Subaru WRX and STI Differentials

What Each Differential Does

Torque leaves the transmission and hits the center differential first. That unit feeds the front differential and the rear differential. Each of those then feeds its two axles. If the center is the weak link, a strong rear limited-slip never gets the torque you just paid for. The front axle takes it instead.

An open differential sends torque to the wheel that is already spinning. The tire in the air, or on ice, gets the drive. The tire with grip gets almost none. A limited-slip fights that. How it fights depends on the type, and the type is different in the center, the front, and the rear of these cars.

A viscous unit is rated by how much torque it reacts with for a given speed difference. The stock WRX center is about 4 kgf-m at a 100 rpm difference. A gear unit and a clutch unit are rated by bias ratio: how much of the low-grip side’s torque can be sent to the high-grip side. A gear-type unit is usually in the range of about 1.5:1 to 4:1. A clutch pack built for racing is often set much higher, up around 6:1, and it can carry preload so it still does something when one wheel has no load at all.

What Came on the Car

These are the manual WRX and STI layouts. A 2015-and-later WRX uses a different transmission, and its rear ratio is not the early WRX ratio. A 2015 WRX rear is geared 4.111. A 2018 WRX rear is geared 3.900.

The 2004 STI front is a cam-type limited-slip, the Suretrac. It is a sealed unit: a differential case, cam followers, and two face cams. It is replaced as an assembly. It is not a viscous coupler, and it is not the front used in 2005. From 2005 on, the STI front is a helical gear unit, also replaced as an assembly. The 2005, 2006, and 2007 cars share front assembly 38410AA100.

The 2004 and 2005 STI rears share assembly 38410AA080. That unit is a clutch pack, not a gear differential. It is a mechanical limited-slip with a hypoid gear, ratio 3.545 (39 ring teeth over 11 pinion teeth), identification HN. The 2006 rear assembly is 38410AA120. The 2007 rear assembly is 38410AA140. Through the 2008 STI, that rear is still a mechanical limited-slip: friction plates, pressure rings, and spring discs, replaced as an assembly. These part numbers stop there. A later STI is a different center and a different rear, and these assemblies do not carry forward. “Torsen,” “helical,” and “mechanical” are not three different rears on the 2004 and 2005 cars. Those two years are the clutch pack.

The WRX front is open on the five-speed. Swapping one open front for another open front from a different year does nothing. A real front limited-slip is a transmission job, and it is the expensive one. The center and the rear are the units a careful owner can reach with the service manual. The front belongs with a transmission shop.

Viscous center and viscous rear

The plates in a viscous unit do not have to touch. Silicone oil and air sit between outer plates splined to the case and inner plates splined to a side gear, sealed with X-rings. When the two sides turn at different speeds, the oil shears and drags torque toward the slower side. Turn the wheels by hand and it feels open. Hand speed does not shear the fluid hard enough to lock anything. That feel is not a mileage death sentence. Silicone fluid does not have a 60,000-mile expiration.

The stock center reaction is about 4 kgf-m per 100 rpm of speed difference. The upgraded center people chase is about 20 kgf-m per 100 rpm. A higher number reacts harder to the same speed difference, which is useful when one end of the car is on snow or gravel. In a tight dry corner the same reaction is understeer, because the inside and outside tires already turn at slightly different speeds. A 12 kgf-m center exists. It does not bolt into the later WRX and RS cases this layout is built around.

A 2002 through 2005 WRX rear is the viscous unit in the 160 mm housing. The rear assembly that fits those WRX years is 27011AB010. From 2008 the WRX rear is open. The viscous rear on the earlier car is thin, packaged beside an open carrier, and slow. It will not step the tail out the way a clutch pack will. For a daily driver that only wants both rear tires to move in snow, that stock viscous rear is still the part that lasts. An aftermarket rear on that car adds noise and a rebuild, and it changes almost nothing in normal driving.

Viscous, Gear, and Clutch

A gear-type limited-slip, whether it is called helical, Torsen, or Quaife, uses the tooth load to bias torque. Most of them have little or no preload. That is why a front gear unit stays quiet and does not push the car on corner entry. It is also why a wheel with zero traction defeats it. Bias ratio times zero is still zero. The other wheel gets nothing until the unloaded wheel has some grip again, or until the unit was built with preload. A wheel that is merely slipping, with some load left, is a different case. The bias still works.

A clutch pack is a Salisbury differential. The spider shaft rides on ramps in a pair of pressure rings. Torque pushes the rings apart, and the rings squeeze a stack of plates. Preload, from a spring or from stack height, keeps some clamp even before the ramps move. That preload is what lets the pack transfer torque with a wheel in the air. It is also what chatters and chirps a tire in a tight parking-lot turn. The noise is the preload being forced to slip so the car can turn. More preload helps on ice and makes the street clumsier.

