So, you've already gotten a turbo-back exhaust and are tuned for it, what's next? First of all you need to realize that different years have different needs, particularly with the 08+WRX's.
This is more of a generic FAQ, if there are any questions, be sure to post them and I or someone else will do what we can to help.
Year specific info:
Pre-06 WRX's (specifically): If you haven't already, a catless up pipe is a very highly recommended mod. It won't net you much power, but that's not the main purpose of it, the pre-turbo cat in your cars have a tendency to break apart and eat turbos.
09+ WRX's: At a typical protuned "stage 2" setup, you'll be near the max on your injectors. Before you can make much more power, you'll need new ones as well as a new fuel pump. For many owners in the midwest or wherever available, this provides a great excuse to switch over to E-85.
Additionally, the VF52 is known to have boost creep issues. To help combat this, you can opt to use an aftermarket boost controller. A 3 port can often times minimize boost creep at the typical stage 2 setup in conjunction with a ported internal wastegate or an external wastegate setup. An external wastegate will eliminate the issue altogether, but is also the most expensive option. Some users say that if you get a 3 port boost controller and port the stock internal wastegate, you should be pretty good.* Remember, with these mods, you need a tune.
*Note: I'm unaware of anyone doing porting of the stock internal wastegate with an aftermarket boost controller and their results. That statement is based off of personal theory and opinion. Take my opinion with a grain of salt.
08+ WRX's: Typically you'll hear that the stock intake is fine until 350hp or whatever. For 08+ owners, this isn't quite the case, these cars use a different intake style than previous years. This intake is more restrictive. A cold air intake will net some gains as well as an increase in sound. Any cold air intake will need a tune. If you don't want the sound increase, you can use a panel filter without having to worry about a tune.**
Additionally, 08+ WRX's are notorious for bad intercoolers and BPV's. The plastic end tanks on the intercooler can blow off at even stock boost pressures. Remember, these cars have a different intercooler fitment style, so gone are the days of the STi intercooler swap. The BPV's also tend to leak at higher boost pressures.
**Note: Some have stated that a drop in panel filter will change air flow through the MAF sensor, this could cause your car to run lean/rich. I haven't seen any proof of this claim, but it is worth noting.
These mods may not require a tune, but as with all mods, you won't experience the full power benefit without a tune.
Header: An aftermarket header or ported exhaust manifold can help or hurt spool. One important factor is heat retention. The stock cast manifold will hold the heat much better than any of its steel counterparts. Often times what people will do is wrap or coat them to keep the heat held within. As we all know, hot air expands, so the hotter the air that reaches the turbo, the faster it will spool. An important thing to keep in mind with aftermarket headers is that with heat retentive coatings or wraps, metal fatigues faster. Another thing to keep in mind when selecting an aftermarket header is whether or not to stay with unequal length. The UEL header is what gives your Subaru it's trademark rumble; an equal length header will remove the rumble, but keep the majority of the tone. Some owners don't mind it, some will miss the UEL rumble. The benefit of going with an ELH is increased torque, power, and spool. Although marginal at lower power levels, the difference becomes more apparrent when you start to make more power.
Up Pipe: An up pipe can provide a little more power and quicker spool because it is less restrictive than the stock one. This mod is a perfect opportunity to switch over to an EWG up pipe. Even if you don't have the cash to do the EWG right away, you can buy the up pipe with a block off plate. Personally, I love this idea.
Turbo Inlet: A turbo inlet can unlock a little more power by getting rid of the restrictive and often times unreliable stock inlet. The stock inlet is known to crack and break, so replacing it may be considered simply a reliability upgrade. When considering which inlet to get, you must consider that some require TGV deletes. Additionally, if you plan to run a larger turbo with more boost, you may want to look at a solid inlet versus the typical silicone one. There have been occasional reports of collapsing inlets. Some manufacturers have started to put wire in the inlet to combat this issue.
Turbo Porting: Porting and Polishing your turbo will speed up spool and possibly up the power. Results can vary based on who does the porting.
Intercooler: Obviously the enemy of a gasoline engine is heat. One way to minimize heat in the intake system is to use a better flowing and larger intercooler. There are two basic options for intercoolers: front mount and top mount. Stock intercoolers are mounted on top of the engine and are cooled by forcing air through the hood scoop through them. Heat rises, so during stop and go traffic, heat soak is inevitable. There are things that you can do to minimize this on a top mount, but you can't get rid of it completely. A turbo wrap will help keep the majority of heat in the turbo and help keep engine bay temps down. A front mount intercooler will almost completely get rid of heat soak even in the worst stop and go traffic, however, spool may be marginally affected.
Turbo Wraps: As previously stated, a turbo wrap or blanket will help with heat soaking as well as reduce engine bay temps. Colder intake temps means more power.
