Originally Posted by
Alphius
1. Easy. Drive by cable. No benefits to DBW really. The long WRX throttle cable is useful, but the short Legacy one will still reach. You would need a 32-bit ECU and different heads as well as a different wiring harness in order to run DBW.
2. EJ207 or AVCS EJ205 ECUs are your only options. You could block the AVCS ports and run non-AVCS cam gears for proper timing, but you lose the benefits of AVCS. If you did that, you could use any old 02-05 USDM WRX ECU. The 02-05 USDM ECU lacks the control circuitry for the AVCS solenoids.
3. Crank gear and cam gear need to match the ECU you use. The sensors are the same. The JDM AVCS EJ205 will likely already have the 36-tooth crank gear necessary to work with a USDM WRX ECU.
4. Transmission should be OK if you don't beat on it. No launching, no powershifting, no launch control. Shift completely and release the clutch fully before applying power after shifting. A cost effective clutch setup is to convert to the 06+ WRX push clutch. The clutch generally needs to match the transmission style, pull vs push. 02-05 USDM WRX clutches are pull style, and will not work with your push style transmission.
Clutch setup, I used this on my car before I swapped in a 6-speed:
06+ WRX flywheel, clutch disk and pressure plate
06+ WRX clutch fork, pivot pin and throwout bearing
You can use stock master and slave cylinders with this setup, and your stock starter will work as well.
You have to use a grinder and clearance one of the ribs on the transmission for pressure plate clearance, as the WRX parts are significantly bigger than stock.
Your other clutch option is an ACT heavy duty clutch and pressure plate for the 99-01 Impreza 2.5RS, but I wouldn't necessarily recommend that since you'll break your stock clutch fork unless you reinforce it, it doesn't hold any more power than the OEM 06+ WRX clutch and it costs significantly more.
5. USDM 02-05 WRX ECUs are cross-compatible between manual and auto. Not sure at all on the JDM stuff, but my thought would be that it'll likely work if you flash a manual calibration onto it.
Other stuff:
Use your stock radiator, gut a rad cap to put on it, and use the overflow nipple on the rad as the straight through connection to the coolant tank on the engine. Run a stock cap on the tank, and route that to the overflow bottle on the radiator. A GD WRX radiator will not fit in a BD/BG/BK without modifying the radiator support. If you do that, stock rad hoses with a little trimming will fit perfectly.
CarBerry is awesome, no doubts there. In my case, I'm still running MAF instead of Speed Density, and it is still much smoother than the factory WRX tune. The other major feature I'm using that I really love is the per-gear WGDC compensation, which means I can hit my boost target spot-on in every gear, instead of compromising the low gears to keep the high gears safe and spike-free. I also use the CEL-as-knock-indicator feature, which I enjoy but which also means that if I set a code I don't get any notification.
Exhaust: Hood exit is dumb as hell. GD fitment exhaust is close to correct. The midpipe needs about a 4" extension, but fits the body hanger like stock. The axleback needs no piping mods, but all three hangers have to be redone. I bent up and welded some 3/8" steel rod into hangers. I have an OEM '06 STI exhaust on my car and love it. Nice and quiet, with good tone. Stealthy.
If you're having someone else do the wiring, it's completely straightforward to get it back and just plug it all in. I did mine myself because I can't stomach paying that much for what amounts to a few hours of work.
Have you thought about the crossmember? A 91-94 Legacy Turbo crossmember fits like stock but gives you clearance for your uppipe. A GD+ crossmember fits, but you have to shim your control arms or use WRX wagon control arms as they're different width than the earlier stuff. JDM GC crossmembers work the same as 91-94 Legacy Turbo ones, direct fit. You also have to use a turbo swaybar with the turbo crossmember.
You can use your stock alternator and A/C compressor along with A/C lines. One A/C line off of the compressor needs to be bent an inch or two upwards to clear the intake manifold, and the A/C compressor bracket needs some grinding on the back corner to fit as well. The power steering system needs to be from a remote reservoir system in order to clear the turbo inlet. That'll be an 02+ WRX for best fitment, and the WRX PS lines will bolt right to your factory steering rack. Another option which may work is the 00-04 Legacy PS pump and lines, but that's untested by me.
You'll probably have to relocate your cruise control actuator, since it most likely occupies the space behind the turbo where the downpipe goes. I've seen a few OEM cruise actuators that mount tight to the strut tower, but I haven't been able to pin down what cars that style comes on. The normal one is way the hell out in the middle of the downpipe space behind the turbo for some reason. I ended up bolting mine to the intercooler bracket.
That's all for now. If I think of anything else I'll be back.
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