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The Single Turbo Conversion Thread 2.0


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1. Introduction.

This thread has been made to store all the tips, tricks, and how-to's on the Legacy Twin Turbo, to single turbo conversion. The following content is based on my conversion, with some research from the ClubSUB forums, and other people who have done the conversion before me. This conversion applies only to the B and C revisions of the BG5/BD5 legacy, that have the EJ20R motor. It is not applicable to BH/BE's, or BG5/BD5 That have EJ20H's. Nor does it apply to the AUTO B's and C's as they have a single plug ECU that is not compatible with the single conversion (unless you reloom). However this doesn't stop you from gaining a bit of knowledge on the how to's of this conversion. I ask you to please read the entire thread through before getting started too. I will be assuming you will be lifting you engine for this, and assuming you know how. It will make life a whole lot more easy for you, but it isn't actually necessary. In fact, it's perfectly possible to leave the engine in for the conversion. Up to you. And lastly, this is only a rough guide, with a few pointers in it, not a manual. Please remember that!

And from hereafter i'll be using "V3/4" to simple refer to Version 3 and 4 WRX's, and STi's, and Forresters with the same engine (EJ20K). Also, the source i used for defining what BG/BD models came with EJ20R's was here http://homepages.ihug.co.nz/~ccgrant/leggymodels.htm I am pretty confident it is ONLY -MANUAL- GTB's and RSB's.

First thing you'll need is a shopping list, expect to pay between $1500 - $3000+ for this, but with luck, that is ALL you'll spend. I've tried to list every last washer and rubber pipe and bottle of fluid, so you don't get any hidden costs, and can run a tight budget. Item's marked with an * are not required, but do make things go allot easier, or make your engine bay tidier etc. I strongly recommend getting everything on the list, and you will see more about them in the thread.

A Version 3/4 STi ECU

A turbo. Such as a VF22, 24, 30 etc. From a single turbo subaru.

Single turbo Coolant supple and return lines. You can make your own

Single turbo Oil return line (rubber pipe) There is a pipe from the TT system you can use, but it's short and cut's things fine.

Single turbo Oil return line (Metal pipe in block) This will be one hard SoB to find, but you need it. Read stage 3 to find out more. (This pipe, to replace this pipe)

Single turbo exhaust manifold. A 2.5L equal length one works too, and offeres better fuel economy. But you won't get that boxer rumble ;)


Downpipe. Caution: Some don't bolt up to the factory midpipe, such as the rage downpipes.

Single turbo gear box cross member. Needed if you get a bolt on DP, not if you make your own downpipe.

Single turbo up-pipe bracket. (the one by the turbo)

Single turbo intercooler Y pipe. Anything from V3/4 and 5/6 will work

If you get an STi intercooler with your y pipe, you need to find a V3/4 intercooler bracket, they are damn hard to find!

Turbo outlet to Y pipe hose

*V3/4 Intake manifold, complete. If you do not get a complete one, here is what you'll need for it:

*Idle control valve supply line (metal thing that goes on top of the manifold)


*BOV Return line/rockercover breather lines (Don't forget the little rubber hoses that connect it to the rockercovers!)(it's a metal pipe hudden on the underside of the manifold)

Turbo Inlet/intake pipe

Crank case breather line (I had to make my own)

*Right hand fuel rail if you want to do the split fuel rail, more on that later

2 meters of vac line. You may be able to make do with the leftovers from the TT system. Budget for this anyway

*Single turbo solenoid bracket (This is so you can replace the BBoD)

*Pressure select solenoid (a 3 port solenoid from a single turbo, this is only to tidy up the wiring, you actually already have one in the BBoD that you can use - but it has a great big plug with like 15 wires.)

Bolts to block off the primary turbo's Oil supply, return and coolant return lines on the left hand head

Gaskets for:

Intake manifold

*Throttle body (only $3)

Exhaust manifold





Oil filter + Sump plug washer

*Silicone radiator hoses

*Silicone coolant hoses (nows a good time to replace them! .... DO IT!)