The “way” is the off-throttle ramp, not a power rating. A 1-way has no decel ramp, so it locks only while you are on the power. A 2-way uses the same ramp angle on both sides. A 1.5-way uses a steeper decel ramp, so it takes more torque to make the same plate pressure while you are braking. Some stacks are sold as 1.6-way or 1.7-way. That is the same idea with a different angle, not a fourth type. Plate order changes how many friction faces are actually working, which changes the bias. Fewer active faces, lower bias.

On the STI rear, the stock pressure-ring angle is 45 degrees. The later STI ring set raises that to 60 degrees. A steeper ramp spreads the pack sooner, so the clutch takes a set with less throttle. That is the change people make for gravel, where a lighter throttle still needs the rear to lock. Shimming the same rear tighter raises the breakaway. Too tight and the car rotates early. The tail gets loose before the driver asked for it, and the plates wear faster.

The STI Center Differential

The STI center is a planetary gearset with a clutch, not a knob that picks a front-to-rear percentage. With the clutch released, the tooth count sets the split. On the 2004 and 2005 STI that open split is 35 percent front and 65 percent rear. On the 2006 through 2008 STI the planetary is a double-pinion unit and the open split is 41 percent front and 59 percent rear. Locking the clutch drives the front and rear output shafts toward the same speed. With equal traction at both ends, that locked state is 50/50. The dial changes clutch engagement. It does not turn the car into a front-driver or a rear-driver.

The 2004 and 2005 center unit is 38913AA160. The 2006 unit is 38913AA190. On the 2006 through 2008 STI the center also has a mechanical limited-slip that still works when the solenoid is off. That mechanical portion covers the first part of the lock, about 0 to 30 percent. The solenoid clutch covers the rest, about 30 to 100 percent. In manual mode the dial moves the solenoid inside that 30 to 100 percent band. Auto mode moves the same solenoid from steering angle, yaw, and throttle. A fail-safe turns the coil off and releases the solenoid clutch. The mechanical portion on a 2006-and-later unit is still there.

Full lock on dry pavement in a tight turn binds. The car hops, the same way a locked truck hops. Opening the clutch while it is bound shudders. That is the plates letting go, not a gear breaking. In snow, an open center lets the car arc. A locked center lets it pivot closer to its middle. Plates slipping against each other is how the limited-slip works. Overheating them is the failure, not the slip itself.

The rear-oil temperature light is the overheat warning. It comes on at about 150°C (302°F). The center control then reduces solenoid engagement until the oil cools. A parking-brake signal on the 2006 through 2008 system releases the solenoid clutch. That is why an STI can be slid on the parking brake without the center clutch staying clamped. A WRX does not have that release.

Which Limited-Slip Goes Where

Front, center, and rear want different units because a locked front understeers harder than a locked rear. A gear-type front is the predictable choice for the street, an autocross, and a road course. It has little preload and little lock on decel, so the nose still turns in. A 1-way clutch front can match it when the ramps and the preload are right for that driver and that surface. Set badly, the same clutch front pushes. On snow, ice, or gravel, a 1.5-way clutch front with preload helps the car stay straight while you brake. A 1.5-way with low preload is the tarmac version of that idea, for a car that gets loose on the brakes.

The 20 kgf-m center holds the rear longer when the surface is loose, and it is a reasonable drag-launch change. On a tight dry course it fights the turn. Do not fit it as a street upgrade and expect a sharper car. An open center with a strong rear bias, the sort sold for tarmac, only earns its keep after the front and the rear are already limited-slips. Without those, it is an open diff that puts more torque on an axle that still spins one wheel.

The STI center is already the adjustable unit. The useful change is a controller and a map, not a different mechanical center. A map that locks early stops the car waiting for a big speed difference. A map that locks too hard brings back the tight-corner bind. The rear clutch pack is the racing rear because the ramps, the preload, and the plate count can be changed. The street rear is the gear unit, because it does not need a winter rebuild and it does not chatter in a parking lot.

Order of work

1. Name the surface. A snow car, a track car, and a daily driver do not want the same front or the same center.
2. Fix the end that is open. On a WRX that is the front. A rear limited-slip behind an open front and a soft center still gives the drive to the spinning front tire.
3. Leave the STI center in place. Change how hard it locks. Do not replace it to chase a split the gears already set.
4. Match the rear ratio before it goes in. Then set preload. Tight is not automatically better.

Ratio and Housing

The front final drive, the center, and the rear ring and pinion have to agree. A rear that looks like it bolts up, with the wrong tooth count, will wind the driveshaft and the center every time the car rolls. The failure is noise, bind, and then broken parts. Count the teeth on the ring gear and divide by the pinion teeth. 39 over 11 is 3.54. 37 over 9 is 4.11. The 2004 STI rear is 3.545, which is that 39/11 set.

