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Part 1: General Subaru & Legacy Info


Considering there are generally many questions, what body style is what, what did this model contain, yada, yada, here is some info to explain things. It will be updated as time presents itself and details become available. I will do my best to explain info. If something somehow seems inaccurate, feel free to voice!


Basic Terminology & Acronyms

4EAT – Four Speed Electronic Automatic Transmission

5MT – Five Speed Manual Transmission

5EAT – Five Speed Electronic Automatic Transmission (includes Sportshift)

6MT – Six Speed Manual Transmission

ABS – Anti-lock Braking System

AT – Automatic Transmission

ATF – Automatic Transmission Fluid

AutoX – Autocross

AVCS – Active Valve Control Sysem

AVLS – Active Valve Lift System

AWD – All Wheel Drive

BHP – Brake HorsePower (measured BEFORE transmission)

BOV – Blow Off Valve

CAI – Cold Air Intake

Cat – Catalytic Convertor

CEL – Check Engine Light

CF – Carbon Fiber

CO2 – Carbon Dioxide

Diff – Differential

DOHC – Dual Overhead Camshaft

EBC – Electonic Boost Controller

ECT - Electronically Controlled Transmission

ECU – Electronic Control Unit (engine control unit)

EFI – Electronic Fuel Injection

EG – Subaru 6-cylinder engines (up through mid to late 90’s)

EG33 – Subaru 3.3 litre 6-cylinder engine code (SVX)

EJ – Subaru 4-cylinder engines (early 90’s and newer)

EJ20 – Subaru 2.0 litre 4-cylinder engine code

EJ205 – Subaru 2.0 litre turbo engine (US WRX)

EJ22 – Subaru 2.2 litre 4-cylinder engine code

EJ25 – Subaru 2.5 litre 4-cylinder engine code

EJ255 – Subaru 2.5 litre turbo engine (US Forester XT, Baja Turbo)

EJ257 – Subaru 2.5 litre turbo engine (US WRX STi)

EZ – Subaru 6-cylinder engines (post EG series H6 engines)

EZ30R – Subaru 3.0 litre 6-cylinder engine code (BL/BP Legacy and new Outback)

FDR - Final Drive Ratio

FMIC – Front Mount Intercooler

FWD – Front Wheel Drive

H4 – Horizontally Opposed 4 cylinder (boxer)

H6 – Horizontally Opposed 6 cylinder

HID – High Intensity Discharge (headlights)

JDM – Japanese Domestic Market

LSD – Limited Slip Differential

MAF – Mass Air-Flow Sensor

MAP – Manifold Absolute Pressure Sensor

MBC – Manual Boost Controller

MIL – Malfunction Indicator Lamp (CEL)

MT – Manual Transmission

MY – Model Year

OE – Original Equipment

OEM – Original Equipment Manufacturer

RallyX – Rallycross

RWD – Rear Wheel Drive

SOA – Subaru Of America

SOHC – Single Overhead Camshaft

TMIC – Top Mount Intercooler

TT – Twin Turbo

UDP – Under Drive Pulley

VDC – Vehicle Dynamic Control

VIN – Vehicle Identification Number

VTD – Variable Torque Distibution

WHP – Wheel HorsePower

WOT – Wide Open Throttle

Thanks to Uncle Meat of the Cobb Tuning WRXForum.com



JDM (Japanese Domestic Market) Info:

Twin-Turbo: This is one of the most loosely used term. Everyone wants a twin-turbo because Japan has had it. It was a sequential setup with one turbo used for the low end, while a larger turbo was used for the high end. However, it wasn’t the most optimal solution being its cost, complexity, and between the 1st to 2nd turbo transition, there was a decent dip in power. It has been proven that other solutions are more cost-effective and produce better results in general.


More on turbo tech below.


B4: The term used for sedan since the 1998 model year.


