Stock ECU compatability (newer DOHC into older DOHC chassis)

[AJ711]

Since I feel there are a greater quantity of knowledgeable nerds/geeks/motorheads here, and since I haven't posted in forever, I'm posing this question to you guys.

Current situation: 1998 Impreza RS, NA, ported/polished heads, valve work, special grind cams from Delta, and a host of other bolt-ons. I have an Apexi S-AFC to help "tune" things, but I want more options and abilities. However, I also want to be able to pass emissions tests (OBD-II connection). As far as I know, any stand alone EM will NOT allow this to happen.

Reflashes are not an option, as the '98 ECU is dumber than rocks. It does make "tuning" with the S-AFC an option, but that isn't good enough for me. I'm getting some stumbling and rough low throttle/low RPM issues that, I think, only messing with the ECU's data will fix. It looks like the PP6 is quite the capable unit, but there doesn't appear to be a whole lot of support. This thread on NASIOC details a lot of members' headaches; fried coil packs, generalized installation and tuning, etc.


So what I'm wondering is this. Could I take one of the newer DOHC ECUs that could be adjusted by ROMRaider or something similar, and wire it in to my vehicle's harness? I guess the larger issue here would be making sure that the voltage values the ECU expects to see are the same. I won't mind playing cut/solder/shrink wrap with those wires if it means I can start altering fuel maps accurately and getting a bit better performance out of my motor.

Or is my best bet the PP6 setup and getting a more robust piggyback unit to get the job done?

And where is the little black box that mimics the ECU output that we probably talked about over chilli so many years ago?

[zaxrex]

I don't know much (anything) about PP6. Just wanted to get that out in the open.
What I do know is that it would be a major undertaking to mimic/install all the sensors input on systems that the newer ECU expects to see in order for it to pass OBD-II based inspections. I would think that would be an even bigger task than just grafting together the sensors and looms needed to just get it to run...
But then, what have I done?

[Sabre]

I'm in agreement with Zak. That could be a hell of an undertaking. Not saying it isn't possible, but you might have better luck going aftermarket. I know some of the newer standalones (like the Hydra, but I have heard the Hydra sucks on the RS) can pass the ODB-II signals from the stock ECU while actually running the engine on the standalone.

Little blackbox: We're still talking about it, haha. With the advent of the newer microcontrollers (like the Propeller and Arduino), it really wouldn't be too difficult.

[AJ711]

I guess it can't hurt to take a look at ECU pinouts and see what sensors are the same and which are different. Figured I would ask here before any forward motion. Thanks for the input, and I'll put something up here when I get a chance to sit down and look into things.

The car runs quite roughly at cold start. It takes a good couple of miles to get things where it doesn't pull timing while accelerating. I think the first thing I need to do is grab some data logs with the PocketLogger and see what that says for cold vs. warmed up driving.

[Sabre]

I'm willing to bet you are running lean at startup. Running lean at startup causes stumbles that feel like timing is being pulled, where as running rich (unless WAY rich... think 9:1) is almost always is smooth. If you have temperature compensation in the Apexi S-AFC, I'd richen up the lower coolant temps.

[AJ711]

Most certainly it runs lean. The wideband jumps way up to the high teens, even pegs, and things get really rough while the motor is cold and idling. I just didn't think there was a way of changing the tuning that doesn't monitor the O2 sensor outputs. I can't remember if it is closed or open loop. Either way, as I understood it, when the ECU isn't looking for the input from the O2 sensors (based on throttle position, RPM, and maybe a few other things), it relies on settings that are programmed into the ROM. And I'm sure the cams and head work is altering how well the stock mapping keeps things running stoich.

Hence my desire to grab an ECU that ROMRaider can handle, or even ECUExplorer. If I can take the base map and add a bit more fuel for cold conditions, it'll be perfect. And it should allow a bit better tuning of high throttle conditions. I imagine tuning the ROM is a bit finer than via SAFC.

