Beginner’s Guide to the WRX

There were supposed to be pictures included, but copy/paste didn’t go according to plan, here is the full document in PDF form with lots of photos:
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MS Word DOCX format


2004-2005 WRX/STI GD chassis specifications:


Curb weight: 3165lbs (manual transmission wagon), 3263lbs STI

Engine displacement: WRX 2.0L, STI: 2.5L

Engine block: WRX open-deck, STI semi closed-deck

Stock horsepower: WRX 227hp, STI 300hp


STI only: AVCS: Active Valve Control System (variable valve timing)


Turbocharger: WRX: FHI TD04-13T, STI: IHI VF39

Stock boost pressure:  WRX: 13.5lbs, STI: 14.5lbs



WRX: front: open differential, center: viscous LSD, rear: viscous LSD

STI:  front: helical-gear LSD, center: DCCD, rear: clutch based LSD

2008+ WRX: front: open diff, center: viscous diff, rear: open diff (uses traction control)



WRX: 5 speed manual (60mph can be reached in 2nd gear)

STI:  6 speed manual with shorter gear ratios, much stronger gears (60mph requires shifting to 3rd gear)


***STI transmission cannot be easily swapped into a WRX***



Brakes: STI has larger Brembo brake discs and calipers.  STI brakes do not fit with some 17” wheels and do not fit on 16” wheels.


Wheels: WRX: 16×6.5″, STI: 17×8″

Wheel bolt pattern:  WRX: 5x100mm, 2005+ STI: 5×114.3mm

***STI wheels do not fit on a WRX (only 2004 STI wheels will fit WRX).***


Sway bars: STI has slightly stiffer sway bars

Springs: STI has slightly shorter and slightly stiffer springs


Headlights: WRX: halogen, STI: HID (High Intensity Discharge, a.k.a. Xenon)




Subaru Impreza WRX years and models:  Match the picture with the year or chassis code


WRX Chassis codes:  GC – 1992-2000 GD – 2001-2007, GR – 2008+


2.5RS Impreza 1999-2000 (GC chassis) 2.5L engine (first generation WRX was not sold in the US, but looks like this)



Bug-eye WRX 2002-2003 (GD chassis) 2.0L engine



Peanut-eye/blob-eye WRX 2004-2005 (GD Chassis) 2.0L engine



Hawk-eye WRX 2006-2007 (GD chassis) 2.5L engine



2008-2010 WRX (GR chassis) 2.5L engine



2011-2012 WRX (GR chassis, wider fender flares)



Saab 9-2x Aero:

2004-2006 produced as a re-skinned WRX.  2004-2005 2.0L engine, 2006 2.5L engine, 5-speed manual transmission or 4-speed automatic transmission.




Car parts:


ABS: anti-lock brake system.

Traction-control/Stability control:  computers sense that one wheel is moving much faster than another and either applies brakes to the one faster wheel, or engine cuts power to prevent wheels from spinning.  Newer WRX’s cannot have the traction control completely turned off without modifications to the car.


Anti-lock brake module looks similar to this:



Brake caliper:  This piece holds the brake pads and has one or more pistons that push the brake pads against the brake disc when the brake pedal is pressed.



4-pot brakes:  brake caliper that uses 4 pistons (two on each side of disc) to push the brake pads against the brake discs

2-pot brakes:  brakecaliper that uses 2 pistons (one on each side of disc) to push the brake pads against the brake discs



Brake disc: This piece turns at the same speed as the attached wheel and is what the brake pads grab onto in order to slow down the car


Drilled brake rotors/discs:  brake disc has holes drilled into it to supposedly allow gases to escape from the brake pad under hard braking, it is debatable whether they have any real benefit



Slotted brake rotors/discs:  brake disc has slots cut into it to supposedly allow gases to escape from brake pad under hard braking, it is debatable whether or not they have any real benefit


Carbon-ceramic brake discs:  Found on very some exotic cars and some race cars.  The brake disc is made from a carbon-ceramic material instead of steel.  They are very light and will frequently glow bright orange for a few moments when brakes are applied hard during a race.  They can be found on American LeMans series GT cars.



Differential:  a mechanical device allowing left and right wheels to turn at different speeds and still stay connected to the engine.  This is important particularly when turning around a corner, the outside wheel must turn faster than the inside wheel.



