STI 6-Speed Swap into a Subaru WRX: What Must Match

What a 6-Speed Swap Changes

The STI six-speed replaces the WRX five-speed. First gear is 3.636 on every box in this list. The split that matters for daily driving is fifth and sixth, and the split that matters for the install is the rear final drive and the shape of the front axles.

The swap adds about 100 lb. A Spec C uses the same ratios as the JDM STI set below. The five-speed starter can stay. Ring-gear diameter does not change, so the starter nose still reaches the flywheel.

Three things have to agree before the transmission goes in the car. The rear differential ratio has to equal the rear final drive of that six-speed. The inner end of the front axle has to match the transmission, male or female. The clutch has to be the pull type these six-speeds use. A 2006–2007 WRX fails the third test until the clutch and slave cylinder change.

A 1993–2001 final drive is not one number. Check the owner’s manual or the VIN. A 2002–2007 automatic is 4.11 front and rear, so an automatic WRX still needs a rear gear change to match a 3.90 or 3.54 six-speed. The automatic driveshaft is the shaft these swaps use. The clutch pedal, hydraulics, and shifter still have to come from a manual car.

Legacy and Forester swaps use the same three checks: automatic driveshaft, front axle gender, and rear ratio. An Impreza driveshaft length does not carry over to a longer chassis. On a 2008 Forester XT, the stock axles and brakes can stay with a 2004 six-speed. That 2004 case changed axle gender mid-year, so some of them need the stubs removed and the later seals installed.

Which Six-Speed

USDM 2004–2006 boxes share the close 3rd and 4th (1.761 and 1.346) and the taller 5th and 6th (0.971 and 0.756). The 2007 USDM box keeps those top two gears and opens 3rd and 4th up to 1.521 and 1.137. Second gear is 2.375 on the STI boxes and 2.235 on the 2007 Spec B. JDM 2001–2006 5th and 6th are shorter, 1.061 and 0.842, which is the set road-race cars want and the set that holds rpm up on the highway.

On one JDM-geared car, around-town mileage improved by almost 2 mpg over the five-speed, and highway mileage went the other way. The driver stayed at or under the limit because the engine was already turning too fast. That is one car, not a fuel-economy rating.

Case numbers for the USDM units are 32000AG880 (2004), 32000AH150 (2005), 32000AJ760 (2006), and 32000AJ350 or 32000AJ100 (2007). The 2007 Spec B is 32000AJ390. JDM axle gender changes in the middle of model year 2003. USDM 2004 changes in the middle of that year. Read the case you have, not the year painted on the listing.

A later six-speed often costs more as a complete unit and skips a ring-and-pinion job, because a 2002–2005 WRX rear is already 3.54. An earlier six-speed is cheaper until you buy and set up 3.90 rear gears. Those two paths land near the same money unless the early case is a rare deal.

Front diffs on these STI boxes are limited-slip, with exceptions: STI Forester, some EDM cars, and the V7 wagon. V7 and V8 wagons and Foresters without DCCD show up both open and with a Suretrac. The Spec B front diff is open.

Rear Gear Match

Match the rear differential to the rear final drive of the six-speed, not to the front. A 2004–2005 USDM STI and the usual JDM STI are 3.90 in the rear. A 2006–2007 STI and the Spec B are 3.54 in the rear. The 3.54 cars use a 1.1:1 drop in the transfer gears so that rear still lines up with the 3.90 front inside the transmission. Do not “correct” a 3.54 six-speed by installing a 3.90 rear.

A 3.54 ring and pinion from a 2002–2005 WRX fits a later R160. You do not have to replace the whole carrier, though swapping the assembled diff is the easier job. An R180 ring and pinion is larger and does not go in an R160.

Some JDM six-speeds are also given as 4.44 front and rear. A separate count of these boxes puts every six-speed at 3.90 in front, with the rear at 3.54 or 3.90. A 4.44 final on a Version 6 Type R five-speed is a different transmission. Read the tag, or count the gears, on the unit in front of you. A 4.44 rear behind a 3.90 front is still a mismatch.

The R160 is still the smaller rear diff after the transmission is upgraded. One hard-driven WRX, on a rear-biased six-speed, overheated an R160 during a night of drifting and broke a second one launching. That is one car. The six-speed does not make the R160 into an R180.

Axles and Driveshaft

Female axles slide onto a stub and lock with a roll pin at the inner cup, next to the transmission. Male axles lock with a circlip and have to be popped out of the front diff. “Male” and “female” describe that inner end, not the hub.

When the genders do not match

A male six-speed in a female-axle car needs stubs, clips, and the stub seals. Five-speed stubs do not fit a six-speed case. The seal pair is 806730042 on the left and 806730041 on the right, plus stub 38415AA110 (two) and clip 805329010 (two). Left and right are not the same part. The right-hand seal is the one that arrives bagged as 806730041.

A female six-speed in a male-axle car needs one each of 806735230 and 806735240, and the stubs come out. Those two numbers are not marked left and right. The lips look alike and are not. The micro-grooves spiral one way on one seal and the other way on the other, so oil is pushed back in while the axle turns. Installing both from a single number, or swapping the sides, is a leak.

Driveshaft flange

The 2002–2007 WRX automatic shaft bolts to an R160. It does not bolt to a USDM R180 until you add the small-pattern yoke, 38358AA000. A 2004–2007 STI shaft does not bolt to an R160 until you add the large-pattern flange, 38358AA040. That flange goes on the differential. The STI shaft stays as it is. The two yokes swap between R160 and R180, so you can keep whichever shaft you already have.

On a 1993–2001 Impreza, the automatic shaft from that generation is the better fit because the center-bearing mount lines up. A 2002–2007 automatic shaft still bolts in, and it sits slightly lower. If that car gets an R180, it uses the same 38358AA000 yoke.