B4 RSK: The name given to the turbocharged sport Legacy Sedan in the 1998 to 2002 model years. Remember, Japan has been even further ahead in model changes. They receive their models 2 model years (or MY) ahead of the U.S.


Note: Previous to the BE-generation B4 RSK, the sedan turbo models were called ‘RS’.


GTB: The name given to the turbocharged sport Legacy Wagon in the 1993 to 2002 model years that used the sportier Bilstein suspension.


S401: The ONLY STi production Legacy to date. This 2003 BE sedan model was produced in a short-run and were rather expensive. It contained much gear from the current year Impreza STi though mostly in drivetrain and brakes. It still retained a sequential twin-turbo setup though with a few more ponies. It also received some suspension modifications including Bilstein struts along with BBS 18x7” wheels.



Body Styles (by USDM MY):


BC (1990-1994)

BD (1995-1999)

BE (2000-2004)

BL (2005-2009)



BF (1990-1994)

BG (1995-1999)

BH (2000-2004)

BP (2005-2009)

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Part 2: Turbos & Engine Tuning

A lot of people have wondered why not twin-turbo, or why not this setup or that. Here is my best definition of the systems Subaru has recently used and is more likely to.


There are 4 main types or turbochargers and layouts applicable to Subaru's:

-Single Scroll Turbochargers (most basic - used in all USDM applications)

-Twin Scroll Turbochargers (used in foreign 2.0L WRX STi & Legacy GT)

-Sequential Twin-Turbochargers (used in former late-model Legacy turbo models)

-Parallel Twin-Turbochargers (non used so far in a Subaru but mayb be with an H6 variety, best description is in the previous generation Audi S4 with 2.7TT)


Single-Scroll Turbocharger

The single scroll turbochargers are as noted above, the most basic in that they have a single exhaust volute. Most stock Subaru (produced by Mitsu or IHI) units do not run ball bearings (the preferential bearing type), which help reduce lag and increase theoretical longevity while being slightly more expensive. To have minimal amounts of lag with this type of turbocharger in a 2.0L application you will sacrifice top-end power for response, that being from a smaller turbocharger, and for higher output levels, low-end response will be sacrificed. There is little happy medium with this turbocharger and 2.0L displacements, big power or zero lag and ultimate response. The 2L displacement class can be fairly responsive with up to around 300chp running a VF28/29 sized turbocharger while the 2.5L variety has around up to 400chp while still being reasonably responsive on the low end. Our 2.5L pushes enough exhaust gasses to not need the following type of unit.


Twin-Scroll Turbocharger

The twin-scroll unit was devised to produce the benefits of a small turbocharger (near-zero lag, plenty of low-end torque), and allow for more power up top (though they don’t have quite the up-top power the mid to larger sized single-scroll units do.) Mitsu has been using this on their EVO’s for a number of years now IIRC and it has shown good results overall. This setup must be used with an equal-length header that is in a 4-2 configuration (not a 4-1 or 4-2-1) where one set of cylinders is directed to one volute, and likewise, the other set is directed to the 2nd volute. This way, there is a very even set of exhaust pulses on the turbine which gives it a much better opportunity to keep the turbo spooled with less volume of exhaust gasses. This setup has allowed Subaru to do away with the Sequential Twin-Turbo setup found in many BD/BG and BE/BH Legacy turbo models found abroad. It is mainly aimed at the 2.0L market as far as Subaru/Fuji is concerned though Litchfield Imports in the UK have produced a 2.5L STi with this type of turbocharger system producing a very responsive 350hp & 350lb-ft of torque. This is also more expensive than single-scroll units.


For more info regarding this type of turbocharger and how they work, you may want to visit Garrett’s website.


Sequential Twin-Turbochargers

These work just as it sounds, in sequence, one after the other. Used for several years in the BD/BG and BE/BH Legacy B4 RSK’s, RS, RS-B, GT, GT-B; this setup was alright, but not entirely effective as it had a large dead zone in the factory tuning between the first turbo cutting out and the second, larger turbo engaging. Additionally, it was overly complex and wasn’t the most cost effective solution. Thus the twin-scroll units have taken over.