The WRX rear is the T-type, with a hypoid ring gear of 160 mm (6.30 in). That is the R160. The STI clutch rear, the one the 45-to-60 degree pressure rings fit, is the larger housing. They are not the same pumpkin, and the axle splines have to match the housing you install. A 2002–2005 WRX rear assembly is 27011AB010. The 2006 and 2007 WRX rears are still viscous; they are not that same assembly. A 2015 WRX rear is geared 4.111. A 2018 WRX rear is geared 3.900. A 3.54 ring and pinion from an early WRX does not belong in either of those later housings.

Rear oil follows the limited-slip, not the badge on the car. A rear without a mechanical limited-slip takes GL-5 75W-90, about 0.8 L (0.8 US qt) in the 2004 spec. A rear with a mechanical limited-slip takes GL-5 90, about 0.9 to 1.1 L (1.0 to 1.2 US qt). Do not mix brands in the same housing. Each brand’s additive pack is its own. A clutch pack that was filled with the wrong oil is the usual reason a fresh unit chatters harder than it should, or locks late.

The center on a WRX sits in the transmission tail, in the transfer section, not in a separate box you can see the way you see the rear. Replacing it means splitting that tail. The rear pumpkin is the easier of the two. Different tire diameters, or a long drive on a flat, load that center the whole time. There is no mileage at which that becomes safe. Match the tires and set the pressures.

What Breaks Them

A viscous center that has been slid on the parking brake binds when it is hot and frees up after it sits. The first sign is a hop in a slow turn, not a bang on the highway. Dark fluid and a burnt smell after a fresh fill mean the silicone has already been cooked. Clutching the car before the lever comes up does not take the load off that coupling. The fronts and rears are still tied together.

A gear rear or a gear front that explodes does it when the power is high and the grip is high: a clutch-drop launch, or a donut on a tire that hooks. The broken teeth do not stay in the gear. They go through the case. That is a high-power failure, not the normal life of a street helical. A clutch pack fails the other way. It wears the plates, gets noisy, and stops locking at the preload you set. Metal paste in the STI rear oil is a clutch pack that is done, not a fluid that needs a shorter drain interval and nothing else.

An STI center that clunks and binds on a hot takeoff in a slight turn, then feels fine the next cold morning, is the center clutch. Cold, the pack has not grown. Hot, it drags. That is a replace-the-unit problem on a stock car, not a reason to convert the car to a different center. The 2004–2005 unit and the 2006 unit are not the same part.

• Hop in a slow turn after a slide. Viscous center overheated. Stop using the parking brake as a turn tool.
• Bind only when the STI is hot. Center clutch dragging. The cold drive home does not clear it.
• Chatter in a tight turn after a clutch-pack install. Preload, or the wrong GL-5. A gear rear does not do this.
• Rear temperature light. About 150°C (302°F). The STI reduces center lock until the oil cools. Let it cool.
• New rear, immediate wind-up. Tooth count does not match the transmission. Pull it back out.

FAQ

Does the STI dial change the torque split?

It changes how hard the center clutch locks. Unlocked, a 2004–2005 STI sits at 35 percent front and 65 percent rear. On a 2006 through 2008 STI the unlocked split is 41 percent front and 59 percent rear. Full lock drives the front and rear shafts toward the same speed, which is 50/50 when both ends have grip.

What is the difference between a 1-way and a 2-way differential?

A 1-way locks on throttle and stays open when you lift. A 2-way locks the same amount on and off throttle. A 1.5-way locks on throttle and locks less when you lift. The front of a street or track car wants the 1-way or a gear unit. A locked front on decel is understeer.

Will a 20 kgf-m center fix a street WRX?

Not as a street upgrade. It reacts harder to a small speed difference, so a tight dry corner understeers. It belongs on snow, gravel, ice, or a drag launch. The stock center is about 4 kgf-m per 100 rpm.

Can I slide a WRX with the parking brake?

A few times will not automatically kill it. Repeating it overheats the viscous center, and the car starts hopping in slow turns. A 2006 through 2008 STI releases the center solenoid when the parking-brake switch closes. The WRX coupling does not.

Do the front, center, and rear have to be a matched set?

The limited-slip type does not have to match. The final-drive ratio does. A rear limited-slip with the wrong tooth count winds the driveline up even if the housing bolts on. Read the ring and pinion before it goes in.

Why does a helical differential quit with a wheel in the air?

Without preload it can only multiply the torque the unloaded wheel is reflecting. Zero times a bias ratio is zero. A clutch pack with preload still transfers some torque in that situation. A wheel that still has some grip is not the same as a wheel in the air.

What oil goes in the rear differential?

No mechanical limited-slip: GL-5 75W-90, about 0.8 L. Mechanical limited-slip: GL-5 90, about 0.9 to 1.1 L. Do not mix two brands in one housing.

What does the rear differential temperature light mean?

The oil is about 150°C (302°F). On an STI the center control reduces clutch lock until the oil cools, then it returns to normal. Hard use is what puts it there. Let the housing cool before you keep repeating the same pull.