Thought that this would be a helpful thread for those who have just purchased a WRX or STi.
--Uppipe - For the WRX this will allow the turbo to spool a few hundred RPM sooner. Great first mod. The STi and 2006 WRX has a catless uppipe so there is no need to switch it out.
--Downpipe - Getting rid of this cat will give you a little more top end. However, for the STi and 06 WRX, a HF cat is recommended to prevent boost creep.
--Turbo back exhaust - This is the downpipe, mid pipe and catback (or axleback). This is a great mod that will free up the exhaust flow from the turbo. Huge gains from this mod. If you have an STi and run a TBE that is totally catless there is a chance that you will get boost creep when hard on the throttle in the later gears. Use a HF cat to prevent this.
--Catback or axleback - This will bring a very small hp increase but will bring out that lovely boxer rumble.
--Engine Management - This is a great first mod as it gives you a very safe base for all future mods. If you get to the point where you are considering a turbo upgrade engine management is highly recommended so that you don't blow your engine up.
--Wheels/Tires - One of the best first mods to increase the handling of the car. Get off the stockers. If you have an STi and are in an area where you get snow you will absolutely need to switch out the tires.
--Sway bars - Adding a rear sway bar will greatly decrease the amount of understeer the car has coming from the factory.
--Springs - Adding a set of lowering springs will decrease the body roll and boost the handling.
--Gauges - If you are modding the engine in any way you should really have a set of gauges to monitor the engine vitals. You should at least have boost and exhaust gas temp (EGT).
--SS braided brake lines, upgraded pads, and high temp brake fluid will do wonders for your braking. For those with a WRX, big brake kits should be researched carefully, fully understanding the compromises associated with rim selection afterwards (think STi).
Also...since we have so many folks asking about an actual modification "path", you can use the following to answer many of your questions.
Before Break-in (if desired)
--Short shifter and bushings
--Steering rack bushings
--Suspension mods (see below)
Post break in (Engine)
--Oil Change-get rid of the crud
--High flow air filter-best air intake mod without issues of CAI or short ram
--Up Pipe (pre ‘06 WRX only) – get rid of a catalytic converter
--Down Pipe – get rid of a catalytic converter
--Catback – open up the downstream side of the turbo turbine side.
--Engine Management (I prefer to do this before UP/DP mod personally)
--Gauges-Boost, EGT, A/F are very good ideas. Oil Press., Oil Temp., and Water Temp. if you plan on tracking. Datalogger software if you can is always a good idea at this point.
--Tuning – for those EM systems that don’t provide for ready made maps.
--Plug wires - take care of some basics before you go nuts.
--Brakes: Before upgrading to a better turbo, please consider being able to stop better first. As great as the STi brakes are, they are easily improved, much as the WRX. SS lines, Dot 5 fluid, and better pads make a HUGE difference. For WRX, a bigger brake kit will completely alter your stopping characteristics.
--Turbo Upgrade, including turbo, fuel pump and injectors. Depending upon needs/power demands and safety, also consider upsized TMIC, FMIC, or water-to-air system. FMIC will need CAI or short ram intake installed. More exotic considerations will include a race type radiator/fans, headers, fuel rails, valve/head work, intake manifold work, and water or alcohol injection.
--Sparkplugs (for some turbo upgrades this is a necessity
Ignition system – This will help, believe it or not.
--IC sprayer for WRX.
--Fluids: Full Synthetic for Engine Oil, Front Diff/Tranny Oil, and Rear Diff. Water Wetter for Radiator.
--Tires (mainly WRX as the stock STi are excellent if kept at higher pressures)
--Snow tires for those in winter climates. Stock WRX or adequate for snow. STi stock tires are a nightmare in snow.
--Post or pre-Turbo upgrade: RA Gear set (WRX) and Clutch (300 whp for WRX, 400 WHP for STi).
--Springs (linear or non-linear, depending on preference).
--Harder bushing: PITA but if you race, will be needed.
--Tranny and Engine Mounts (WRX only)
--Rear Sway: hollow construction, adjustable
--End Links: An aftermarket rear sway will break the stock rear sways (yes….even the STi)
--Coilovers: Adjustable are preferred, since you are spending the money, go for the best.
--Top hats (to go with the coilovers)
--Frame supports (interior and exterior, including A bars, Strut tower bars, roll bars, harness bars, etc.
--Weight reduction. Toss out the back seat, stereo, AC, crap in trunk.
Please note: There is no mention of a BOV anywhere here. If you must get one, get one that is adjustable to allow 100% recirc when you intend on getting 100% of the performance from your car. There are aftermarket intakes out there that do not affect the Impreza ECU, but most will. None, to the best of my knowledge, increase it’s performance. However, for a FMIC an intake is needed so choose wisely and be sure it will work with our sensor and ECU. Grounding harnesses are very good theory, but very hard to demonstrate that they actually do anything (unless you have a very high end stereo or are running a computer in your vehicle somewhere). For the money, just make your own, but don’t spend over 100 bucks on grounding wire. It’s just money down the drain. Higher octane fuel will always be a good idea to give a boost in performance, but without specific tuning, your car will not be able to truly take advantage of it. Octane boosters are mostly junk. Depends on the percentage of acetone or toluene in them.