Information can be found HERE for starters. More to come...


Parallel Twin-Turbochargers

These turbochargers work together in unison, one per bank of cylinders. Audi used this successfully with their 2.7L V6 in the Audi S4 during 2000-2002 as seen here:


It’s not the most effective setup for 4-cylinder Subaru’s considering the lack of exhause header piping, but considering the H6 would require even more piping, a parallel twin-turbo setup may work the best. I would say either using a setup similar to Audi’s small FMIC's (one per turbo) for a scoop-less H6 twin-turbo terror, or both ducted into a single larger top-mount would work all right. The B4 Asterope-like bumper would work better for the previous method. All in all, this isn’t really required unless you’re pushing more displacement and 6 or greater cylinders.


Conclusion: Based on the general info laid out above, for markets limited to 2.0L of displacement, twin-scroll units work rather well for a good mix of power and drivability, while those of us who can get the 2.5L of displacement have enough gas flow to effectively use single-scroll turbos which are the most effective in price and most widely available. The sequential system is likely to have seen it’s last days with Subaru, while we may see a parallel setup with a turbocharged H6, though it may be some time before we’ll actually see it.




Modern Subaru turbocharged engines do not require turbo timers (in stock configuration) as the cooling system is design to continue circulating coolant through the turbocharger post shut-down to preserve component life. Aftermarket applications may require a shut-down timer system.




Subaru has been adding some 'active' systems that in addition to the normal operating action of the engine are able to vary certain components to increase power and efficiency. Two of these systems, AVCS (Active Valve Control System) and AVLS (Active Valve Lift System) are very new to Subaru, with the latter being new this year and only available on the EZ30R as far as we know.


AVCS is a system that works with the cams (usually only on the intake side where the most benefits are shown) to adjust the duration of the intake valve opening, closing, and rates of change depending on what the computer determines according to sensor input. The AVCS system is actuated through oil pressure from the engine, which is sent through a computer controlled valve and then directs oil into an AVCS actuator to advance or retard cam timing in respect to the cam sprocket. The best thing about the system is that, the same heads can be used with or without AVCS activated. It’s just an auxiliary system in essence.


AVLS is another new technology to help improve volumetric efficiency. While AVCS can alter the cam duration and angle, either advancing or retarding the valve timing, AVLS is able to use two different profiles to alter the valve lift. The EZ30R is the first Subaru engine to use this technology. Instead of using a system like Honda’s VTEC with two cams, Subaru uses a double-profile single cam to control the same parameters. Much like the AVCS setup, AVLS actuates the switch between low and high lift lobes through an oil-actuated system. This system uses a double-tappet (the components which is in contact with both the valve and cam as shown below.) The switchable tappet switches between the low-duration and high duration generally in the 2,000 to 4,000 rpm range depending on ECU input (much lower than VTEC) allowing the EZ30R to breathe better at high rpm and improve low-rpm torque as well as emissions.





The main components of the Legacy exhaust are laid out above:




...although the downpipe and cat-back sections can be split into multiple sections as seen above.


Note on exhaust manifolds; the stock unequal length unit is considered rather good for most applications and is rather trouble-free. Generally for most it is only considered a necessary upgrade when going with a much higher-performance turbo setup or moving to a twin-scroll unit. So for most practical purposes, it's not worth the hassle of messing with.


Some of the things to look for in these components:

Up-Pipes come in a number of materials, with and without flex joints. There are some decent units out there with flex joints, but the consensus appears to be that if the tuner is able to hold tolerances tight enough, that a fully cast pipe is just as reliable and leak-free as one using a flex joint. Down the road you won't have potential issues with the flex joint itself also as miles add up.



Downpipe turbo outlet shape: Bellmouth is preferential, some say divorced is just as good. Some of the higher quality units used cast turbo outlets to better resist the effects of the high exhaust gas temps.


