Guys, plagorising myself from RSLC here, but this is a guide i wrote a while ago for RS/GT gen1's
Alot of the info is relevant to all subaru's, the engine stuff is all single turbo related though.
If i find time ill write one for gen2,3 and 4's as ive owned them all, but for now its copy past 
Hope this helps someone out.
Hi Guys, ive been meaning to do this for a while, its a basic overview of how i have built a few RS’s on a budget, with very healthy returns on performance/handling/braking for the money.
Its by no means the yard stick of measurement for RS builds, and in not saying this is how it should be done, its just the way i do it, and how i like the car to be setup.
I will add approx costs to the components to give an average indication of prices
This will be mainly in point form to keep it easy to read
Photos will come
Im making an assumption here that any one reading this has at least some mechanical knowledge/aptitude, or is willing to at least follow instructions.
So ill start where i usually start with a stock RS.
Whenever modifying i have a rule to upgrade the brakes, and suspension before i start chasing power, so when the time comes, the car is ready for it.
Suspension:
One of the best bang for buck mods is going to late model STI suspension, there are many variants, with the pick of the bunch being 04 Inverted monotube Red struts, with STI lowering springs, this will be the most noticeable improvement you will do to the car, budget up to $800 for a set of struts/springs in good condition. You can also go to other branded springs eibach/whiteline for a similar performance result.
HERE is a guide i did a while back to installing them in your RS
Other options include coilovers, but unless its a track car, it may not be financially viable to install on a streeter.
Also not all coilovers are bult the same, and im yet to see anything under 1500ish mark be any good
i use Tein Flex coilovers, with Swift Springs, this is over $2500 worth of suspension, so not cheap at all, but it is great for the track, and street.
having said this the STI suspension is also very useable on the track
Bracing:
Front and rear strut braces, and H brace will harden up the ride a bit, and prevent the chassis from flexing under load, this will help with turn in, and controlled cornering, but tends to also make a few rattles appear, or become more prominent. Most brands are similar for rear, and h brace, but alot of front units are flimsy, i recommend Cusco, if you can afford it/find a bargain, budget up to 600 for all 3.
Swaybars/links Misc handling:
A heavy duty/adjustable rear swaybar is one of those great mods that you notice immediately, i use the Whitleine HD unit that has an effective 18/20/22 adjustability, and run 20 on the street, 22 on the track, this provides good turn in, minimum body roll, and controllable power oversteer. Budget 200 for this
Add to this a Front swaybar(driver style dependant) up to 22mm i find ok, any more then this and understeer becomes an issue. Budget 150-200 for this
Both front and rear swaybars should have good links in use, for the rear use Whiteline HD C links/Drop links, and the front GC8 sti units are great from the factory. Butdget 200 for these
I also change to Alloy lower control arms immediately to lower unsprung weight(plus they look sexy budget around 2-250 for a good set
Brake System:
4pot Subaru brake callipers, budget around $3-400
RDA Slotted Rotors 295mm, budget around $200
Quality Brake pads(personally i use Ferodo DS2500’s, and they blow every other pad ive ever used out of the water, but there expensive) so budget $250 for ferodo’s, or $80-150 for descent bendix/ebc/lucas etc
Braided Brake lines(this eliminates any brake spongyness) budget up to 200 a set
Brake master Cylinder Stopper, this prevents firewall flex when brakes are applied hard, and gives a more direct pedal feel, budget 60-150 brand dependant(although there all the same)
Add to this quality brake fluid, good 5.1 synthetic is nice
This combination will give you outstanding braking and you will be track ready(assuming you have suspension mods), it is however at a price, you can get away with just 4pots, standard rotors, and street pads, and still be doing well, but completing the brake upgrade is noticeably better, as with all of this, its just my opinions on a good build.
If you happen to find a set of rear GDA 2pot brakes with hubs/handbrake etc grab them, they do compliment the 4pots well, and provide a bit better brake bias balance.