Lightweight/Aftermarket Crank Pulley: Lightweight crank pulleys can help improve off boost throttle response and reduce power loss. There are down sides to an aftermarket crank pulley, sometimes they will trigger a check engine light due to knock. An important thing to keep in mind with crank pulleys is that if you're going to run a lightweight flywheel, you really shouldn't have one. Only one of the two should be used.
Lightweight/Aftermarket Flywheel: A lightweight flywheel is much like a lightened crank pulley; the whole purpose of it is to reduce rotational mass and thus freeing up power and increasing throttle response. As with a crank pulley, it does not create or add power, per se, but it does reduce drivetrain loss.
Fuel Pump: You won't gain any power from a fuel pump, but if you plan to upgrade injectors in the future, it's never a bad idea.
Suspension: Obviously you won't gain any power from it, but it can make driving more enjoyable. A set of front and rear swaybars and endlinks are a great place to start.
Phenolic Spacers: These help combat heat transfer from the engine block to the TGV's and intake manifold. Simply put, they are used in an attempt to keep intake air temperatures down. Some people swear by them, others are unsure. From my point of view, it's a relatively cheap mod that isn't going to hurt, so why not?
Mods that require a tune:
Never install these mods without tuning for them, detrimental effects WILL ensue.
External Wastegate: An external wastegate can improve spool as well as boost control capabilities. You can either plumb the wastegate pipe back into your downpipe, or vent to the atmosphere. Atmospheric dumping adds a very unique and extremely loud sound to your car when you're in full boost. Some people love it, others hate it. I have heard of some people putting a motorcycle muffler on a VTA EWG.
Injectors: Obviously an engine can't make more power without the fuel needed to make the power. A set of higher flowing injectors may not add power, but they can add headroom in the event that you upgrade your turbo.
TGV Deletes: Although they don't require a tune, they require some sort of engine management to turn off the check engine light that will pop up. This mod will get rid of a lot of the restriction within the intake tract.
Boost Controllers: An aftermarket boost controller can help increase spool and better control the turbo. Typically if you want to go to an external wastegate setup, you should get an aftermarket controller. The most common aftermarket EBCS is a 3 port MAC valve. Another option you may consider is a 4 port solenoid. The difference between the two is that the 4 port will force the wastegate to close as opposed to relying on spring pressure (in an EWG setup). This will allow you to run a much lighter spring and still make the same boost pressure, for example, one may be running 22lbs of boost and only need to use a 3lb wastegate spring. Oftentimes people will be running 15lb or heavier springs. A 4 port boost controller will be able to make quicker adjustments, however it can be more expensive depending on where you get it. I've seen 4 ports being sold for as much as $170, but I paid $40 plus shipping for mine and it included a mounting bracket.
Cold Air Intake: Although you may not need it, a cold air intake system will provide headroom for further mods. Don't expect any added power from an intake system though.
Water/Meth Injection: This cools the intake charge and effectively lowers the knock threshold of whatever fuel you're running in your car. This can be a worthwhile modification; however, any failure in the system can have catastrophic effects. Methanol is quite corrosive, so some tuners really don't like meth injection and others like it quite a bit; be sure to consult your tuner if you are considering meth injection on your car. As with many modifications, this really comes down to weighing out the benefits and detriments in order to make an informed decision.
The stock 2.5L engine isn't the strongest in the world. Many people will say that the limit is somewhere between 350-400whp depending on quality of tune and fuel used. Personally, I wouldn't recommend going above 300whp without having some sort of plan for a built motor. Built motors cost more than you think, especially if you're going to have someone else put it in for you.
The 5 speed tranny in the WRX's is notorious for being unreliable at upgraded power levels. There are many people that maintain that it isn't a matter of how much torque and power you make, but how you drive. If you plan on drag racing, don't expect it to survive beyond 300wtq. It seems to me that the newer tranny's (08+) have been made stronger than previous ones. I am unaware of any real proof to this, so if you modify beyond the 300wtq level, beware.
*Update* Upon further research, I've seen at least one 08+ 5mt fail at stage 2 power, but I've also seen a few that last till 390+whp. Still no clear verdict on how much stronger, but this has put my mind at ease a bit.
The 6 speed tranny in the STi's are more than adequate for 99% of all people. If you're reading this for advice, you don't need to worry about the tranny.
So you've done an assortment of those mods I've listed above, maybe not all, but the important ones: fuel pump, injectors, intercooler. What comes next? Well, first it depends on your car; a WRX owner will have to worry about the tranny where an STi owner won't. Second, you should sort out your suspension if you haven't already done so. How about brakes? Your car needs to be safe before you want it to be going any faster than it goes now. Don't get me wrong, I know we all love the head jolting, mind numbing rush of power that our cars can give us, but you need to be safe about it.
A nice set of brake pads and rotors along with a change in brake fluid should do well for most. Personally, I run D.O.T 5.1 brake fluid, but it is overkill, especially for a car that doesn't see any track time. D.O.T. 3 and 4 will be more than adequate for most people. The H6 rear brake upgrade may be something you want to look into as well. Unless you're tracking the car, don't worry about getting a big brake kit, they're damn expensive and won't really do much.