Quick perusal of NorthUrsalia's site and some other google searches has me thinking this might work. I need to find NA specific 05 Impreza pinouts, as all the ones I've found have pins for tumbler valves and the like, which NA didn't come with.

Worse yet, the NA engines are all SOHC. Which means I'd need to get a FI ECU to get the DOHC issue taken care of, but brings into it the issues of included signals that I won't have, notably tumble valve positions and other FI specific readings. That does solve the MAF vs. MAP issue that most SOHC engines will utilize.

This may be a worthless venture. If I can turn off values and things inside of ROMRaider, I should be good to go. Otherwise, I don't think there is any chance of getting it to work.

[Sabre]

Worse yet, the NA engines are all SOHC. Which means I'd need to get a FI ECU to get the DOHC issue taken care of, but brings into it the issues of included signals that I won't have, notably tumble valve positions and other FI specific readings. That does solve the MAF vs. MAP issue that most SOHC engines will utilize.

This may be a worthless venture. If I can turn off values and things inside of ROMRaider, I should be good to go. Otherwise, I don't think there is any chance of getting it to work.

You can turn off the CEL's associated with a lot of things, including the TGV's. I didn't have TGV's on my STI and I just turned off the CEL's for them. Vola, no problems

[AJ711]

Well, that certainly opens things up. I may look in to this option down the road if I can source an ECU on the cheap.

That said, I reset the ECU this weekend and there has been a marked improvement. No longer running nearly as lean (highest AFR on the wideband has been 17) and it no longer stumbles. Now I start fiddling with the SAFC to see what can be done there.

[OtisSeldon]

I attempted to contribute to this thread in a positive way, but failed.

[AJ711]

As an update, there is a possibility for me to run something that is open source tunable. This thread on RS25.com has been a HUGE source of information.

I can swap phase II parts in, as well as some WRX parts, and run a 02/03 WRX ECU in place of my MY98 ECU. There are a few snags that I am looking in to.

1) the WRX cam sprocket may not work with my camshafts. WRX camshafts have a pin on them, where my camshaft has a slot. As well, the timing marks are different, and I'm currently trying to determine if the important parts of the camshafts line up between the EJ25D and EJ205. If those line up, then I might be able to modify my camshaft sprocket to work.

2) EGR valve. I have one, the WRX does not. I need to use a WRX intake plenum, so I will have to remove and plug my EGR system. Just another annoyance with this process.


The ability to run a WRX ecu, lightweight flywheel (phantom misfire codes galore), and potentially not worry about misfire codes (disable them for a short time) and still pass emissions (set all DTC codes to "passed") would be awesome. Anyone have any EJ205 camshafts lying around? I may swing by Andrewtech and see what they have taking up space. I just need to take a quick look at them and compare them to EJ25D camshafts.

[AJ711]

OH yeah, I'm definitely more interested in this project now. I swapped a 5MT for the 4EAT the car came with, so driving is much more fun. The lightweight flywheel and the stock ECU did not agree, but a stock flywheel and lightweight crank pulley seem to be doing okay at the moment. I would LOVE to run the lightweight flywheel with the lightweight pulley, as those coupled with the NA motor (and an increased redline thanks to RomRaider) would make that car so much fun on the track.

[Sabre]

No CAM shafts here My first thought is that Zak might have some.

Sounds like you have things pretty well worked out! Once you get the ECU's swapped, things will be much easier.

[zaxrex]

Sorry, only had EJ207 cams.
I'll look in the garage this weekend when I am back home and see if my memory has failed.

[AJ711]

No worries, though I wonder if the 207 cams are similar to 205? I have zero idea about that aspect of things. I may just give Andrewtech a call and see if Sam has anything lying around.

The car passed MD VEIP last Friday, only after swapping a stock flywheel in place of the lightweight one. That gives me a cushion (and ability) to pass again in a few months. So, I need to start researching whether this project will work or not. I may just have to wait until the car is old enough (almost there) to not require VEIP testing and THEN make it fun to drive again.

[Sabre]

Did it not pass because of a CEL? For lightened fly wheels, I see mis-fire codes a lot. I just turn them off