And Budget an extra $200 for crane hire, and tools you don't have etc.

If you wish to do the split fuel rail mod while you're at it (estimated cost, $150), which i recommend you do since the intake manifold will be coming off anyway, all you need is

8 feet of EFI (Electronic Fuel Injection) hose. 8 ft really is just enough.

A few hose clamps (6 should do it, you can reuse the originals for the rest)

2 T sections

A second right hand fuel rail, with luck you picked one up with the intake manifold

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2. Getting Started.

As mentioned above, you do not actually need to pull the engine to complete this conversion, but i think it makes things easier. I'm going to skip how to do it for this thread though. This, mainly cause that's too much writing, and i'll probably forget 90% of what you need to do! If you chose to lift the engine, make sure you have a mate who's done it before to give you a hand, and/or use the service manuals. Also, get yourself a pack of zip lock bags, some tape, and a sharpie to keep all your bolts with their associated part. I didn't, it sucked.

Tool's you'll need, will be a socket wrench (set) with a short and long extension, a 14mm impact socket, a power bar, pliers, spanners etc.. and even a little 12mm ratcheting spanner is a huge help for getting the aircon out.

Once you have bought all the bits you need, Rip into it! You'll need to take your intake manifold off, your downpipes off, exhaust manifold and turbos. up to you which order. You also need to swap the gear box cross member over if you're getting a bolt on downpipe. You'll want to drain your oil, and coolant too.

Onto the conversion 8)

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3. The conversion part 1.

First things first, Get rid of everything on the left side (passengers side) of the engine that is to do with the primary turbo. This includes brackets, turbo, turbo oil supply and return lines, and coolant supply and return lines. The oil supply is in an odd place behind the cambelt, i can't recall if you need to take the cambelt off or not to get at it. Blank the oil return and supply with a bolt, and a bit of gasket glue. put the glue around under the head of the bolt after you're put the bolt in, but before you're tightened it. we don't want bits of silicone blocking the coolant system now do we? At least that was my reasoning. The coolant return is also an easy one to block off with a bolt, but unfortunatelly the oil return is a right old bitch. Some pics of these steps:






Once you get off the oil's return line, you have to block the holes. Now i tried doing it with a frost plug. First, i couldn't find any, then when i did, none of them fitted!, i needed a 15.5mm plug. I gave up, and just welded what was left of the return, and put it back in after all that effort to pull it out! the other option is to simply slice the rubber line for it, cram a bolt in, and do it up with a hose clamp. that was too messy and rangi for me to even consider. Give the left head a good look over and see if there's anything else left, i think i coverd it all, it seemed to be pretty damn easy.

Now, onto the primary side. You will need to keep the turbo's oil supply line. The coolant supply you will need to make up, unless you get a turbo from the '98 ish era, such as a VF24 or 22, In which case you can buy a proper line from a dealer. The coolant return line you can make your own as well. BUT, the oil return line in the block, you really should steal one from a single turbo. I imagine that will be the hardest piece to find in the whole conversion! One from ANY single turbo will be fine, just get it from a pick apart. You will need to take the TT one out, and hammer the new one it. This is because the TT's sticks out the right side and conflicts with the new up-pipe, but the Singles sticks to the left and up:



Once you have the coolant and oil returns sorted, time to stick on the exhaust manifold, and up-pipe. Don't forget the heat sheilds. With that in place, on goes the bracket. Do get one, you could damage something if you don't have it secured properly. People will try talk you out of it some of these seemingly useless things cause it's too much hassel, but seriously, do the job right first time round and bother with the little things.. Now stick on the turbo! Make sure you got the rubber pipe for the turbo oil return on. There is a pipe from the TT's system you can use (i think its from the secondary BOV) but it's short, and only just fits. i would recommend getting the subaru part for this. Then on goes the intercooler bracket

Right, so new problems: The up-pipe bracket i found conflicted with the oil supply line. The angle grinder fixed that. Then the intercooler bracket conflicted with the turbo outlet. this is because i got a newer model turbo with an outlet that points towards the back more, The older ones (VF24/23/22 etc) point forward. the angle grinder fixed that too.