Material/size: 3" is the standard and preferred size for optimal flow. Stainless is the material of choice for it's metallurgical properties including corrosion resistance. Titanium is also available at a premium price though it does weigh less by a nice amount. Mandrel bends are also highly recommended over crush bends, which hurt flow ability.

Edited by SUBE555
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Part 3: Drivetrain

Transmissions & Gearsets


The first and foremost part of the drivetrain is the transmission. This is the component mounted just rear of the Subaru boxer engine. This key assembly hosts several very important parts including but not limited to the front differential (located at the front lower portion of the case), the center differential (located at the rear of the unit just ahead of the tail shaft), and the complex geartrain. Subaru offers a few different flavors of transmissions depending on application and user choice:

*5-Speed Manual

*6-Speed Heavy-Duty Manual,

*4-Speed Automatic

*4-Speed Automatic with Sportshift

*5-Speed Automatic with Sportshift.


Though the each manual or automatic transmission has a good deal of similarities to the others of it’s type (ex. 5MT and 6MT), the automatic and manual types in themselves vary greatly from each other in both components and how they function.


Manual Transmissions

While many know Subaru has been using 5MT’s for quite some time, many don’t know the history behind the current unit in use. The overall design of the 5MT dates back to the early to mid 1980’s as the basic design was conceived to be used with the naturally aspirated 1.8L engine of the day producing around half of the power as the current WRX produces. This gearbox has evolved plenty over the 20 or so years of its lifespan for increased strength, reliability, and smoothness. The 5MT design in general is rather reliable given modest power levels and being driven smoothly.


The problems only arose after the WRX started to become widely available, the USDM in particular. While the 5MT is just fine in a very high percentage of instances, problems have arose with that small percentage of owners and drivers who like to slam through gears, do hard launches, and flog these machines just like any 2WD car. It’s not like a 2WD car where traction is going through one or two wheels and then up in smoke, the WRX has 3 traction surfaces (the open front + viscous center and rear differentials transmit power effectively to a minimum of 3 wheels) which places much greater loads on the internal components of the transmission (which few owners that destroy their transmissions understand.) Previous to bringing the car to the USDM, the rate of failure (including high-output STi models with the virtually the same tranny) was MUCH lower, which shows how abusive the American crowd is in general.


The new 5MT units placed in the ’04 Forester XT first, now ’05 Legacy 2.5GT, and WRX turbocharged models are said to have received a strengthened case to reduce case flex which may have contributed a substantial effect to the proverbial ‘straw that broke the camels back’, though that doesn’t place any less blame on those who clearly abused their transmissions. From all evidence so far about the Forester XT and from the models in Japan that have supposedly started receiving this new case revision as early as MY02, it is substantially beefier and harder to break leading evidence of malfunction more directly to abuse.


The STi-exclusive 6-speed manual transmission is similar in function to the 5MT, but has received a major overhaul in design. While it operates somewhat similar to the Subaru 5MT in feel, it has recieved a much beefier case, shorter ratios (6 gears instead of 5), a transmission oil cooler with pump built-in, and segregated oil galleys to reduce the potential of chips to be spread around as most notable upgrades over the basic 5MT design. This transmission is built to handle a substantially greater amount of power over the 5-speed unit, though same effect if drivers flog them in similar order. The STi transmission also includes a Drivers Controlled Center Differential (DCCD) that has the ability to bias power where needed (automatic mode) or desired (manual mode); anything between a full 50:50 lock and nearly a 2:1 rear bias is available. For those requiring the most durable and performance-oriented solution that is just as much at home on the streets as it is at the track, this is currently the butcher’s prime cut of meat.