DCCD: Driver-Controlled Center Differential – controls power distribution between front and rear wheels.





Rear differential is bright silver and between the two green pieces:



LSD – limited slip differential – when wheels start to slip, power is transferred from the slipping wheel to the wheel that has traction.  There are a few different types of LSD’s.


Helical LSD (Torsen aka Torque-Sensing) – uses an arrangement of helical gears to provide LSD functions.




Clutch-based LSD – uses clutch discs to provide LSD functions.



Open differential – wheel that loses traction keeps spinning uncontrollably, power is not transferred to the other wheel that has traction.  Without Traction-Control, these can easily lose traction on slippery road surfaces like snow.  Found in the front of all WRX’s, front and back of 2008+ WRX’s, and most other Subaru’s except for STI (which uses all LSD’s).




Strut:  a.k.a. ‘shock absorber’ or ‘damper’, controls the speed at which the suspension springs compress and expand, keeps the car from continuing to bounce after hitting a bump.




Boxer engine:/horizontally-opposed engine:  Pistons move horizontally in opposite directions.  Engine is not as tall (vertically) and gives the car a lower center-of-gravity (car less likely to roll over).



V-engine:  cylinders form a V-shape




Inline engine:  cylinders are all upright and in a straight line





Rotary Engine:  The ‘rotor’ is the triangular shaped part which rotates in an elliptical path around the center (letter B).  This engine can be found in the Mazda RX-7 and RX-8.




Sway-bar/Anti-roll bar:  a solid or hollow bar that connects both sides of front or rear suspension to limit body roll during turns



Sway-bar End-link:  connects the sway-bar to the wheel hub assembly.



Strut-bar/strut-brace:  A bar that connects the tops of the front or rear shock towers if the body of the car requires extra stiffness.  It is debatable whether or not they have any real benefit on a WRX or STI.  A rear strut bar on a WRX wagon may offer some benefits since a wagon doesn’t have a metal frame connecting the sides of the body behind the rear seat (like the sedans do prior to 2008).




Turbocharger:  An air compressor that pumps extra air into the engine intake, driven by exhaust gases.




Supercharger:  air compressor that pumps extra air into engine, driven by belts connected to the engine pulleys.



Stoichiometric air to fuel ratio:  14.7 parts air to one part gasoline, 14.7:1 ratio of air to fuel.  This tends to run very hot, so a slightly richer mixture is frequently used to keep engine cooler.


Rich fuel mixture: too much fuel and not enough air.  Not all of the fuel gets burned.  This is sometimes used purposely to keep the engine cooler and prevent detonation/knocking.


Lean fuel mixture:  too little fuel and too much air.  Engine runs hotter and can cause detonation and knocking.

Knocking:  A condition in which the cylinders get so hot, that when fuel is injected, it ignites before the spark plug actually fires.  This can cause the fuel to explode while the piston is still moving upward (instead of pushing the piston downward like it should).  This can cause a piston to crack or actually end up with a hole in it.  A higher octane gas can be used if knocking is present.




Octane:  A rating applied to gasoline that indicates how readily it will explode.  A higher octane gasoline is less willing to combust. Higher octane gas is useful in fighting knocking/detonation since the heat from the cylinders is less likely to cause it to ignite prematurely.



Intake:  a tube or other inlet where air from the atmosphere can enter the engine.  They often contain a MAF sensor.



MAF sensor:  Mass Air Flow – this electronic sensor measures how much air is entering the engine.  The ECU uses this to calculate how much fuel for the fuel injectors to spray into the cylinders.


Fuel injector:  electronic valve that allows pressurized fuel to pass through it in a “mist” spray pattern.  It has two states:  fully open and fully closed.  To control the amount of fuel injected into the engine, it is opened for a shorter or longer period of time (measured in milliseconds).


ECU:  Engine Control Unit – The computer the controls the engine’s ignition timing, how much fuel is injected into the cylinders, and many other events to keep the engine running.  It is located in the passenger foot-well, behind the glove-box and behind a metal plate.



OBD-II : Onboard Diagnostics revision 2:  a standard port used to connect computer diagnostic equipment to a car.  On the WRX the ECU can be reflashed or ‘tuned’ by connecting a computer to this port or with a Cobb AccessPort, Tactrix cable, etc…




Boost:  measured in PSI (pounds per square inch) or bar (a multiplier of the actual barometric pressure.  Standard atmospheric (barometric) pressure at sea-level is 14.7 PSI.  1 bar of boost would be 14.7 PSI above atmospheric pressure, 1.5 bar of boost would be approximately 22 PSI above atmospheric pressure.