Pay upto $500 for rear hubs/handbrake/rotors etc.
if you cant find these, then upgrading the rear pads to something with bite is a good idea
Fuel Delivery/ECU:
Walbro 500HP 255LPH Fuel pump, this is fine for almost all applications, can be had for $150 anywhere, and a MANDATORY upgrade
Most JDM ECU’s are great, but the v2 STI/ V2 WRX RA/ V2 STI-RA variants have a very aggressive tune out of the box, and can deliver the goods up to roughly 190kw atw(dyno dependant etc) Budget $300 and up
The JDM chipped ECU's are again head and shoulders above the factory ones, they are very aggresive, and deliver more go, but worse fuel economy, but the right foot feel is worth it, they also costa bit more.
Parallel rails are a good upgrade for anyone going for more then 1bar of boost(14.7psi), this is a cheap eays mod that guarantees both injector banks have consistent and equal fuel pressure.
Components are:
2 brass t pieces in 3/8 $15
A good fuel pressure regulator $150
EFI hose x2M in 3/8 $10
a barb to replace factory FPR $20
Good info on this mod HERE
Engine:
If you have the stock RS engine, i would either run it into the ground, or upgrade straight away, a few good choices are below, and are best bought in half cuts, ill explain why now:
Basically in a half cut you get a good engine/gearbox/turbo/brakes sometimes/clutch/ecu/steering rack/alloy control arms etc etc, and at a good price, so it saves you money in the long run.
Back to engine choices: the following will bolt into an RS:
RS/GT Leggy engine – Ok engines, not real good heads/breathing etc, but cheap
V1/2 wrx engines - very tough engines that breath well, and deliver the goods
V1/2 STI engines, as above, but forged bottom ends, and better cams
V3/4 wrx engines, a revision of the engine with an open deck block, but again better heads
V3/4 STI engines, as above with great heads and cams,
***V3/4 engines require the inlet manifold to be changed from your RS and put onto the later engine.
Halfcut costs range from $2500-5000 depending on what you get, where and when you get it
If they are available my choice of early engines is the ej20G from a v2 stira, they are a closed deck block, with aggressive cams, and they are tough as nails.
ANY engine you get second hand should have a major service done, this mean timing belt/water pump, crank/cam seals, rear main seal, and check all hoses etc for perishing, and replace where needed. Not doing this could end in tears later.
Turbo:
The stock RS/or Gt comes with a vf8/10/12, these are tiny, and for real fun/power you can bin it fast.
if you have bought a JDM halfcut you usually get a nicely suited turbo with it. turbo guide can be foundHERE
One of the best, and commonly overlooked turbo is the stock TD05, they can handle good boost(ive run one at 25psi, which is admittedly past its efficiency range), but they are cheap, and almost bulletproof.
if your going the oller bearing core, review the IHI turbo's explained, and work out whats best for you, they are all "similair" in performance.
turbo budget is from #300 and up
Clutch/Flywheel
I run lightened flywheels(4.3kg) and absolutely love them, they increase the cars ability to redline faster, however they are not everyones cup of tea, so perhaps test drive a car with one first. Budget around $400
in addition to this, Excedy HD clutch's are great, alot of people like the standard clamp load clutch, but this is style dependant, again, a personal choice. budget around $550
Intercooler:
I highly reccomend a Front Mount, any core in the 270-300x600x75mm size range is an easy fit to the RS.
You can get these on ebay, just jap, and a heap of other places. Ive had big brand name APS/PWR, and the cheap chinese copies are just as good any day of the week for the power levels were talking about here.
i have a preference to go for alloy pipework as it is cheap, dissapates heat quickly(unlike stainless) and looks good, and is easy to keep shiny(if thats what your after).
If your willing to doa half days work, you can custom fit a FMIC no problem at all.
Budget 400 for a FMIC kit inc pipework, and another $100-150 for extra pipes silicone, and Extra 50-70 for welding if needed.
Gearbox/Diff choice:
usually what comes with your halfcut :0
but any 4.44 ratio JDm box after 96/7 if treated properly will last, and they have great ratios for the street.