USDM Legacy 2.5GT 5MT Gear Ratios

(with 4.111 Final Drive Ratio) by: Gear, RPM, MPH

1st 2nd 3rd 4th 5th

Ratio 3.166 1.882 1.296 0.972 0.738


2000 11.6 19.5 28.3 37.8 49.8

2250 13.1 22.0 31.9 42.5 56.0

2500 14.5 24.4 35.4 47.2 62.2

2750 16.0 26.8 39.0 52.0 68.5

3000 17.4 29.3 42.5 56.7 74.7

3250 18.9 31.7 46.1 61.4 80.9

3500 20.3 34.2 49.6 66.1 87.1

3750 21.8 36.6 53.2 70.9 93.3

4000 23.2 39.0 56.7 75.6 99.6

4250 24.7 41.5 60.2 80.3 105.8

4500 26.1 43.9 63.8 85.0 112.0

4750 27.6 46.4 67.3 89.8 118.2

5000 29.0 48.8 70.9 94.5 124.5

5250 30.5 51.2 74.4 99.2 130.7

5500 31.9 53.7 78.0 103.9 136.9

5750 33.4 56.1 81.5 108.7 143.1

6000 34.8 58.6 85.0 113.4 149.4

6250 36.3 61.0 88.6 118.1 155.6

6500 37.7 63.4 92.1 122.8 161.8



Upgraded Gearsets

For those who require an even stronger 5MT gearbox there are two main options; doing an entire converstion to the STi 6MT unit, which is very comprehensive, complicated, and over the top for most, another option is available, just replacing the gearset with a stronger synchro set.


Similar to the standard synchro gearset used as original equipment, but replace the gears with ones that use a smaller gear pitch meaning you will have less teeth in the same diameter, however, each tooth will be thicker and therefore, stronger. Of this type of upgrade, the most popular solution happens to be the STi RA gearset shown below left in comparison with OE gears to compare tooth profiles. Other companies that make good synchros gear sets include but are not limited to: APS, MRT, STi, and Quaife to name a few of the most well known manufacturers.


Automatic Transmissions

For a basic understanding, check out this article at European Car.


USDM Legacy 2.5GT 5EAT Sportshift Gear Ratios

(with 3.270 Final Drive Ratio)

1st 2nd 3rd 4th 5th

Ratio 3.540 2.264 1.471 1.000 0.834

2000 12.8 20.0 30.8 45.3 54.3

2250 14.4 22.5 34.6 50.9 61.1

2500 16.0 25.0 38.5 56.6 67.9

2750 17.6 27.5 42.3 62.3 74.7

3000 19.2 30.0 46.2 67.9 81.5

3250 20.8 32.5 50.0 73.6 88.2

3500 22.4 35.0 53.9 79.3 95.0

3750 24.0 37.5 57.7 84.9 101.8

4000 25.6 40.0 61.6 90.6 108.6

4250 27.2 42.5 65.4 96.2 115.4

4500 28.8 45.0 69.3 101.9 122.2

4750 30.4 47.5 73.1 107.6 129.0

5000 32.0 50.0 77.0 113.2 135.8

5250 33.6 52.5 80.8 118.9 142.5

5500 35.2 55.0 84.7 124.5 149.3

5750 36.8 57.5 88.5 130.2 156.1

6000 38.4 60.0 92.4 135.9 162.9

6250 40.0 62.5 96.2 141.5 169.7

6500 41.6 65.0 100.1 147.2 176.5



Differentials & Electronic Input

Differentials are a very important part of any drivetrain. They are the components that move power between the transmission and axle shafts, changing the direction of power and final gear reduction (through the ring and pinion gears as seen below.) These aspects all help to give you the proper power balance to keep the vehicle under control. While 2WD cars use a single differential to transmit power to two axles, Subaru’s and other AWD cars require three; the center, which transfers power fore and aft from the transmission to front and rear units which transfer power side to side. However, not all differential types are created equal as each is tailored to a specific type of driving and set of conditions.


