Subaru WRX STI Modification Guide

Modified 2004 Subaru WRX STI

The Subaru WRX STi is a fantastic package with solid Brembo brakes, a high-performance suspension, and around 300 turbocharged horsepower being put to the ground with pure all-wheel-drive traction. It is a deadly back road weapon as-is, but there is a lot of room for improvement should you want to take your rally car the the next level.

While better tires will improve any car, overall, the STi is already very well set up for handling and braking. To raise the excitement level, you can just dive right into adding horsepower.

This post is meant to be a generic, easy to understand guide for anyone who is looking into modifying a Subaru STi. We hope this will help save people the hassle of having to dig through online forums just to find the basic information. Look for more modification guides for other types of cars in the future.

Stock STi Info:

Engine HP: 300bhp

Wheel HP: 240-250awhp

Boost: 14.5 PSI

The Easy Stuff

Stock Subaru STI engine bay
Stock Subaru WRX STI Engine Bay

Stage 1: 

This is a reflash of the car’s ECU that will raise the boost pressure a little to give you more horsepower and torque without any physical modifications to the car. For newer Subarus, this is a good way to make your car a little quicker without voiding the warranty.

Cost: $400-800

Engine HP: ~320bhp

Wheel HP: 250-260awhp

Boost: ~16 PSI

Subaru STI stock and aftermarket downpipe comparison
Stock Downpipe (left) vs Aftermarket Downpipe (right)

Stage 2:

The next step is to replace the restrictive stock exhaust with a larger 3-4” exhaust from the turbo to the rear bumper (Turboback Exhaust). You can also add short-ram/cold-air intake at this point. You will need further tuning or an ECU reflash to realize the gains from these modifications, and many kits are sold with hardware and software included.

Stage 2 really helps the engine breathe a lot better. In addition to the car being noticeably louder, you will notice that power no longer seems to fall off after 80mph. Because of the increase in top-end performance, I’d recommend that every Subaru owner modify their car at least to this point. It will be significantly faster than stock, while offering you great value for money spent.

Cost: ~$2000 new, but a lot can be saved if you buy used exhaust parts.

Engine HP: ~350bhp

Wheel HP: 270-280awhp

Boost: ~18 PSI

Subaru WRX STI Front-Mount Intercooler

Stage 2 Plus:

Many people often throw in a few extra modifications to their cars when going stage 2 that can allow for even more power without compromising driveability or reliability. These modifications include larger fuel injectors, a better fuel pump, a front-mount intercooler, equal-length headers, a boost controller, and an external waste gate.

Cost: as much as $3,500 on top of stage 2, but you don’t have to do all of it at once or buy everything brand new. 

Engine HP: 350-400bhp

Wheel HP: 290-330awhp

Boost: 18-24 PSI (22 PSI is the reliable limit for the stock turbo, though)

*Note. You should start thinking about getting a stonger clutch here. 

Subaru WRX STI Hatchback

Stage 2 Plus is sort of the end of where things are simple and cost-effective for Subaru WRX STis. At this point, you are pretty much maxing-out the stock turbo, and things are about to get much more complicated. 

Many folks, including myself, tend to stop here because ~300awhp is a really nice place in an STi. Your car will blow the doors off of any stock, or lightly modded, STis and Evos, but it will also stay reliable. It’s plenty fast, especially on winding back roads — 0-60 in around 4 seconds flat, 1/4 mile in the high 12s, and it’ll touch 160mph easy. 

It’s really all about cost-benefit analysis from here. Do you really need 400 or 500+awhp? Sure it’s a hell of a lot of fun, like having your own private aircraft carrier launch, but there are drawbacks. 

The obvious reliability issue is definitely something to think about, but also consider how it will effect the car’s driveability. If you’re like me, and love ripping rally-style back roads, then adding that much power will probably ruin the experience for you. Conversely, if you just wanna be able to run with motorcycles during open highway pulls, then maybe 500awhp is for you. Just consider everything, because you will have to spend a lot of money to go further. 

The Harder Stuff

Subaru STI Rotated Turbo Kit

Bigger Turbo Time:

The stock turbo maxes out around 350awhp, and is losing top-end boost well before that. If you want a strong 330awhp or more from an STi, you need a bigger turbo.

