Early and Late ECUs

[sammydafish]

What's different between the 90-91 and the 92-94 ECUs? I know the late models use a plastic AFM and have different style injectors. What in the ECU is different though? Are they plug and play? I have a 92 parts car here and I'm wondering if there is any benefit to swapping the ECU. Is the newer (plastic) AFM compatible with the older one (metal).

I thought I read somewhere that the old ECU was 8 bit and the new one was 16. Not sure about that though, can't seem to find that now.

[Legacy777]

The 90-91 ECU's are 8-bit.

Unless you're planning to swap injectors as well, don't bother swapping ECU's.

[sammydafish]

well... I have the injectors and could swap. Engine is out of the car right now, so if there is any benefit, it wouldn't take long to do.

In the fuel milage thread, a few people noticed that 90-91s got better economy than 92-94s ... anyone care to guess as to why?

[vrg3]

It's a little more complicated than that --

There are two engine management manufacturers used: JECS and Hitachi. JECS ECUs are 16-bit and use the black plastic MAF sensors (which are also manufactured by JECS). Hitachi ECUs are 8-bit and use the aluminum MAF sensors (manufactured by -- surprise! -- Hitachi). You need to use the correct sensor for the ECU you're using.

Models with JECS ECUs:
90-91 automatic non-turbos
92-94 non-turbos

Models with Hitachi ECUs:
90-91 manual non-turbos
all turbos

The external wiring's mostly the same (aside from turbo vs. non-turbo differences and the fact that no 93-94 ECUs output a barometric pressure signal).

The ECUs are each calibrated for their injector type (of which there are four: 90-91 non-turbo autos, 90-91 non-turbo manuals, 92-94 non-turbos, and turbos), so as Josh indicates you might have troubles mixing and matching them. They all have the same basic flow rate but their opening and closing times (and maybe the relationships between these times and voltage) differ.

I don't think any of us knows whether or not you could see achieve any kind of improvement just by swapping the ECU and/or injectors. If you try it out, let us know.

[Manarius]

So, my 91 non-turbo auto has a 16bit ECU? Why? Why wouldn't the turbo have the 16 bit ECU? Is there any advantage to this?

[andys2]

I think my 92 JDM EJ20T has a JECS - but I am not 100%. What I do know is that is has the same MAF as my 93 impreza L FWD auto. In fact I am using the impreza maf now since it was cleaner on the inside

[vrg3]

Don't get too caught up in the 8-bit vs. 16-bit difference (wow, flashback to the video game console wars of the early 90s).

They're just made by different manufacturers, and Subaru phased the JECS ones in.

Yes, the EJ20T ECU is made by JECS. It's very similar to the JECS ECU used on American Legacies.