If your serious, and can afford it, GDB 6 speed gearbox.
in both cases, if you can get your hands on a rear plated(mechanical clutch pack) LSD, you wont look back
Budget 1-1500 for a 5speed box, 3-4000 for a 6 speed, and 4-700 for a plated diff.
Im currently running a GDB 6 speed with a suretrack front diff, viscous centre, and plated 2 way mechanical rear diff, the amount of traction is amazing compared to early 5 speed box, and diff setup's, and well worth it if your willing to spend the money
Steering:
Best best is grab a2.7 rack out of any GC8 half cut, put in new bushes, and tie rod ends if needed, this will provide point and shoot steering with a very friendly lock to lock ratio. budget up to $250 for a rack, and 30-40 for bushes, tie rod ends are $15 each trade
Add to this a nice steering wheel with good to excellent hand grips(personal preference here) i run a momo 300mm unit and love it,
Exhaust: 3inch turbo back witha high flow cat, and descent muffler that keeps the growl, but quietens as much as possible. This is a mandatory upgrade to allow the exhaust to flow, when you start pushing 18psi plus you need flow, and the stock setup is quite restrictive.
Budget 400-1500 depending on style, i prefer Mild steel over stainless due to cost and exhaust note
Extras:
Electronic boost controller, simply put, a great invention, allows you to change boost from in the cabin, personally i think blitz DSBC Spec R are the best on the market, budget $3-600
Aftermarket/late model high Energy Coils, this is a good upgrade as it both replaces the usually tired coils with new/er high energy Coils, its an easy install job taking an hourand a half with a beer in your hand, and will have you covered to run big boost and not have spark break up.
Budget $80-150
Engine Oil Cooler, if your going to do track work, a descent aftermarket oil cooler is a good idea, this will increase oil capacity, and improve the oil life, and keep temperatures where they should be.
I run a nice 8 row cooler with stainless braided lines via a flow driven sandwich plate, and oil capacity is at 5.5L
Budget 200 and up for a kit, and common sense to install it.
Fluids:
Oil;
Engine:i run Fuchs Titan GTO fully synthetic 10-40, and change it every 5000k's and use genuine oil filters, for an everyday street car magnatec works, but i like the fuchs gear budget 35 for oil, $11 for filter
Gearbox: Fuchs 75-90 synthetic, works very very well, and lasts, budget $60
Castrol multitraxx with Nulon G70 will do the job though for half the price
Same fluids in the Diff unless you have a plated diff, then some good 85-140 will do the job fine.
have run redline, and it didnt live up to the hype/price tag for me, some however love it
Coolant: nulan Super Concentrate, once a year, or if needed earlier
Seats:
Any GC8 seats will bolt into an RS, and are a huge upgrade over factory, the bolsters are larger, the seat is deeper, and holds you in the driving position alot better.
You can get a clean set of STI seats for around $300
The Version 1&2 seats are smaller then the later models, and suit people 5'10" and under, anything later then this suits taller/rounder people.
Add to this a racing harness for track/pan events, and you have a cheap et up that will hold you where you need to be, budget ADR 5 point harnessed range from $100 up
Headlights:
The factory RS headlights arent great at lighting the night road, a great upgrade is changing to a HID lights, a good H4 Hi/Lo set can be had for Roughly $100 these days, and the difference in illumination is simply amazing.
Try to stick between 5-6000 heat ran ge, and higher will actually give you less usuable light.
If you can afford 2 sets, get a second set of single beam H3 5000K for the foccies, and you will have great night time/rain visibility
Tyres:
One of the most important parts of any build.
First point: Never Skimp on Tyres, Ever, there the only part of the car on the ground.
i have run many variants, street and track.
On the street i like Kumho KU31's
choice usually comes down to cost, so search around, and find good tyres with good treadwear, temp ratings, and make sure they are 91W or above load rated.
Almost every manufacturere has good top quality stuff, so simply buy the best tyre you can for the money, not the most expensive tyre(reasearch is key here)