Obviously one of the best ways to improve the driving experience is to get more feeling through the car to the driver. This means harder, stiffer bushings and more noise, vibration, and harshness (NVH). Some drivers won't appreciate the increase in NVH, but many won't mind it. It also depends on which bushings are replaced. Engine and transmission mounts are likely to be the noisiest mods in this catagory, but they can also be very beneficial. The bushing material also plays a huge part in this. I don't know nearly enough on this topic as I wished I did. I'd like to put together a list of what bushings do what exactly, but I'm not have more to learn. I'll certainly be updating this section as I learn more.
Anti-Lift Kit: An ALK is rather counter-intuitive. Quite simply, it INCREASES the amount of lift and dive the front end of the car sees. This is supposed to be more predictable in the corners as well as increase weight transfer. Increasing weight transfer will help drag racers as well; we've all heard of the term "squat."
Positive Steering Response Kit: As with all OEM bushings, the control arm bushings are made of a soft, forgiving material. The PSRS eliminates side to side movement from the stock units with a much stiffer and more responsive unit. This equates to more responsiveness, control, and stability.
Rear Subframe Lockdown Kit: This eliminates/minimizes flex between the differential and chassis. This can be a huge benefit to autocrossers and drag racers as it will slightly help acceleration and throttle response.
Shifter Bushings: The first part of shifter bushings are the front and rear ones within the car itself. These are often done while upgrading to a short throw shifter. While upgrading the transmission mount, a Positive Shift Kit will stiffen up the mount underneath the transmission; this provides a crisper, cleaner shift.
Steering Rack Bushings: These bushings hold the steering rack in place better than the OEM bushings. OEM ones allow the rack to move around as you turn the wheel, this causes a disconnect between you and the front wheels. Stiffer bushings will give you better steering feel as well as quicker turn in response.
Swaybar Bushings: Almost every aftermarket swaybar will come with its own bushing set, but in the event you want/need to retain a stock swaybar, a stiffer set will help stiffen the car up. The stock bushings have some play in them, getting rid of that play makes the bar more effective.
I really don't know much about suspension choices and am trying to keep this impartial, but recognize that when it comes to suspension, or anything for that matter, you get what you pay for. I would suggest talking to some vendors or maybe even your tuner about it. I know my tuner had his own STi and was very willing to offer suspension advice alongside power. When talking to other forum members, take their experiences with a grain of salt. What one person thinks is acceptable as ride stiffness will break another's back, for this reason, it is a great idea to get a ride-along with another person when considering suspension. Who knows, maybe they'll even let you drive (but don't hold your breath).
Ahhhh, the weak link of the WRX...How we owners hate it. If you're shooting to make anything above 300whp/wtq, you really should be looking into a built tranny or a 6 speed swap. Neither of these options are cheap, but that's the problem with owning a WRX.
The EJ25 isn't a very strong engine as I stated above, a set of forged pistons is enough for most people, but while you're in there, you may want to consider adding more parts for safety/head room. Obviously there are parts that should be replaced whenever you open up the block: gaskets, bearings, timing belt, etc. Some of these you can upgrade. You can also do head porting while the engine is apart. The way Dominic at Maxwell Power Services described it to me is that the reliability and safety is built into the bottom end (crank, rods, pistons, block) and the power is made in the heads (headwork, cams, valves/springs). Obviously some power can be made in the bottom end by lightening up the rotating assembly, but that shouldn't be your primary goal. Unless you're building a dedicated track car that you'll be rebuilding often, don't worry about lightening the rotating assembly too much (If you are building a dedicated track car, why are you reading this? I'm no expert).
*Note: There has been some talk of the 704 EJ25 casting having weak cylinder walls. Some tuners have expressed their worry with these blocks, yet more have theorized that it is more of a tune related problem. It is something worth researching on your own as more information surfaces and certainly is worth discussing with your tuner/engine builder.
The EJ20 is similar to the EJ25 as far as power limitations go. Around 300whp, your reliability will become questionable. This makes sense as the EJ20 must work harder for its size than an EJ25.
Picking a turbo depends on many many factors. There is no "one size fits all" turbo. In order to pick a correct turbo, you must figure out your power goals and how much spool time or lag you're willing to deal with. Additionally your rev limit will factor in. Always consult your tuner before you pick one.
Generally speaking, in order to get more power, you will give up on spool time. There are exceptions to this as the stock VF turbos are relatively slow to spool for their size. It is possible to get a slightly larger turbo that spools faster than stock if you know what to get.
This shouldn't replace your research on the parts you're considering to buy. Always do 4x the research you think you need and always get a professional's opinion. Always consult your tuner for advice on any and all modifications.
Please refer to Unabomber's Manifesto for further information on these mods.
If there is any information you want added to this FAQ, please, don't hesitate to put it in. I don't claim to know everything, but from my research, this is what I've found. If anything is incorrect, please let me know.