Hubs, wagons, and the R180

Keeping the WRX front hubs is normal when you convert axle gender instead of swapping to STI axles. An R180 does not accept WRX rear axles. The usual path is STI rear hubs, knuckles, and axles with that diff. Custom axles that keep the WRX hubs exist, and they cost more than the hub swap. A 2004 STI rear is 5x100, so it can keep a matching bolt pattern. A 2007 STI rear diff is 3.54, the same rear ratio as a 2002–2005 WRX, and it has been paired with 2004 STI rear hubs for that reason.

Track width follows the lateral links, not the axle length. Sedan lateral links are longer than wagon links and can hit the wheel or the quarter panel. STI sedan rear axles have been run in a wagon on the wagon’s own links without that problem. One wagon on 2004 STI rear links, dropped about 1.5 inches, cleared 205 and 215 tires, and needed a fender roll at 225/45R17. Spec B rear axles, sometimes used because the inner end suits an R180 and the outer end suits a WRX hub, measured about 3.5 cm too long per side for an Impreza. That length is a Legacy part. Sway bars can stay; only the links change if you want more track width.

Rear calipers that stay with the original hubs need a bracket and a rotor that match the hub you kept. The caliper does not bolt to an STI hat by itself.

Clutch and Starter

Every six-speed in this list is a pull clutch. The release bearing snaps into the pressure plate, and the fork pivot has to come out before the transmission splits from the engine. The slave cylinder points away from the firewall and pulls. A push clutch leaves the bearing and fork on the transmission, and the slave points toward the firewall.

• 2002–2005 WRX. The clutch and flywheel are shared across these years, except a very early 2002. That clutch fits the six-speed and is a smaller diameter, so it has less friction surface. The WRX slave cylinder can stay.
• 2006–2007 WRX. These cars are push clutch. They need a 2002–2005 WRX clutch or a 2004-and-later STI clutch, and the matching slave. The push setup does not operate a pull pressure plate.
• Starter. The five-speed starter fits. Flywheel ring-gear diameter does not change.

A USDM STI clutch and pressure plate has been run on the flywheel that came with a JDM V8 six-speed. These six-speeds stay pull-type, including the JDM V9. The push change is the 2006 WRX with the 2.5 L engine, not the STI.

While the case is out, the cheap seals and fork parts are the ones that leak or leave you with a bare used transmission. Output-shaft seal 806735210, fork shaft 30532AA020, fork-shaft plug 30550AA000, and fork spring 30546AA060 cover the clutch fork and the driveshaft snout. The plug is the pull-style piece used on 1993–2005 five-speeds as well. A stripped case has also needed pitch-stop bracket 32034AA001, return-spring bracket 30548AA010, axle nut 28044AA000, and pan bolt 010406200.

Center Diff and Speed Sensor

DCCD is the electronic center clutch. Open, a 2001–2004 STI and the 2004–2005 USDM box bias 36% to the front and 64% to the rear. From the 2005 JDM split, and on the 2006–2007 USDM box, the open bias is 41% front and 59% rear. Locking past that split takes a controller. A WRX harness does not include that circuit, so the controller is a separate install. The 2007 Spec B is not DCCD. It uses a viscous center and an open front diff.

Swap Checklist

1. Name the transmission. Year, 5th and 6th, rear final drive, and whether the front axles are male or female. Mid-2003 JDM and mid-2004 USDM are the years that changed gender.
2. Name the car. Rear ratio, axle gender, and push or pull clutch. A 1993–2001 car needs the VIN, because 3.90, 4.11, and 4.44 all exist.
3. Match the rear. 3.90 stays 3.90. A 3.54 six-speed gets a 3.54 rear, including the 1.1:1 transfer drop that already matches it to the 3.90 front. A 3.70 rear does not stand in.
4. Match the front axles. Install the stub kit or the male seals. Do not reuse five-speed stubs. Put left and right seals on the sides the numbers call for.
5. Pick the driveshaft and the flange. Automatic shaft to an R160, STI shaft to an R180, or the yoke that adapts the one you have: 38358AA000 or 38358AA040.
6. Match the clutch. 2002–2005 WRX can keep its pull clutch and slave, with less disc area. 2006–2007 WRX changes both the clutch and the slave.
7. Plan the center diff. DCCD needs its controller. A Spec B does not. A 2008-or-later case needs an older bellhousing for the speed sensor.
8. Seat the axles. Mark the retainers, then confirm each circlip locks past the dust shield before you fill the transmission.

FAQ

Can a 6-speed swap keep the rear diff in a 2006 or 2007 WRX?

No. That rear is 3.70. A 2006 or 2007 STI six-speed needs 3.54. Use a 2002–2005 WRX R160, a 3.54 gear set, or a 3.54 R180.

Do I have to swap hubs to fit a six-speed?

Not for the transmission itself. Convert the front axle gender and the original front hubs stay. An R180 is the part that forces new rear axles and, unless you buy custom axles, new rear hubs.

Will my WRX clutch work on the six-speed?

A 2002–2005 pull clutch will, and the disc is smaller than an STI disc. A 2006–2007 push clutch will not. That car needs a pull clutch and the matching slave.

Can I reuse the five-speed starter?

Yes. The flywheel ring gear is the same diameter, so either starter fits.

Are JDM fifth and sixth gears better on the highway?

They are shorter, 1.061 and 0.842 against 0.971 and 0.756. On one car that held highway rpm up, used more fuel at speed, and picked up almost 2 mpg in town. They suit a road course more than a long freeway trip.

What is different about a 2008 STI transmission?

The speed sensor is no longer on the front differential. Using that case means swapping on an older six-speed bellhousing, and that is a large extra job.