust fueling and timing. Even TRD and other major tuners produced custom piggybacks for these ECUs back in the ’80s and ’90s.

*Note: Some individuals have reportedly reverse-engineered the firmware and claim to support reflashing, but so far no fully verified or publicly working solution other than piggyback boards exists.

I made some non-mcu boards myself before i upgraded to an standalone.

Oldschool Modboard made by me : Adjustable T-VIS RPM, Fault Code LED, Adjustable MAP and CLT Signal offset, an Logic and an 5A Output (Speed Chime and/or RPM Switch) and IGF (Ignition Coil Field/Fault Signal) Simulator to use other Coils and Drivers 

First version of my adjustable clock-speed board. Changing the processor’s frequency alters ignition timing, fueling, and the RPM limit — effectively letting you "fine"-tune the engine without a piggyback or standalone ECU.
It’s a bit messy but also a simple way to tweak the factory tune.
Managed to get a clean 104 kW on an Bigport with just slight head and intake work done

The OEM transistor (NEC 2SD1694) on Blacktop ECUs is a common failure point.
It’s rated at 3 A @ 60 V. Thats enough for the VVT solenoid which draws around 1.3 A, but it cant handle the inductive kick when switching off.
There is a 1Ω Resistor and a protection diode (MPG06J) on the pcb, but it seems like it doesent help at all, leading to repeated transistor failures.

 
Note:
Some people claim the 20V uses variable control for valve timing — it doesn’t. There’s no PWM, just a simple ON/OFF signal.

While PWM control might be possible with a standalone ECU for finer tuning, there’s little reason to do so. It’s also unclear whether the solenoid (specifically its internal spring) can tolerate PWM operation. The pulley itself would likely be fine, as it moves relatively slowly — full activation reportedly takes about 1.5–3 seconds.

Anyways, i replaced it with an IRL540N logic-level MOSFET (100 V, 36 A) — total overkill, but intentionally so. It’s avalanche-rated and much more robust. I also added a 10 kΩ gate pulldown just to be safe (under the heatshrink, secured with epoxy).

See image.

I used those for my ECU (89661-1A860) but this will most likely correct for other ones. I didnt write down the amount of each caps needed, but buying in 10x is cheaper anyway.

HINT: PCB is a 4 Layer one with huge ground and power planes internally. Use a desoldering gun or an iron with a big thermal mass, otherwise the now 30 year old plating/tracks can lift of the Board due to excess and too long heat.