There are a number of types of differentials applicable to the Subaru drivetrain with two main categories: Open and Limited-Slip. The main difference being limited-slip units maximize traction by sending torque to the wheels with the most traction which they essentially lock while still allowing a variation in speed for turning. Limited Slip differentials comprise of several different designs including, but not limited to: Viscous Coupling, Torsen, Clutch Type, and Electronically Controlled units. The main types of differentials applicable to Subaru drivetrains are described in further detail below. One main thing to keep in mind is the size of the rear differential however, as most general road applications use an R160 (160mm ring gear), if more power is desired and especially if an STi 6-speed manual transmission is being installed, an R180 sized rear differential with associating shafts and hubs is nearly a must to keep reliability in check. An R200 size is available but is only required for the absolute highest output race machines.


Open: This is the most common type of differential used for it’s relatively low cost, lighter weight, and generally predictable vehicle dynamics. This type of differential is used in the front differential in nearly all Subaru’s other than more recent STi models. It is also still used in many of the lower level models for the rear unit as well. This type of differential distributes power fairly evenly to both wheels in a straight line, but when one wheel starts to slip, power follows the path of least resistance, which is unfortunately the slipping wheel. This type of differential is therefore NOT considered a Limited Slip Differential. It is not considered optimal for racing and frequent driving on low traction surfaces though adequate for most transient driving conditions.

Please check out THIS animation to see how open differentials work.


Torsen (TORque SENsing): The Torsen differentials (also known as Helical differentials because of one of the main components- helical cut gears, also known as worm gears) are a mechanical type of differential, no clutches, fluids, or electronic controls involved. This type of differential is excellent because it has nothing to wear out and does not wait for the loss of traction to take effect. Torsion units use a pair of helical cut gears that work like an open differential when power is equal. When a difference in traction is sensed, the helical gears go to work and bind together sending a predetermined amount of torque to the wheel with more traction. The torque bias is predetermined by the design of the helical cut. The only drawback of this type of differential is that when one wheel or set of wheels looses traction, the torque bias defaults to zero torque transfer. A simple remedy of tapping the brake while staying on the throttle produces enough of a level of traction that shifts power to the wheel with traction. Despite this small and less than likely occurrence for most drivers, this is the preferred type of differential for many drivers and some manufacturers due to it’s fast acting characteristics, safe and predictable handling, and it’s very positive ability to reduce torque steer. Audi’s Quattro system has used this type of differential for several years (though information points to just as the center differential lately with Electronic or some other forum of LSDs front and rear) because it works well with ABS sensors, it allows power to move smoothly until things get tricky, and then sends power to where it’s needed most. The Quaife ATB (Anti-Torque Biasing) units are probably the most well known, though STi also sells Helical units and others are available as well.

For more info on this type of unit, consult the Quaife America website.


Viscous Coupling: This type of differential is used in most of Subaru’s performance-oriented vehicles (RS’s, GT’s, WRX’s, etc.) in the rear location, while being used in the center location for nearly all late model Subaru models. Viscous coupling differentials have the ability to better transfer power to the ground in situations where wheel slip occurs versus open style differentials while still being cost effective.


How does this work? The viscous coupling units use two sets of plates close together that are surrounded by a thick fluid. When one set of plates starts turning faster than the other (one wheel or set may be slipping), the thick fluid in the differential heats up and brings both sets of plates to the same speed essentially locking them together. The only drawback is that this system doesn’t kick in until wheels actually start spinning at varied speeds (greater than just a normal turn) while some other units start working when a difference in torque is detected.



This type of differential works similar to the open differential design in normal driving, as do most of these types of limited-slip differentials. However, as viscous units use fluid and torsen units use mechanical gears to essentially lock two output shafts or bias them to a given ratio, this system uses clutch packs as seen in the image above. These clutch packs are made up of alternating friction discs (splined to the back side of the Side Gear) and friction plates (splined to the differential case.) These two clutch packs (one per each axle) reside between the end of the case and a cast pressure ring (which rotates with the case it’s splined to), which essentially butts up to the side gear.


