ok this serves as a rough outline of good modifications steps that people who are new to modding should take note of...here we go..yes its a long read....ignore the fact that im talking about the old wrx motor, it has good info regarding the new wrx motor and the sti motor as well...
WRX mod guide
Going Faster - Engine and Drivetrain modifications
Here is your complete guide to getting more horsepower from your WRX motor. These steps are best followed in order, although they don't have to be. Have you got a NON-turbo late-model Impreza? See our Non-Turbo Guide.
The Subaru EJ20 turbo 2.0-liter motor is an amazing high-tech power plant with a huge capacity for increased performance. Straight out of the box, it generates 227 horsepower, but simple bolt-on modifications can increase that to over 400!
To unleash the increased power from your turbo-charged motor, you need to allow the turbocharger to breath more freely, on both the intake and exhaust sides of the motor. Generally, you should start from the ends of the system and work inward. When you have opened up the intake and exhaust as much as you can, you can switch to a larger turbocharger for even more power. Then there are additional ways to add power to the motor, including electronic add-ons. Note that none of these modifications involve taking apart the motor itself ?most of these add-ons can be put on by an amateur mechanic.
All horsepower figures in this catalog are measured at the crankshaft (as opposed to at the wheels) unless otherwise noted. Horsepower figures we quote here are approximate; as always, our mileage may vary.
Here we bolt on the simple modifications. These are generally easy to install and get some quick horsepower, but they don't change the basic nature of the car. It remains civil and easy to live with; it just has 30% more horsepower.
The key to getting horsepower from the turbocharged motor is to increase boost pressure produced by the turbocharger. More pressure means more air and fuel can be stuffed into the cylinder on each stroke, and more fuel in the cylinder means more power produced.
But too much boost can be bad. At some point the fuel system will not be able to deliver enough fuel to match the air that is being forced into the cylinder. The fuel-air mixture will become too lean, and will burn very hot. Those extremely hot temperatures can severely damage pistons and valves, and can destroy your engine.
This is not much of a problem early on in the modification process, but to be safe we urge you to install a boost gauge (if you don't already have one) as the first modification to your WRX.
The boost gauge can be installed in a number of locations, but the A-pillar (the one that connects the dash to the roof) or the center dash are great locations.
High Flow Intake
This used to be an easy way for owners of turbocharged cars to add gobs of power. Just slap an open-element air filter on a turbo Eclipse, for example, and you get an instant 10 or 15 horsepower. Technology advances, though, and the newest factory-built intakes are much better. Horsepower gains from intake systems on the WRX are smaller, but there still are a few horsepower (maybe 5) to be gained, and as other modifications increase, the added power will go up as well.
Rear Section or Catback Exhaust
The stock exhaust system uses small-diameter tubing and a fairly restrictive muffler in the effort to combat noise. We hard-core performance buffs actually like that trademark Subaru boxer motor exhaust thump, so it's an added bonus when you can bolt on 10 horsepower and get a nicer sound at the same time.
The exhaust system is built in multiple sections. The smallest section you can replace extends from about the rear axle to the outlet. (Blitz and TurboXS make systems like this.) You can also get "cat-back" systems, which add a rear-mid pipe to extend all the way to the rear catalytic converter.
Downpipe and catalytic converter
The front two sections of the turbo-back exhaust are the downpipe and the front-mid pipe.
The downpipe bolts directly to the turbocharger outlet, and collects exhaust gas from the turbine wheel as well as the wastegate. The wastegate outlet part of the downpipe has one of the worst layouts ever in exhaust-dom, with the wastegate gas running directly into a flat plate. That gas has to somehow find its way over to the main section of the downpipe. It's obviously not a high-flow layout, so upgrading this pipe is great for performance, as well as for preventing "boost creep" (uncontrolled increases in boost) from poor wastegate flow.
The factory downpipe has a catalytic converter in it. Some aftermarket downpipes also do. Some don't. A cat-less downpipe will generally out-flow one with a catalytic converter in it.
The front-mid pipe has a catalytic converter after it. Many manufacturers offer larger-diameter replacement pipes with higher-flowing catalytic converter sections, or with no catalytic converter. It's good to keep at least one cat in the systems for a lot of reasons, one of which is good citizenship and keeping the air clean. Another is that there are two oxygen sensors in the exhaust system, and having a cat between them keeps the ECU happy and not throwing "Check Engine" lights.
Of course, it is against federal law to remove any working catalytic converter from the car.
Boost Controller or ECU upgrade
At this point we add power by increasing the turbo boost pressure. There are two ways of doing this, both of which have some tradeoffs.
The "easy" way is with a boost controller. You basically remove turbo boost control from the car, and put it in the hands of a mechanical or eletronic device that YOU set, so you can specify whatever boost pressure you want. The benefits of this are complete control, low price, and ease of install and use.
The downsides are...well, possible destruction of your motor if you aren't careful. Sending too much pressurized air into the motor, especially at high rpms, can have very serious consequences.
The other approach is to get a whole ECU upgrade. This will increase boost levels, as well as change a whole bunch of other stuff, including timing maps, rev limits, speed governers (bye bye!), and more. The benefits are that you get a known-quantity ECU program with proven results, and you get the most horsepower you'll probably be able to make with a certain set of modifications on the car.
The downsides of this choice are lack of control (many ECU mods don't allow you to adjust the boost), and large to VERY large cost. Few ECU modifications cost under $1,000.
These modifications start to carry the car away from your average street-driver, into the realm of the street/track warrior. More horsepower awaits!
Header(Exhaust manifold) and Up Pipe
The factory manifold and up pipe are another source of restriction in the flow of exhaust out of the car. The manifold is the stuff that gets exhaust gas out of the heads, and most of the way to the turbo. The up pipe is the last piece of tube that consolidates all the exhaust gas from the four manifold tubes and carries it into the turbine housing.
Tubular header manifolds are very expensive to build. There are many bends, and the tolerances are tight. For this reason, the header upgrade is a pretty expensive way to make power.
The up pipe is not very long, but it does contain yet another catalytic converter. If one were (not legally, of course) to remove this cat, one could gain some more horsepower. Theoretically.
The factory turbo has limited air flow capability at high rpms, and high boost at high rpms is how you make lots of horsepower! There are many turbo choices you can make to solve the high-end wheeze. Most of them are IHI products, and bolt directly onto the factory intake and exhaust piping.
Fuel Injectors (or a complete fuel system)
The factory injectors probably won't be able to deliver once you bolt on the bigger turbo, so you'll want to thread in some larger ones.
The stock fuel pump seems to be able to do quite well even with high horsepower demands, so that can stay in place for the time being.
Front Mount Intercooler
For ideal intake charge-cooling, the top-mount intercooler just doesn't cut it. Airflow from the hood scoop is limited, and the core size isn't very big. A front-mount intercooler is just what the doctor ordered. It gets a blast of fresh cool air, and we can fit a huge core up there.
There are downsides to the front mount i/c, of course. The much longer hose routings will lead to some additional turbo lag. And if you're planning to use the car on a road race track, you may run into problems with the intercooler blocking (or simply pre-heating) the radiator. Not to mention that installing an intercooler means removing your fog lights, and in most cases carving up the steel core of your front bumper.
Upgraded Flywheel and clutch
Your factory clutch is probably slipping pretty badly by now. You'll need a tougher one, probably with a stiffer pressure plate and a more heat-resistant friction surface.
While you have the tranny off the motor, you might want to pop in a lightweight flywheel, which allows the car to accelerate faster in low gears. There's a little sacrifice in slow-speed driveability, but it's not bad. There's also a little sacrifice in your cash flow, but hey, you knew that, right?