By popular request we have compiled a small tech & FAQ section for the Subaru WRX/STi. There are a million ways to modify a Subaru and this section contains examples of how WE like to build cars but is by no means comprehensive.
While it would be great to throw every performance part imaginable on your Subaru most people cannot afford to build their cars all at once. Instead it's best to choose to build your car along one of three main paths; handling, power, or balanced. While we cannot possibly include everything you can or should do to your car in this article we hope to give people a good place to start from. Always be sure to research any part before you buy it. Some parts work great alone but poorly in conjunction with other parts and conversely; some parts don't do much on their own but can make major gains when combined with others.
Handling
This is for the people who want to get the most out of their cars handling capability. The Subaru AWD system makes suspension tuning different than many other cars so it's key to emphasize turn-in and understeer prevention.
Our upgrade recommendations:
Wheels and Tires. Probably the most overlooked handling component on any car is it’s wheels and tires. Lightweight wheels reduce rotational mass more than any other drivetrain component. Reduced wheel mass helps with both acceleration and braking. Couple that reduced mass with a performance or track tire and you can really take advantage of your vehicles suspension components.
We recommend the Enkei RPF1 wheel for its extreme light weight and low cost. For tires we recommend the Kuhmo XS which is the top rated street tire. For all around track use the Kuhmo V710 is a great tire as is the Hoosier A6 for autocross.
Sway bars, front then rear.
Subaru swaybars
These will help keep the car level through the corners balancing the amount of work each tire is doing during a corner. You will get crisper turn-in and improved front end grip from the addition of a front bar and more consistant rotation from the addition of a rear bar. This combination will reduce understeer and help keep the car composed during series cornering and when switching directions.
We recommend the Hotchkis hollow sway bars to our customers for their combination of handling characteristics and weight. We also recommend the use of aftermarket end-links which help transfer force to the swaybar and won't break during repeated hard cornering like the stock ones tend to do.
Full coilover setup.
Subaru Coilovers
Power is nothing without control. A properly tuned coilover setup will allow you to adjust your ride height to the optimal level for stability and a lower center of gravity. Good coilovers keep the car from squatting during acceleration and diving during decel which allows you to carry more speed through a corner. They keep weight evenly distributed across all 4 tires during a corner to maximize grip levels. They are fully adjustable to cater to each driver’s unique driving style.
We recommend the JIC FLT-TAR (formerly FLT-A2) coilover as a good entry level coilover for track day or auto-x use. If you tend to drive your car on a daily basis outside the track then the Endura-Tech Pro Series will probably suit you better.
Big brake kit.
Subaru Big Brake Kits
Larger brakes will let you brake later and deeper into a corner which lets you get the most out of each straight. Larger brakes also dissipate heat better for repeated hard braking like what you would see on a track. A high quality big brake kit is a MUST for anyone looking to do heavy track driving or time attacking. The STi came with a great Brembo brakes from the factory and many people choose to just upgrade the rotors to slotted or slotted & drilled for better heat dissipation. Pad choice is also very important in brakes, it is not recommended to use a street pad on the track as they will burn off in a few laps and track pads are a very poor choice for the street because their operating temperature is higher than what most people will ever see on the street.
We recommend the Rotora 6 piston Kit for the front and 4 piston kits for the rear of your WRX. We also recommend the Hawk HP+ pads for anyone who wants to be able to repeatedly stop hard while still being a decent pad for regular street driving. If you plan on hard track-day or time attack a full racing pad is recommended such as the Hawk racing series. We also greatly recommend an upgraded brake fluid with a high dry boiling point such as the ATE Super Blue racing brake fluid.
Bracing and bushings.
Subaru Chassis Bracing
Subaru Bushings
The chassis of the WRX has many week points that flex and deflect the suspensions ability to properly work. Urethane bushings and aluminum braces help isolate suspension loads to their respective suspension component and prevent deflection across other parts of the chassis. These upgrades greatly help with steering response and car balance. Another advantage to aftermarket bracing is many braces replace heavy flimsy factory steel components with structural aluminum that is more ridged and significantly lighter.
We recommend a full polyurethane bushing kit for the front end, strut tower bars (front and rear), and rear differential lock out bushings and bolts. Above that subframe braces, fender braces, and any multi-point tie brace will improve handling. We carry the entire line of GT-Spec and Beatrush subframe braces as well as a few others.
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Power
If what you really want is that peddle down, slam you back in your seat, pure acceleration exhilaration then a power build is for you!