Check over the engine and make sure everything engine wise is delt to. Next up is the intake manifold and vacuum system.

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4. The conversion part 2.

Right, now to deal with the intake manifold. Strip the TT manifold of all it has. And you will be doing a split fuel rail while we're at it :P. Take photo's of how the loom is arranged. Also, if you break this, don't worry. leave the vac lines off it, it's not needed. But leave it plugged in so you don't get a CEL (it's to do with emissions, it goes off to the charcoal cannister)

Split fuel rail mod:

This is a very easy mod, you need:

8 feet of EFI (Electronic Fuel Injection) hose. 8 ft is really just enough. I think you need 9 feet for BE's and BH's as the filter is not in the engine bay

A few hose clamps (6 should do it, you can reuse the originals for the rest)

2 T sections

A second right hand fuel rail, with luck you picked one up with the intake manifold

Now, cut your fuel rails like so:


Now take the injectors out of your old left hand rail, and swap them over. TAKE CARE. There was a great thread on how to take them out, but sadly it was lost in the sever F*** up of 09. Basically, it's a c*nt of a job, and it's easy to brake something. just be patient, and use force.

Bolt both fuel rails onto the intake manifold, and cut your EFI to length. I recommend you arrange them under the BOV return/crank case breathers. it makes things tidy and easy to install. Sorry, no pics of how to arrange it. You'll see what i mean though.



And this is from my third time doing it.



Now put the final length of EFI hose on the 2 T sections, loop it around as you don't know what length the supply and returns should be yet. You'll cut it once the manifold is installed, and the engine is in the car. Mark which line is the fuel return, that will be the line with the fuel regulators on it. This line will be longer FYI. Once the manifold is on and the engines in the car, don't vorget to hook up the vacuum lines on the regulators too.

That's the split rail mod complete. simple huh :D

Now put everything back together on the new intake manifold. Note: Remeber the crank case breather hose to the turbo intake pipe, the TT one is different, either make one or buy the subaru part. Also, if you're using a V3/4 Intake manifold, you need the TT's throttle cable bracket. Now there are some parts not actually required from a single turbo here. Those being the rockercover breather hose, and ICV line. If you used a V3/4 Intake manifold, and not your TT's intake manifold, you must use a V3/4 ICV line, but you can use the TT's rocker cover breather line. You have to hack out the old TT rockercover breather line from the other 2 lines it's attached to, From this to this. You won't have to do any of this hacking if you used BOV return line/rocker cover breather line from the V3/4 manifold (recommended method! :D)

Ok, so now we have a complete single turbo intake manifold of some description, and split fuel rail to ensure that Cyl 3 doesn't melt after you inevitibly crank the boost up... Put the sucker back on the engine and ensure you connect everything up, like the crank case breathers and purge control solenoid (that green thing you broke... ;))


Sweet, that's simple aint it. Engine done and dusted! Hardly any work at all is it ;). And this coming from a damn n00b ;D

Now give the engine a once over, check everything is in order, all things plugged in, all lines plummed correctly etc. Now you can drop the engine back in, or move on to the next step if you never took it out.

There are a few things to note at this point. You'll be wanting to pop the intercooler back on by now, but just remember this. The line that goes from the water pump to the heater core on a V3/4 runs around the side of the clutch housing, but in the TT's it runs up and over right where the intercooler Y pipe wants to be (would normally go between the two TT intercooler pipes). If you want to be really pedantic, you can replace that line, but it's a waste of time as the TT one just gets squashed and pushed around, with no ill effect (been running it for a whole year now, all is fine). Be mindful of that however, as you re-install the intercooler! Don't forget about the O2 sensor either, the TT's one will stretch over and fit behind the turbo just fine, but putting the intercooler back on can stretch the wire, i'm lucky mine didn't tear off... Just give it pleanty of slack turbo side before putting the intercooler back on, and you'll be fine.