NA – Naturally Aspirated – a name for a non-turbo engine.  Air is sucked into the engine by the natural vacuum of the pistons moving downward in a cylinder.


Why does a turbo make more power ?

Horsepower is limited by how much fuel can be burned inside a cylinder.  In order to burn more fuel, more oxygen is required.  An NA car is only capable of sucking in a certain volume of air into its cylinders.  A turbocharged car, however, can inject extra fuel into its cylinders because the turbo is pumping more air in than the engine would naturally suck in by itself.  More fuel can only be burned if more air is supplied.  If too much fuel is injected without the required 14.7 times as much air, the extra fuel will not be burned and will simply be expelled from the cylinder as exhaust.  Extra fuel only produces more power when there is enough air.


Intercooler:  a radiator that cools the air exiting the turbo before it enters the engine intake manifold


TMIC: top-mount intercooler, sits on top of engine underneath the hood scoop, less piping and less turbo lag, but more susceptible to heat-soak from engine.


FMIC: front-mount intercooler, sits in lower front grill, uses more piping and can create more turbo lag.  only really necessary for very high horsepower



Downpipe:  the exhaust pipe that connects to the back of the turbo’s hot-side, the stock downpipe contains a catalytic convertor.  This piece is considered to be “after” the turbo.



Up-pipe:  the short exhaust pipe that connects the exhaust headers to the hot-side of the turbo, the stock up-pipe contains a catalytic converter on models made before 2006.  This location is referred to as “pre-turbo.”



Muffler: Sole purpose is to make quiet down the exhaust by echoing the sound through a series of chambers to cancel out certain frequencies.



Wastegate:  keeps the turbo from over-boosting by letting exhaust bypass the hot side of the turbo, stock turbo uses an internal wastegate



External Wastegate:  wastegate is built into the up-pipe, some turbos do not have an internal wastegate, or sometimes they are welded shut.



Wastegate Actuator:  controlled by the Boost-control solenoid (BCS), it opens or closes the wastegate flapper to prevent turbo from over-boosting.





BPV: bypass valve – attached to front of intercooler, releases pressure from turbo when you suddenly let off the gas. This allegedly extends life of turbo and prevents “compressor surge.”  The pressure that is released is re-circulated back into the intake tube.



BOV: blowoff valve – replaces stock BPV and vents excess pressure to atmosphere (instead of re-circulating it back into the intake tube). They can cause car to idle rough or backfire if not tuned for it.




Radiator:  a large metal cooling device made up of tubes carrying coolant/anti-freeze with metal fins connecting the tubes.  As the tubes carry hot coolant through them, the heat is dissipated into the fins which are then cooled by the air from cooling fans and from driving the car.


Compressor surge – condition when driver lets off of the throttle suddenly, the high pressure created by the turbo suddenly has nowhere to go when the throttle plate shuts, and the air backs up causing the turbo compressor to slow down very suddenly, there is some controversy that this may be bad for the turbo, but some will dispute this


WRC:  World Rally Championship, the pinnacle of rally racing in the world.  These cars are worth $250,000 and up.  They begin life as a stock vehicle and are fully stripped down and rebuilt.  All components including the engine, transmission, all-wheel-drive system and suspension are replaced with specially built racing parts.

Subaru pulled out of WRC after 2008 season.  Subaru cars participating in WRC events today are not sponsored by Subaru.

Current WRC cars are the Citroën DS3 WRC, Ford Fiesta RS WRC and Mini WRC.  The engines are turbocharged 1.6L and limited to 300hp.  They have sequential gearboxes with straight-cut transmission gears (make a loud whining sound similar to when you drive in reverse).  Anti-lag technology is a closely guarded secret but is present on these cars as well.



Famous Subaru Rally drivers: 

Colin McRae (1968-2007) WRC driver for Subaru World Rally Team (SWRT)

Richard Burns

Petter Solberg



Anti-lag:  WRC cars have an extra fuel injector of some type placed in the up-pipe that creates small explosions keeping the turbo spooled up when the driver off the throttle.  This makes frequent loud popping sounds.




