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Thread: Engine/Transmission build question

  1. #1
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    Engine/Transmission build question

    Hi everyone, brand new to the forums and about to embark on a first build with my son. Actually his first build and first car! He's planning on building the 818C, and I really need some advice/info/help.
    We bought a 2007 Impreza with an unusable motor and auto trans for next to nothing. My understanding is we could put in an Sti motor with either a 5 SPD or 6 speed. I know if we use a 6 speed it has to be an 06-07. My questions are
    1. Will any 02-07 motor work with the wiring harness?
    2. Will I have any issues going from an auto to manual with the wiring harness?
    3. Anyone have a preference on the 5spd vs 6spd?
    4. Are the motor mounts the same for all years/models?

    Any help would be greatly appreciated.

  2. #2
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    Quote Originally Posted by 10secondcobra View Post
    Hi everyone, brand new to the forums and about to embark on a first build with my son. Actually his first build and first car! He's planning on building the 818C, and I really need some advice/info/help.
    We bought a 2007 Impreza with an unusable motor and auto trans for next to nothing. My understanding is we could put in an Sti motor with either a 5 SPD or 6 speed. I know if we use a 6 speed it has to be an 06-07. My questions are
    1. Will any 02-07 motor work with the wiring harness?
    2. Will I have any issues going from an auto to manual with the wiring harness?
    3. Anyone have a preference on the 5spd vs 6spd?
    4. Are the motor mounts the same for all years/models?

    Any help would be greatly appreciated.
    I can help with a few answers.

    1. Not likely as different motors have different pin assignments for the main connectors. ex: you can't plug an STi motor into an Impreza N/A motor wiring harness.

    2. Not sure about the harness, but you will have issues with the ECU as it expects input from the auto transmission. Other's have done it though.

    3. For stock power levels use the 5 speed. It's easier, cheaper, and doesn't require frame modifications. If you expect higher power levels and don't mind modifying the frame go for the 6 speed.

    4. Yes, the motor mounts are the same for most EJ motors and years.
    MK3.1 Roadster completed 2011
    818R built with EZ36R H6 completed 2018
    818R rebuild with a JDM Honda K24A

  3. #3
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    I can give some insight into this since my build is kinda made with parts from all kinds of WRX/STI. If you're buying a motor, do the opposite of what I did and buy a 100% stock drivetrain. They can be hard to find in the modify-everything WRX world. Even if you plan to modify, starting with a known quantity is best.

    1. Wiring harness and ECU is not interchangeable between WRX and STI because of AVCS and the STI having a higher-spec ECU with additional features. Similarly, 06-07 WRX (which has AVCS) is not compatible with earlier WRX or STI. But all 02-05 WRX wiring is compatible and ECU's can be made compatible by flashing the correct software via a Tactrix cable.

    2. I would just find a manual transmission harness, personally. I think some of the pinouts are different on the ECU harness and it might be more trouble than it's worth to decipher all that but I have not looked into it too much.

    3. The six speed is definitely the better transmission. However, unless you're planning to do big power mods then it's probably not worth the trouble and cost.

    4. They're all the same, just stiffer on STI.
    Last edited by lsfourwheeler; 03-21-2019 at 11:11 PM.

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    When referring to wiring harness, are you just talking engine wiring harness or entire harness? Assuming it's just engine harness.
    If I'm understanding all of this correctly, if I get an 05 with engine and transmission harnesses I should be able to connect to main harness, and then flash ECU? Alternatively, if I got an 07 motor/trans with harnesses would this connect to main harness?

    Appreciate the help everyone, just trying to get my son pointed in the right direction and this is all new to me.

  6. #5
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    You want the main harness and the engine/trans harness to match. If you are going to use your 07 main harness, you will also want to use your 07 engine / trans harness or some other 06-07 engine / trans harness. As Isfourwheeler stated, you can not mix an 05 engine / trans harness with an 07 main harness because of the AVCS wiring, unless you want to do a lot of research and re-pinning.
    MK3.1 Roadster completed 2011
    818R built with EZ36R H6 completed 2018
    818R rebuild with a JDM Honda K24A

  7. #6
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    Quote Originally Posted by 10secondcobra View Post
    When referring to wiring harness, are you just talking engine wiring harness or entire harness? Assuming it's just engine harness.
    If I'm understanding all of this correctly, if I get an 05 with engine and transmission harnesses I should be able to connect to main harness, and then flash ECU? Alternatively, if I got an 07 motor/trans with harnesses would this connect to main harness?

    Appreciate the help everyone, just trying to get my son pointed in the right direction and this is all new to me.
    Your main harness would need to be WRX rather than NA Impreza. Everything I stated only applied between WRX models as the NA models are different.

    06-07 Engine harness and ECU would not be compatible with an earlier main harness or with an NA Impreza main harness.

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    Thanks everyone for all of this so far!
    It looks like there were 2 engine choice for 2007, I think EJ207 and EJ257. I'm assuming the EJ257 is the STi motor. Is the bulk wiring harness the same for both turbo motors? Any advantages to either motor (besides the EJ257 making more power)?
    Also, does the 5 speed connect to the EJ257, or only the 6 speed? And is the 5 speed reliable to about 350HP?

  9. #8
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    Quote Originally Posted by 10secondcobra View Post
    It looks like there were 2 engine choice for 2007, I think EJ207 and EJ257. I'm assuming the EJ257 is the STi motor.
    In 2007 there were 2 turbo motors. an EJ255 (WRX) and an EJ257 (STI) The short blocks are the same except for a small difference in piston design. The STI makes more power (bigger turbo, different tune etc...)

    Quote Originally Posted by 10secondcobra View Post
    Is the bulk wiring harness the same for both turbo motors? Any advantages to either motor (besides the EJ257 making more power)?
    The bulk of the harness will be the same but there are small differences like for the gauge cluster and some of the engine sensors I think. Like we mentioned earlier, stick with a single model year for everything, engine, harness etc.. You will save yourself many headaches.

    Quote Originally Posted by 10secondcobra View Post
    Also, does the 5 speed connect to the EJ257, or only the 6 speed? And is the 5 speed reliable to about 350HP?
    Subaru's are like legos, you can bolt up any engine to most any Subaru manual transmission (5 or 6 speed). And no the 5 speed is not reliable to 350 hp. You would need the 6 speed for that power level if you plan on driving hard.
    MK3.1 Roadster completed 2011
    818R built with EZ36R H6 completed 2018
    818R rebuild with a JDM Honda K24A

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