Right, onto the Black Box of Dooooom! Oh noes! ... It's actually pretty simple really... Jason showed me the pic below when i had to do it, it's really all i needed. Basically make your solenoid box look like the one below, and you're set. Now, in the shopping list i said to get a "*Single turbo solenoid bracket" and "*Pressure select solenoid". While you don't have to, it just makes the TT conversion that much more complete. The bracket i said to get for obvious cosmetic reasons, the solenoid, just so you can lose the big whore of a plug with a mess of wires coming out of it that you won't using any more. The picture of my end result shows this well (thanks cameron for the wiring on that one)



If you don't know how to plumb up the single turbo solenoids, simply get a mate, or someone from clubsub over who has a V3/4, and copy their engine bay. Just hook up all the vac lines like theirs, and you're set. I'll point out here that the boost control solenoid there... is NOT the solenoid the V3/4 ECU will talk to. It talks to the solenoid in the passenger side fender. It's just a simple wire change, and is noted in the wiring guide.

Now that's pretty much it. Put everything back in the engine bay make sure EVERYTHING is plugged in, like radiator fans, starter motor ground wires etc. etc. and put that intercooler on. Another note on the intercooler; If you get a V5/6 one like i did, thinking "it's the same shape and size, it'll fit right in". You're wrong, it doesn't :P You need a V3/4 bracket for it, and you'll have to make a BOV adaptor than can fit in behind the ICV, you'll see what i mean. As a result, you'll just have to scavenge the Y pipe off the underneath of it and stick it on your GTB intercooler, untill you can sort out the V5/6 intercooler proper. You'll have to trim some of the long end off to fit it (Short end goes to the turbo). If you got a newer turbo with the inlet that points towards the back of the engine back, you'll have to trim even more off. i took about 30mm off mine in the end, and it was still a tight squeeze, but it's entirely possible, even with the bastard heater hoses.

Ta-da, all done. Now the fun part, the ECU wiring.

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5.1 ECU.

The reason i said in the beginning, that this thread was for EJ20R legacy's only, is mainly because of this; the ECU wiring. You need only solder 4 wires, and you're done! Auto's have a single plug ECU that isn't compatible with any STi ECU, so you're buggered there. My official line on it is you should have got a bloody manual you pansy! Ha ha ;D (Just kidding, auto's are great.. i really mean it ::)) I am also unsure on the ECU's for the Manual EJ20H drivers... And BH/BE drivers. But what i have found out is the 4-plug EJ20H's are somewhat compatible with the 4Plug EJ20G's. I have posted a pinout for you guy's at the bottom of this thread. BE/BH drivers, V5/6 STi ECU's for the EJ208's, i have not seen pinouts for either, so up to you to research that one. But it has been done!

Anyway, the wiring for EJ20R drivers:

Wire A9 (loom) into A10 (ECU). Wire A22 and A23 (loom) into A9 (ECU). Ground B18 (ECU).

Swap A19 with A3 <- makes V3/4 ECU talk to the correct boost control solenoid.

And the not so technical drawing of the above description for us simple minded folk.


It's that easy.

If you compare the links below, you'll quickly find out V3's and 4's Just use a EJ20R loom! All the TT bits are blanks on the V3/4 ECU's.

here's some documents to back everything up

V3/4, Plug B137. Courtesy of http://www.northursalia.com/

V3/4, Plug B136 and B135. Courtesy of http://www.northursalia.com/

Twin turbo 3 plug pinout. Courtesy of UK Twin Turbo Legacy Register

5.2 Coil Pack Mod.

Hokay, quick addition to the conversion how too. I've been asked to do a write up on the coil pack mod, to rid ones self of the coil on plug system, and go to the sexy, cheap, and reliable coil pack system.

