An introduction to the Subaru WRX
The WRX is the model name of a high performance all-wheel-drive Subaru. Refined in the hot fires of rally competition, this four door sedan or wagon makes an unlikely, if highly competent, sports car. It features a turbocharged boxer engine, excellent safety features, high quality construction and a decent interior.
In continuous production since 1992, the Impreza WRX was first brought to the USA with model year 2002. It was an instant success: 18,060 were sold in the first year. American drivers, in particular a generation of young tech-savy gearheads (read: nerds), were buying these cars in large numbers. The reason? The WRX offered the best performance per dollar stock and excellent potential for modification. For a few thousand dollars, the WRX can be transformed into a sportcar with performance rivalling that of the world's most expensive supercars.
The heart of the WRX: the turbocharged boxer engine
The basic model WRX engine is a 2.0 liter design with a claimed output of 227 horsepower. The turbocharger is the Mitsubishi TD04.
The STi WRX features a 2.5 liter engine with an IHI VF39 turbocharger. The claimed output is 300 horsepower.
Both engines respond extremely well to modifications ("mods"). Because they start life as turbocharged motors, they have a relatively low compression ratio. That means that they respond well to running with more intake air pressure or boost. Running with more boost means that more air is being forced into the cylinders. An engine which is ingesting more air needs more fuel and that means better performance.
On a WRX, the factory turbo can be tricked into producing slightly more boost. As well, there are a number of bolt-on modifications which can help the engine "breathe" better. One example of this type of modification is a more free flowing exhaust.
One of the first things many people mistakenly do to a WRX is to swap the airfilter to a high flow version like the one made by K&N. There is no need to do this! In fact there are a number of reasons why this is a bad idea. The stock airbox and airfilter on the WRX is capable of supporting more than 400hp before it becomes a bottleneck. Save your money for beer.
All WRXes require premium gas (91 octane or higher). The Engine Control Unit can adapt to higher octane to produce more power (and better mileage). Because of this, many WRX owners increase their octane using the paint solvent Toluene or with a quality commercial octane booster such a Torco.
Perhaps the best thing about this high-strung engine is the exhaust note. At full power, a WRX sounds a great deal like a WWII fighter. The deep boxer rumble is a welcome contrast to the angry sewing-machine buzz of a Honda or Evo.
Modifying the WRX engine
I'm not a fanboy for the products that I own. I've tried several engine management piggybacks (XEDE, UTEC), standalones (TEC3, Hydra) and am familiar with the AP and ECUTek. I have no vested interest in one system or manufacturer over another.
The first engine mod I suggest to someone on a tight budget is replacing the uppipe. While it is possible to cut and grind the cat from the uppipe, this is generally not worth the hassle. It takes forever, sprays toxic metal dust everywhere and won't ultimately flow as well as an aftermarket uppipe. Uppipes can be had for $170 - $250. The Perrin unit ($199) is probably the nicest of all of them; it's made in the USA of 321 stainless and it's truly a work of art. The cast iron TurboXs uppipe ($175 street) is another good option. I'd go with the Perrin.
After that, you should consider engine management. Though it's possible to get turboback exhaust without engine management, you put yourself at a great deal of risk doing so. We're all computer guys here so there's no reason to be afraid of EM.
The AccessPort from Cobb Tuning is not a bad choice, particularly if you're on a tight budget. There are some concerns with bugs in the software but they seem to be improving. The AccessTuner should be interesting to see and is a welcome addition to the scene.
After that, one should start cleaning up the exhaust system. An exhaust with one high-flow cat does a pretty good job of scrubbing out the exhaust stink, which I presume means that it reduces the emissions reasonably well, at least after the car is warm.
If you have moral issues with increased emissions then don't touch the exhaust. You can still get some performance gains from some form of engine management. Other clean mods you could make are: silicone intercooler hoses and an underdrive pulley. As was previously stated, intakes don't do much until you've got a bigger turbo. Pretty much anything else is going to demand a freer flowing exhaust of some sort. Like I said, as long as you retain one cat, your car won't stink and it won't be very loud.
Once you start modding your car, you need to take responsibility for the fact that you are reducing its overall reliability. Subaru engineers designed your WRX engine to produce 227 HP not 330+. But that's not to say that you can't increase the power output safely.
Modifying WRX handling characteristics
Car makers are often forced to compromise when it comes to setting the handling characteristics of a vehicle. This is especially the case with a car like the Impreza, a 4-door sedan (or saloon as James Montgomery would call it) with offroad and onroad duties, often driven by inexperienced or novice drivers. The WRX is not a pure sports car and the suspension needs a little tweaking to unlock all that the car can offer.