Our upgrade recommendations:
Intake.
Subaru Intakes
Your motor is like a big air pump, anything you can do to free the flow of air into and out of the engine will help make power. Most intake systems are sufficient so long as they have a good free-flowing air filter. The short ram style of intake is better suited to driving in the northwest and also makes fitting front mount intercoolers easier as fmic piping and full cold air intakes usually share the same passage into the fender well.
We recommend the COBB short ram intake or the AEM intake system. We will be offering a custom intake made out of billet aluminum with an AEM dry-flow air filter very soon.
Turbo-back exhaust system.
Subaru Turboback Exhaust Systems
Getting exhaust gas out of the car is extremely important and any restriction will impede the turbos ability to make boost. A good system consists of 3" piping, heavy duty flanges, a straight through muffler, and as few bends as possible. There are many downpipe styles but we find that the bellmouth style and the 4" taper make the least turbulence near the turbo and perform the best. Stay away from divorced gate styles as they often cause interference with proper wastegate operation.
We recommend the TurboXS turboback exhaust or the Topspeed turboback exhaust system. We really like the sound of the Tanabe cat-back system when mated to the Topspeed 4" downpipe.
Header and up pipe.
Subaru Header & Up-Pipe
Simply put, the stock header is insufficient to spool a larger than stock turbo. We have tested nearly all the headers on the market and find that the GTSpec/Tomei style UNEQUAL LENGTH headers make the most power across the entire powerband and keep the Subaru boxer rumble we have grown to love. Many people argue that the equal length headers should make more power but on the Subaru's this just isn't the case. We think the reason the UNEQUAL header performs so well is due to the firing order of the Subaru which allows all the pulses to hit the collector at different times. This increases the exhaust velocity inside the header which aids in spooling the turbo faster and helps it hold boost longer. The factory up-pipe in many early Subaru models contains a cat that will break down and destroy your turbo. The factory up-pipe is also too small to accommodate a larger turbo and upgrading will allow faster spool and more power potential.
We recommend the GTSpec UNEQUAL LENGTH header & up-pipe combination
Front mount intercooler.
Subaru Intercoolers
The factory top-mount is insufficient to run in a high boost application. It's just too small and can't get enough direct air flow across it for a high CFM turbo. Even upgraded top-mounts are insufficient for high boost applications. A front mount intercooler will allow a colder charge of air to enter the motor which will make more power.
We recommend the TurboXS front mount intercooler for its fitment and performance. These kits fit extremely well and are sufficient for even very large turbos.
Larger fuel injectors and pump.
Subaru Fuel Systems
When raising boost and power levels the fuel system must be upgraded. Larger fuel injectors and a high volume fuel pump are required. Additional fuel system upgrades may be required depending on power goals. Examples of these upgrades are twin in tank pumps, upgraded fuel rails, and upgraded fuel pressure regulator.
We recommend modified top feeds which usually flow out to 850cc and the Denso TT high volume fuel pump.
Upgraded turbo.
Subaru Upgraded Turbochargers
You will soon realize that all the supporting mods on the planet won't make 400hp. It's time for a turbo! Turbo choice really comes down to this: How much power do you want? Find what your maximum power goal is and choose the smallest turbo that will achieve your power goal at the peak of its efficiency. This will generally give you the best spool-up characteristics while still hitting your power goals. If you only plan on pushing 400whp there is no need for a GT42R.
We recommend any of the FP series of turbos as well as the Tomei for stock location turbo replacements. If you plan on a rotated setup we recommend any of the Garrett ball-bearing options such as the famous GT35R.
EJ25 block.
Subaru Engine Internals
Let’s face it, to make power on a Subaru you really need to get rid of that old 2.0L and go with the 2.5L. The EJ25 will make more torque rite out of the box and the higher displacement will help spool larger turbos than the 2.0L. The EJ25 also came with semi-forged pistons and forged rods that we regularly run up to 400whp without problems. If you plan on making between 400-550whp we recommend a minor build with forged pistons and some other upgrades we do in house. If you plan on 550whp+ you should sleeve your block in addition to forged pistons and oiling system upgrades. ARP headstuds are a must when putting your motor back together.
We recommend CP pistons, ARP fasteners, and Dart sleeves when building your motor.
Ported Heads.
Subaru Head Parts
Porting the stock heads will allow more air to enter and leave the combustion chamber. Oversized valves as well as upgraded valve train and cams will shift the power band up, increase power, and help spool larger turbos.