Projector:  uses a lens to focus the light beam, may use halogen or HID bulbs.


Reflector:  uses a reflective dish to focus the light beam, some are for halogen and some are for HID bulbs.  Halogen reflectors and HID reflectors are designed differently, you should not put an HID bulb into a reflector designed for a halogen bulb.



HID – High Intensity Discharge – instead of a light bulb with a filament that heats up and glows, these ‘capsules’ use a powdered salt that vaporizes when approximately 35,000 volts are passed through.  Produces a very pure white light.  These require a separate module called a ‘ballast’ that creates the high voltage.  HID capsules must not be used in headlights that were not designed for them, they do not produce good light output and will blind oncoming drivers.  The best bet is to have the headlights professionally converted for HID’s (involves replacing the reflector with a projector unit), or to buy OEM HID headlight housings and replace yours with them.


HID/Xenon capsule:



Halogen bulb:



HID Bulb and ballast:



Bad HID beam pattern (using HID bulbs in non-HID headlight blinds other drivers):



Good HID beam pattern (with factory or correctly retrofitted projectors):




Retrofitted WRX headlights with HID/Xenon projectors:


These are still not technically legal, but they produce an acceptable beam pattern and do not blind other drivers if made correctly.


Carbon fiber hoods and body parts:

Inexpensive carbon fiber parts are typically a metal frame or fiberglass with carbon cloth on top.  They look like carbon fiber but are not strong and are not light-weight. CWII style hood (about same weight as stock hood) $900.

Expensive carbon fiber racing parts are made only of carbon cloth and resin.  They are extremely light and extremely strong.  Also very expensive. Example: Seibon Dry-Carbon hood for WRX, 5.5kg, $2400

A WRX hood is already made from aluminum and very light to begin with.  A carbon fiber hood is not a significant upgrade on a WRX and might only save 10lbs.



Big wheels:

Larger diameter wheels usually weigh more than smaller diameter wheels.  Cheaper wheels usually weigh more than expensive wheels.  Forged wheels are stronger and more expensive than machined wheels.  Tires for an 18” diameter wheel are generally more expensive than tires for a 17” wheel.


Popular wheels:

Enkei RPF1:



5zigen FN01R-C:




Stock (OEM) 2004 WRX wheels 16×6.5”



Rota Torque:




Popular springs (lowering and performance):

Springs can be replaced to lower the car, or to stiffen the suspension (or both).  It is generally accepted that going too low makes car handle more poorly and also takes bumps very poorly (since the springs are almost fully compressed and can’t absorb any bumps).  Lowering the car up to 2 inches is generally accepted depending on the springs.  Lower springs need to be much stiffer.

Stock springs:



RCE Yellow springs (good, about $300, much stiffer, lowered about 1.5” or 2”):



Eibach Pro-kit springs (good, slight stiffer than stock, lowered about 1.5”):



‘Slammed’ (probably using coilovers set very low, looks cool, very impractical and bad for handling, 100% for looks, any minor bumps in the road will be very jarring in the car since the springs are probably fully compressed and have no ‘give’ left in them)



‘Flush/Hellaflush’ look:  tires extend out to or beyond fender.  Very stiff suspension or very stiff sway-bars are necessary to prevent the fender and tire from damaging each other when springs compress.  Purpose is primarily for looks, although in some autocross classes, extremely wide tires are used that extend past the fender, and this also requires very stiff suspension and/or very stiff sway bars in front and back.





16” vs 17” vs 18” wheels:












Rear wings/spoilers:

Rear wings and spoilers do not generally create much downforce at highway speeds.  They might improve fuel efficiency slightly by smoothing out the airflow at the back of the car to reduce drag.  I’m not certain that this has actually been studied and may be negligible.


Vehicle Dynamics/Driving Technique:


Lift-Off Oversteer:  Turning into a corner and suddenly lifting off the throttle.  The weight of the car shifts forward suddenly and the back wheels become light.  The front tires ‘bite’ very quickly and the front of the car slows down but the back wants to keep moving forward.  If the car is not pointed straight ahead, the back of the car will start to rotate around the front and a spinout can result.