Eaaasssyyyy. You will need a Coil pack and leads off any Version 3 or 4 impreza, or N/A BG/BD. A matching 5 pin ignitor, and some additional wires and some actual loom stuff. Refer to the pictures to see what these look like!

For the wiring, I just spliced into the engine loom to get my power and ground, and 2 ignition signals. For the power, I think I just stole the 12v supply that goes to the coil packs, makes enough sense. And for the ground, there's a general ground wire you can tap into. Again, the pictures show which wires I used clearly. Then from there I just made a new branch off from the loom with some wires I had lying around, dedicated to the coil pack and ignitor. It's very strait forward! I think i'll let the pics and wiring diagram do the rest of the work for me.







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6. Epilogue and Notes.

Well. What can i say. It's one of those projects that start out seeming fairly simple... then get very complicated... But once you're done, it actually seemed quite easy... ??? It took me three and a half months to do.. but 3 months of that were taken up by my engine rebuild. The conversion itself, only took one (full time) week! add another day or two on for the engine lift and drop, and ta-da, a 2 week project at max (After posting this up, someone did it in a single weekend, including the engine lift!!). I hope that my shopping list will allow you to gather every last piece you need before even starting.

There's not a whole lot left to do from here. I can't actually remember... Some said beware, the ECU will spit fault codes at you like a monkey on crack, but i did a whole engine rebuild, and conversion at once, and didn't get a single CEL! There's not a whole lot to get wrong!

It really is a fun project, and WELL worth it. Im adding this in a year after doing mine, and i must say, there is NOTHING like single turbo. I now look at TT's with distain almost, i have no idea how i managed to cope with VOD. Having a single turbo really is a joy, it's so so SO much easier to work on, and work with. There is great support for aftermarket engine goodies, since you'll effectively had a V3 STi engine.


Newer model turbo's like VF39's have different outlets, that point rearwards, instead of forwards like the 22's and 24's etc. This isn't an issue, it only requires you to trim the turbo-y-pipe hose down. Any turbo from a single turbo subaru will work! Twin scrolls will require a twin scroll oil sump, headers and up-pipe, which could double your conversion cost easily. Some may deem that an acceptable price to pay.

Version 5/6 intercoolers do not fit V3/4 Intake manifolds. Their BOV, or aftermarket BOV flange conflicts with the 3/4's idle control valve. While it is possible to make a custom bracket to fit, it will be a source of pain for you. Also, a switch to either a V3/4 or 5/6 STi intercooler will require you to find a V3/4 Intercooler bracket (yep, V3/4 bracket to fit a V5/6 intercooler). These are hard buggers to find, i ended up driving 150km's (return) to waiuku to get one from the wrecker there, at a cost of $25 (and probably $50 in fuel at todays prices)

On the above intercooler 'drama' as Jase likes to call it (he wanted me to add this one in). It can be solved by simply buying a front mount. Remember that your car, (to anyone behind a counter), is now a Version 3 STi, and any part that can fit a GC8 wrx, will fit your legacy. Jason got a GC8 bolt on FMIC kit for his legacy when he did the conversion, and that was drama free (as far as FMIC kits are concerned anyway). He lost a fog light to it though (it couldn't fit past the GTB's foggie) and he needed a single turbo coolant over flow tank though (easy peasy to get from a pick-a-part). The downside is this can add another grand to the conversion easily.

Prepare to spend exorbitant amounts for seemingly benign parts from wreckers. For example, that little pipe that goes from the turbo intake pipe, to the ICV cost me $45 from a wrecker! Simply cause it was obscure, and he saw that i HAD to get one. It would have been something like $5 from a Pick-a-part, but none had the applicable model WRX's for my conversion at the time. The fact that no V3/4 WRX's/STi's were at any Pick-A-Part in Auckland probably raised the budget of the conversion by a few hundred dollars at least. I wish you luck ever finding one that lasts for more than a few days... I still managed to squeeze through the whole project at under $1500 though. (not including things like oil, tools, and crane hire; that all went under the rebuild budget ;))

The heater hoses are fine in the TT engine bay, they do work with a single turbo TMIC. But that doesn't stop you finding the line that goes to the water pump from a V3/4, it would make the whole lot go together much more smoothly.

