Converting an OBD-I Legacy turbo to OBD-II

[Matt Monson]

Ok,
VRG, I know you are gonna be the man on this one...Here's the deal. I got more than I expected on my recent STi v6 parts purchase. Not only did I get the expected VF-28, intake manifold,TB, injectors and fuels rails, but I got a complete uncut functional OBD-II engine harness. So, with my planned build I was gonna need stand alone engine management and all the wiring entailed with that.

Since more and more my build is looking to be an STi v6 w/an EJ22 block, why not just wire this thing up and run it off an WRX ECU? I can take the whole thing to Cobb once it is done, put it on their dyno and tune it with AccessPort ECU as if it was a WRX...

So, the big question here is how much of the bulkhead harness will need to be changed? My first guess is that the USDM turbo Legacy is wired real similar to an early JDM/AUS WRX? And I really need to look to guys that have done OBD-II based engine swaps on those cars to figure out the details?

I do have access to service manuals for both newer WRX's and the Turbo Legacy, so I can check and confirm all this, but having people that have done this or explored this would be a great help...

[vrg3]

You're probably right that a USDM Legacy Turbo has similar wiring to an early 90s WRX so you could draw upon others' experience there.

On the Legacy Turbo, the engine management wiring is part of the rest of the car's wiring harness, so you probably wouldn't want to try removing it. You may, however, be able to overlay the new OBD-II harness. It'd be a pain to run it everywhere it goes in the car, but you could probably do it... end up with a WRX ECU under your passenger carpet.

There are a couple of potential catches I can think of off the top of my head:

At some point Subaru changed the design of the cam and crank angle sensors. I believe the WRX ECU will require the new type in order to work correctly. Swapping them in from a newer SOHC EJ25 might be possible, but I don't know.

The WRX ECU will want a Denso wideband oxygen sensor in front of the cat and a standard sensor behind it. It'll also want a Subaru EGT sensor before the turbine inlet. And a fuel temperature sensor. And a power steering pressure sensor. And TGVs (does the STi v6 have them?). And some other similar stuff.

It'll also expect to be able to talk to the TCU.

It may not be worth it... I don't know what I'd do in your situation. I'd probably just wait until I get somewhere with my project of hacking the stock Legacy Turbo ECU and remap it. But in the meantime, maybe you could get a cheap aftermarket ECU. You could even just get one to handle fuel and let the stock ECU handle spark. MegaSquirt's pretty cheap.

[Matt Monson]

Well,
For starters, when I switch over the STi manifold, all the connected wiring around that will need to be disconnected and replaced anyways.So reconnecting all that stuff using the STi harness won't be too bad. I figured there were a few sensors that would be needed. The DP I have has bungs for both O sensors and an EGT, so that is a non-issue. As for the cam and crank, that shouldn't be too hard either. As far as the TCU, it is an 4EAT tranny, so maybe the TCU's never changed, or maybe a newer TCU will be needed to talk to the newer ECU. I will have to look into that...

I have been meaning to talk to you about your Legacy ECU project. My GF is a bit of a geek and good with code and loves Subarus. If you are looking for some help, and wanted to share where you are with it, she is willing to take a crack at it. And if you were looking at it as some sort of commercial endeavor, we would happily sign a non-disclosure and non-competition agreement and only use the info on my personal car and not steal your business...

[vrg3]

If you think you know what you're doing with the wiring swap, go for it; I was just raising some issues that you'll have to deal with. You're right that you're going to have to get messy with the wiring on the intake manifold anyway.

The TCU definitely has changed between 94 and the present day. Your Legacy Turbo's TCU doesn't communicate with the ECU at all, except for tapping off a few of its sensors. The big question is whether the WRX ECU would go into limp-home mode if it can't communicate with the TCU. Or maybe you could just tell the WRX ECU that it's in a 5-speed car?

I don't know when exactly Subaru switched the cam and crank sensors on which engines, but you'll need the correct sprockets. If you're using your stock SOHC heads I doubt you could use sprockets from a DOHC WRX. But, like I said, if the Phase II EJ25 uses the new design you might be able to swap those on.

Keep us posted on how that goes.

I appreciate your girlfriend's offer to help... I've actually gotten offers from a bunch of people to help. My plan now is to find enough free time to organize the work I've done so that additional pairs of eyes will be able to make sense of it and help out. I'll remember her offer when I'm ready to take her up on it.