When one axle spins, slower than the rotating case, the pinion shaft shown in the middle (looks like a 4-way) rotates against the V-grooves of the pressure ring putting increased pressure back on the plates causing them to lock when enough pressure is applied. A spring plate and disc is generally used on each side to lessen the impact when the two plates do lock. To gain the desired results, plates and discs of varying thickness can be combined for desired locking results.


This type of differential needs to be picked extremely carefully as it can potentially have very undesirable results. In many (not all) instances, understeer becomes more prevalent as both wheels are trying very hard to spin at the same speed, thus the car will want to push straight no matter how much the driver tries turning. This type of differential is generally favorable more on low traction surfaces such as gravel and snow where equal power to the ground through both axles may be desired. With the right setup though, such as the Suretrac design found on WRX STi’s, a good balance can be found for street/tarmac applications.


Subaru’s DCCD (Driver Controlled Center Differential:

Better than can be described here, the Canadian Driver effectively describes how Subaru’s special center differential unit available only on STi models works, and works very well!


Dynamic Stability Programs:

Subaru relies mainly on mechanical means of safely propelling their vehicles, at least mainly on the manual transmission models. For the automatic transmission enabled vehicles, they use a combination of VDC (Vehicle Dynamic Control), VTD (Variable Torque Distribution), and ATS (Active Torque Split) additional to the already present Viscous Center Differentials. These systems are all controlled with the center differential located within the transmission case.


VDC- takes input from an array of sensors about data including vehicle speed, engine speed, steering wheel angle, gear, and brake status to constantly monitor the conditions and to try and alleviate a potential danger before the car gets out of hand. Several who have tried this system have said it isn’t geared as much for sport as it is for safety, which is good for the average driver, but not the enthusiast. This feature is exclusive to the high-end Outback H6 models thus far.


ATS- has been around since the early 1980’s in Subaru automatic transmissions. This simple setup though similar to VDC just senses power at the axles, gear selection, accelerator position, and vehicle speed without the vehicle dynamics included in the VDC setup. This system takes all the data and calculates how much power to send fore and aft for optimal safe results. Best said on the Subaru of New Zealand website, in basic terms ‘it automatically transfers power from the wheels that slip to the wheels that grip.’


VTD- is a slightly sportier tune of the ATS system allowing more power (up to a 36:64 F:R torque split) to the rear wheels or up to a 50:50 ratio depending on driving conditions. This is the system used in the Legacy GT (at least through USDM/CDM MY04), WRX’s, and some of the other sportier models.


Some of the best information and diagram sources:

HowStuffWorks - Differentials

Canadian Driver - Differentials

Fuji presentation Comparison

General Differential Info

History of the Torsion Differential



Clutches & Flywheels

The new Legacy manual transmission models incorporate a dual-mass flywheel setup which is designed to reduce jerkiness and noise, and make everyday driving easier. For those who want something a bit more, lightweight flywheels are available, though it is recommended purchasing one at least 12lbs or greater as lower a weight unit may make drivability suffer. Lightweight units allow the engine to rev up faster, increasing performance. Of the current designs available, Chrome-Moly fabricated units appear to have the optimal strength with the ability to be lightweight as well.


Upgrading power in Subaru AWD cars puts an even greater stress load on transmission, and since power is being transmitted to a minimum of 2 wheels (up to 4) in 2 different directions through the transmission case, many recommend keeping the stock clutch, or some type of aftermarket full face clutch with less than 50% greater holding power over stock. 3-Paddle style clutches (a.k.a. 3-puck/button) aren’t recommended for street applications as they tend to shutter and are more abusive to transmissions, particularly in ample-traction environments.


Hardened Mounts & Bushings

These components are designed to reduce driveline and engine movements to transmit power more effectively to the ground, though there will be a noticable increase in NVH with many of these mounts.

Edited by SUBE555
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