Update: I've heard twice now, of TT heater hoses rubbing through and bursting! I now STRONGLY advised you convert to the single turbo style heater hoses (this requires getting the metal pipes that go all the way to the water pump). Turns out i simply got lucky with mine, and the settled before they burst. They have been working for 3 years in their squished state just fine. I still have the single turbo pipes ready to go on when the time comes though.

The turbo oil return... This could quite possibly be the most annoying thing you'll have to do during the whole project. I was lucky enough that "rex-in" had some old RS heads he wasn't using and couldn't sell, so we stole the return from that. any single turbo subaru will have one, and there'll always be a single turb of some description at a pick-a-part (gen 1 leggy's, and V1/2 wrx's), but getting that thing off while stuck in a pick-a-part wouldn't be a fun job at all!!! (i'm sure there's worse... like anything to do with a TT for example ;)) Basically just hammer the things base a few times to loosen it up, then hammer it left and right and jimmy it out. It's very easy to break, you saw the picks of the primary side one i tried to take out... Alternately you can heat up the block and try loosen it up like that as ben did to get mine out.

Rage 3" downpipes don't bolt up to factory mid pipes >:( A rage 3" midpipe from a wrx WILL fit, no matter what the twat behind the counter says (you just need to lengthen it by about 5-10cm's, i did that for a 5er). It pays not to mention it's going on a legacy at all, just lie and pretend it's for your V3 STi au. This applies for EVERYTHING you buy for your engine after the conversion.

If you break the charcoal canister, don't worry, you don't need it. It connects to the purge control solenoid on the intake manifold that you can break very easily too. Besides, it's a waste of vac line hooking it up, because the factory rail it has, is thrown out the with the factory fuel rails! I bitched about doing the right thing and getting obscure brackets for the turbo. This, this is different, it's useless. You will just get to smell petrol fumes every now and then, which i love anyway.

Other model legacy's. If you have a Auto BG/BD. Ha ha, tough, what were you thinking buying a auto GTB... Honestly... If you have a manual with 20H and a 4-plug ECU, you can use an EJ20G ECU, but there's about 10 or 15 wires you need to re-pin. Most wires are fine. If you have a manual BH/BE, (EJ208) Your engine i think is more or less very similar to the V5/6 STi engines. The ECU's have been used, but i can tell you what needs to be done to get it working. I also don't have any pinouts for you.

Update: We have found the compression is higher in the EJ208 than the EJ207, so much higher that detonation is unavoidable. You either need to drop some thicker HG's in, redo your short block, or run an aftermarket ECU with custom tune.


Thanks to Jason (FastFour) and Paul (ScoobyDoo) for advice and info on the single conversion. And to the websites i stole the wiring diagrams from.

And a big big thank you to Cameron (Fused) for helping me out and doing most the work ;), and to Ryan (Boostin) for helping me tare apart the engine, and put it back together! Also to Ben (Rex_in) for the oil drain and help, and to David (Miller) for your help too :D And to everyone else who helped me at some stage. You are all what make club sub a great place

Also thanks to, Strong For Subaru, and Pick-A-Part For supplying me with the bits and pieces I needed for the conversion. Strong ship nation wide, and now offer discounts to clubsub members, so ask for Nathan or Gerald, tell em Reuben sent yah, and you will receive exceptional service and great discounts! When asking for single turbo parts, for the single conversion, do mention that it's for the single conversion! They have now delt with so many, they pretty much know what you need!

And last but not least, the final product, i only realised now, after 2 years, i had never shown this in the conversion thread... lol...



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Right. 4-Plug EJ20H owners. This includes some Auto GT's and RS's. The B specs are the 1-plug wonders (*facepalm*).