Understeer:  when driving fast into a turn, the front tires lose grip and the car goes straight instead of turning.  Understeer is a characteristic of a WRX (but not as much with STI) due to its Open-Differential in the front of the car (STI has an LSD type differential in front).  In a hard turn, the inside front wheel on a WRX will start to lift off the ground, and if throttle is applied the wheel will spin freely, reducing steering ability and causing the car to “plow” or “push” straight instead of turning.



Oversteer:  when driving fast into a turn, the rear tires lose grip and the car starts to spin, often happens if you lift off the gas suddenly in the middle of a turn, because the car leans forward suddenly and the back tires become unweighted, losing grip.  Drifting uses controlled oversteer to point the car in a different direction than it is actually traveling, causing it to “get sideways” and slide.



Opposite-lock (steering):  if a car starts to oversteer or “drift”, the steering wheel can be turned in the direction the car is sliding to attempt to control or regain control of the vehicle.  This is used frequently in ‘drifting’ driver technique to control the car.



Handbrake turn:  Technique used that locks up the rear tires in a sharp turn in order to rotate the car around a hairpin turn quickly.  FWD cars can apply throttle while handbrake is applied, AWD cars cannot without causing damage to the AWD system unless specially modified to do so.



Acceleration: speeding up


Deceleration: slowing down

Alignment: adjusting the toe, camber and caster of the four wheels so that the car drives straight, is easy to control and minimizes tire wear.  Many cars do not have caster adjustments, and many do not have adjustable camber in the rear without using ‘camber bolts’ or ‘camber plates.’


Negative camber:  when the tires are leaning inward at the top.
Why use negative camber?  When turning aggressively, the car suspension on the ‘outside’ of the turn will compress and the tires can roll inward at the bottom, causing them to ride partially on the sidewall instead of the main contact patch.  Negative camber counteracts this to keep more of the tire’s contact patch on the ground.

Positive camber:  when the tires lean outward at the top.  This is generally not useful except with certain types of suspension design because it can cause the tire to ‘roll over’ onto the sidewall even more when cornering, reducing grip.



Toe-in:  the front of the tires are angled inward slightly

Toe-out:  the front of the tires are angled outward slightly

Zero-toe:  the tires are perfectly straight, no toe-in or toe-out



Caster:  the angle at which the steering axis leans forward or backward.  If caster is out of adjustment, it will cause the tires to meet the ground at an angle when the steering wheel is turned.




Active Yaw Control:  electronic traction control on the Mitsubishi Lancer Evolution that causes the outside wheels in a turn to have more power, thereby helping the car rotate more effectively in corners and prevent or reduce understeer.





R-Comp:  “race compound” tires that have a soft rubber, it doesn’t last as long but has better grip.

UTQG:  tire treadwear rating that can loosely be used to determine which tires are stickier than others.  A typical all-season tire:  400, typical autocross tire: 200, typical autocross slick tire:  80



Tire size numbers:  245/45-R17:  245mm wide contact surface, sidewall height is 45% of the width of the tire (adding both top and bottom sidewall heights together), fits on a 17″ diameter wheel.



Tire tread pattern:

High performance tires usually have large blocks of tread with fewer grooves, usually a very stiff sidewall also.


All-season tires will have many more grooves and smaller blocks.


Winter/Snow tires will have soft rubber but taller tread blocks, similar to fingers or toothbrush bristles, so that they can dig into snow, but that the snow will fall out of the treads and not clog them up.



Stretched tire:  when the tire is “too narrow” for the wheel, the lip of the wheel sticks out past the tire sidewall.  it is a stylistic choice that is actually worse for performance, since a wider tire will give more grip than a narrow tire.  Sometimes used by drifting drivers because they have less grip and are easier to drift.



Wheel sizing:  17×8 +48 offset, 17″ diameter wheel, 8″ wide wheel (affects how wide of a tire you can fit, 8″ wheel should generally fit a 245mm wide tire), the mounting plate of the wheel is 48mm away from the centerline of the wheel.



Other pictures:

GR- chassis exhaust underside:



GD-chassis exhaust underneath:

















Camber Plates (for adjusting camber and sometimes caster):



















Pistons, connecting rods and crankshaft for an inline-4-cylinder engine:



Stock intake, turbo and boost controller for WRX.


Cutaway of a turbocharger:





Front lip spoilers:

They may not offer any performance advantage at highway speeds and can be easily cracked or damaged by hitting curbs, parking markers and driveways if the car is lowered.



Tire sidewall information:


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