We recommend a full 5axis cnc port and polish combined with Ferrea valve train and cams based on your needs.
EMS Tune.
Subaru EMS upgrades
While the stock re-flash is good for a mildly build Subaru its resolution and options are lacking for seriously high boost applications. We recommend a full stand-alone engine management system such as the AEM unit for big power builds. This unit allows better 3d mapping, extremely fine resolution, and options for various inputs such as methanol levels, air fuel logging, boost logging, etc. Switching to an EMS also eliminates the need for a restrictive MAF by switching the car to the more "boost friendly" MAP style of engine control. The biggest downside to most stand-alone EMS's is the car will no longer be scanable via the OBDII port and thus will not be emissions legal in most states.
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Balanced (power + performance)
For the person who wants to come hard into a corner and hard out, this is for you. We feel this build is best suited to 90% of our customers.
Our upgrade recommendations:
Sway Bars. Rear then front. These will help keep the car level through the corners balancing the amount of work each tire is doing during a corner. You will get crisper turn-in from the addition of a front bar and better rotation from the addition of a rear bar. This combination will reduce understeer and help keep the car composed during series cornering and when switching directions.
We recommend the Hotchkis sway bars to our customers for their combination of handling characteristics and weight. We also recommend the use of aftermarket end-links which help transfer force to the swaybar and won't break during repeated hard cornering like the stock ones tend to do.
Lowering springs. Inexpensive and extremely effective lowering springs have been used for decades to get better handling out of a car without sacrificing drivability. A good spring will slightly lower the car and usually offer a 20-30% stiffer spring rate.
We recommend the Tanabe GF210 spring for all our customers. It is a very nice riding spring that offers extremely good handling. Go one step further and couple this with KYB AGX adjustable struts for the ultimate budget street setup!
Intake and turboback exhaust. Your motor is like a big air pump, anything you can do to free the flow of air into and out of the engine will help make power.
Header and up pipe. Fairly inexpensive and a great bang-for-the-buck upgrade. This also replaces the dangerous stock catted up-pipe that came factory on many cars.
We recommend the GTSpec UNEQUAL LENGTH header & up-pipe combination.
STi topmount intercooler. If you have an STi already then cool! If not, then try and pick up a tmic from one. The STi's tmic has more than double the fin surface area of the WRX which allows for better cooling and higher air flow. This upgrade will allow you to run more boost and therefore make more power.
Reflash tune. Get your car tuned for the parts are on it. From the factory Subaru tuned the cars to run with factory parts in any state, on any gas. We always see a gain over the Subaru stock tune even if you don't have ANYTHING done to your car! Once you start upgrading your car the stock tune can't keep up with the modifications you have made and can cause dangerous lean conditions and over boost issues. These issues could potentially destroy your engine and turbo if not taken care of with a reflash tune. There are really two ways to reflash your car. There is the COBB accessport and an ECU reflash. Both tools tune your car exactly the same and have the same power potential but the COBB has a few extra features like multi map selection and CEL code clearing.
We recommend an ECU reflash to our customers. While we will be happy to sell our customers a COBB accessport we find that most are happier spending their money on other performance parts. All of our tunes are done on the dyno and custom tailored to your car's specific needs on an individual basis.
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Frequently Asked Questions
I have a check engine light (CEL) what does that mean?
There are many reasons your car can throw a CEL. The first step is to find out what code the car is throwing. Most automotive parts stores and many mechanics will check a CEL for free. Once you know what code your car is throwing you can check our Subaru CEL Diagnostic page here. Do a Google search to see what people are doing to fix the problem you are having or bring your car by the shop and let us diagnose it for you. Some CEL's are common and not a problem but others are warning of potential damage to your car. Don't ignore a CEL!
Can I put an aftermarket blow off valve (bov) on my car?
Yes, you can put a properly adjusted properly installed blow off valve on a Subaru WRX/STi. The key is properly adjusted. A poorly adjusted or improperly installed BOV can damage your motor so be sure you research which BOV's work well with the Subaru motor. From our experience the TurboXS series of BOV's have the least problems when adjusted properly and we have never had an issue with any of the WRX specific Turbosmart valves. Stay always from Greddy RS and HKS valves on the Subaru. They tend to stay open too long and cause serious drivability issues.
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Ask your question by e-mailing erich@preracing.com and we might post it on this page!
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