This is for you, it should be all you need to get a 20G ECU working.

Sorry i don't have a higher res copy


It's colour coded too, uncoloured means it's the same. Um, coloured things are different. yeah. The legacy EJ20G ECU will have things like the intercooler water pump, the WRX EJ20G won't. etc.

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thanks for getting this thread back up. am going to undertake this conversion in a couple of weeks and panicked that i hadn't printed this out before the forums went down as it's great info

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Well. Funny day, i got my very first CEL after the conversion! Nice to know my ECU was not simply incapable of spitting CEL's at me :P (jokes)

When we first got the car on the road, and the new engine run in. It soon became clear i was running wastegate pressure. We didn't know why, but i had bought an electronic boost controller anyway, so we hooked that up, and i've been fine ever since. This afternoon, i had nothing to do, so i thought i should go and removed the boost control solenoid from the front fender that was doing nothing, and sell it to the bloke in the wanted section who's after one. This is the solenoid that did the wastegate on the primary turbo.

I took it out, and surprise surprise, i got CEL code 44 "Supercharging oppression output" In other words, the ECU was bitching that it had no boost control. Very odd! I compared the pinouts for the TT and the STi... Pin A3 is the pin that the STi ECU uses for the BCS. Turns out the engine bay/BBoD BCS was actually wired into A19, and the fender/primary BCS was the one wired into A3! So this whole time i've had them mixed up! The solution is to solder A19 into A3, and that will make my engine bay BCS the one the STi ECU uses. I will amend the wiring section of the thread later tonight... I'm amazed i never picked that up! Damn TT's

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 ReubenH said:

Right. 4-Plug EJ20H owners. This includes some Auto GT's and RS's. The B specs are the 1-plug wonders (*facepalm*).

This is for you, it should be all you need to get a 20G ECU working.

Sorry i don't have a higher res copy


It's colour coded too, uncoloured means it's the same. Um, coloured things are different. yeah. The legacy EJ20G ECU will have things like the intercooler water pump, the WRX EJ20G won't. etc.

Red means you need to remove that wire (these give you wires through the firewall that can be used where extra wires are needed ;) )

Yellow means that they need to be swapped to their corresponding position.

Swapping a 4-plug TT ECU to run a 4-Plug RS or WRX ECU is the biggest waste of time ever. Find some loom plugs for a V3/4 and re-wire to suit a V3/4 STI ECU, add yellow injectors and the right AFM and you will have a car that may actually go fast. A singled early TT running a 4-plug ECU (i.e. ~10psi boost and grey injectors) is slower than a wet week and not worth the effort required.

More 4-plug info:

To make the thing actually go there are more wires that need to be added in the engine bay and some annoying hidden stuff in the loom. DON'T bother. Go V3 or 4!!!

I can email full res version of this spreadsheet if you want to have a better copy of the ECU Pinouts. Just PM me an email address and beg :P

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  • 2 weeks later...
 ReubenH said:

I stumbled accross a good pinout for the 4-plug ECU's, this should hopefully make thing's easier.

I emailed that to someone once.....funny how far they get around.

Also more confirmed info for people with 4-plug ECU's. Someone mentioned on here somewhere that 3 of the plugs from the 4-plug ECU's are the same as the 3 plugs from a 3-plug ECU. I checked this out today and the plugs fit perfectly so that means if you have a 4-plug TT, then to convert to single turbo, use a 3-plug STI ECU and yellows + AFM to suit etc and you'll get great results.

I always thought that you'd have to find the plugs from a 3-plug ecu to do this conversion but its just become alot easier :D

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Someone mentioned on here somewhere that 3 of the plugs from the 4-plug ECU's are the same as the 3 plugs from a 3-plug ECU

That was me that asked about that. Sure the plug fits ok, but if you're going to do this you will have to remove some of the pins from the unused 12 pin plug and add them to the missing pins on the other 3 plugs. Thats what I'm doing at the moment with my adaptor harness, so far so good.

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[quote name='ReubenH said:

I stumbled accross a good pinout for the 4-plug ECU's, this should hopefully make thing's easier.


I emailed that to someone once.....funny how far they get around.

Also more confirmed info for people with 4-plug ECU's. Someone mentioned on here somewhere that 3 of the plugs from the 4-plug ECU's are the same as the 3 plugs from a 3-plug ECU. I checked this out today and the plugs fit perfectly so that means if you have a 4-plug TT, then to convert to single turbo, use a 3-plug STI ECU and yellows + AFM to suit etc and you'll get great results.

I always thought that you'd have to find the plugs from a 3-plug ecu to do this conversion but its just become alot easier :D

Yep, they do get around. That scan i did of the keyless entry page from the owners manual has popped up everywhere.

And yeah, i saw that too, even most the wires match, i compared the pinouts and about half didn't have to get moved.

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  • 2 months later...

Sorry to start up a new thread.....

Hey reuben, im doing the conversion in a few weeks. just wondering if there will be many problems with the new ECU (probably just a V3/V4 wrx ecu) throwing CEL's as it will be running a different engine from what it should?? cheers

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Oh right cheers, was just a bit worried i might start getting random codes thown at me etc.

Ive got a 95 GT - only engine mod is a full 2.5" exhaust.

Approx how long will it take to do by a mechanic? just wondering how much im going to be paying in labour :S

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Either way, your current engine now, is a "4-plug ECU" EJ20H that's running grey top injectors, and you have no ECU that will plug in. You can chose which ECU you want, and then you are going to have to re-pin your loom to run with that ECU. It would be advised to chose a 3-Plug ECU from the 96-98 (V3/4) era.

There are multiple options, there are some ECU's that will just plug right in (after you go 3-plug), or you can upgrade to yellow top injectors, get an orange sticker AFM, then run a V3 or 4 STi ECU.

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  • 1 month later...

hey ive jst got a 1996 gtb legacy and really are new to subis. being young i donot have much cash but im wanting to put an ej20t forester moter in it and need to know the tough bits. can any one guide me through or give me guidlines on what to do and what not to? cheers. p.s hope to make one sick car so help out :D

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 GTBNP said:

hey ive jst got a 1996 gtb legacy and really are new to subis. being young i donot have much cash but im wanting to put an ej20t forester moter in it and need to know the tough bits. can any one guide me through or give me guidlines on what to do and what not to? cheers. p.s hope to make one sick car so help out :D

Please Introduce yourself >> http://www.clubsub.org.nz/forum/index.php?board=33.0

and start a new thread with your intentions.

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Was wondering whats the best option in going from twin to single? the one in the thread or??

price wise im wanting to know what it would cost in labour for some one in the know to do this?

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Labour shouldn't be more than a box of beers and some pizza. Expect to pay around 1500 for the conversion, if you take your time on getting the parts you need for cheap. It's possible to do it for under 1K if you can get a good deal on everything :D

The best way to go about doing it is to find what engine you have in your TT, and going from there. If you have the EJ20H, you can go either way, EJ20G or EJ20K. V3 WRX EJ20G is the closest match, but you still need to do a good bit of wiring to convert to 3-plug. For EJ20R owners, stick to V3/4 STi only, and EJ206/208 owners need V5/6 WRX/STi Parts. You can drop whole STi engines in, but only worth that if you can get a bloody good deal on them. This option really only applies to people who have blown their motor's

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  • 4 weeks later...

Hi Reuben, I am doing a single turbo conversion on my 99 gtb at the moment, Its th original block, sensors, inlet manifold, I have extractors and tdo516g turbo. what dump pipe should I get as the steering shaft is close to the turbo flange(outlet). I should get away with not changing the ecu. If I connect the pressure lines Back to the black box of doom and blank the actuator ports I should get away with it wouldnt I? I am running a electonic boost control.

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