But here’s the problem: The stock Pistons tend to fail around 400awhp, or even before if you don’t watch out for knock. Consequently, making big numbers with an STi becomes quite an expensive proposition… to the point where it may be smarter just to buy a different type of car that is more apt to handle “big power” from the get-go.

How much horsepower you make depends on the size of the turbocharger you go with. However, the more power you want, the more parts of the car will need to be replacement parts you’ll need to strengthen the engine and drivetrain.

In terms of cost, I’ve been quoted roughly $12,000 for a built motor and bigger turbo setup that would be good for a “reliable” 450awhp. Sure, if you do everything yourself, it can be quite a lot cheaper, but it’ll cost you your time. I, however, really only wanted around 380awhp from a 20G turbo setup, and $12,000 just wasn’t worth an extra 70awhp or so. 

It can get much worse, though. Once you start ordering entire new engines with all forged parts, the total cost can get as absurd as $60,000-$70,000 and beyond. Some people are crazy, and happen to have that sort of money just laying around, I guess. For that kind of money, the sky is the limit on horsepower, and you’re basically at the level of building a full-on race car. Anything “can” be done with an STi, like any car, but for what price?

Alcohol Injection:

Many people use different types of alcohol, commonly methanol, to allow them to push the STi’s boundaries a little further. Basically, alcohol burns a little cooler, thus reducing the risk of knock for cars running around 400awhp on a stock engine. It’s not a bad way to go if you’re really trying to cheat that expensive engine rebuild, but it adds more complexity to the car (ie, it’s just something else to go wrong and have to deal with).

Suspension and Brakes

As stated in the beginning, the STi comes from the factory with incredibly good brakes and a solid suspension. Modifications to the brakes or suspension really depend entirely on what you do with your car.

My own car, at Stage 2 Plus level, is still on the stock suspension but with thicker sway bars front and rear that are set up to reduce understeer a bit. I use my car on real roads, many of which are rally-stage-like back roads with varying road surfaces. Because of the unpredictable nature of the roads I drive on, I need the suspension travel of the stock suspension to keep the car well-composed when things get iffy at speed.

Now, if I auto crossed my STi or took it to track days on a road course, then a lower, stiffer suspension might benefit me. Conversely, if I did rally cross, then I might want to raise the suspension. It all depends on what you feel will benefit your chosen activities with your car.

As for the brakes, like the suspension, the stock Brembos work pretty well even for a car with modifications like mine. I do have slightly better brake pads, but otherwise my brakes are totally stock. Now, obviously, if you are going to take the STi’s performance to an entirely different level, then you should upgrade the brakes accordingly.

Blacked out modified Subaru WRX STI Hawkeye

Conclusion

Subaru WRX STis are very solid cars up until a certian point (~350whp). Their sweet spot is right around ~300awhp, where performance is quite thrilling, but reliability is still preserved. It is also pretty cheap and easy to get an STi to the 300awhp level, so it’s accessible for most owners. Doing the basic modifications brings out the Subaru’s full raw character, and is really worthwhile for any owner to do.

More can definitely be gotten from an STi, if you have the wallet, the skills, and the patience to deal with the issues that come up at higher horsepower levels. However, for most people, there are far more cost-effective options out there if a 400whp+ is desired.

So when it comes to modifications, a Subaru STi is a great value until it’s not, and there isn’t much too much middle ground in between. 

Be sure to check out our Volkswagen/Audi Modification Guide as well!

-By Nick Walker

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43 thoughts on “Subaru WRX STI Modification Guide”

  1. Very good basic running guide:). Only thing that you should add, is an upgraded mishimoto radiator, as the stock plastic ones tend to go after about 5 years. Which leads to overheating, and as everyone knows, it’s not the cheapest thing out there to get your head gaskets replaced on a scoob:(

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  2. Thanks alot for the honest article i will start with the basics as you mentioned. I have sti 2015 and wanting to work onit with just basics, am not looking for +400. 350whp is enough for me.. But here in the middle east is hard to find parts, so am planning to buy parts from the USA.. But i have a question, is it worth to lose the warranty?!! As i have 5 years warranty. And what should i worry about if i lose the worranty on the car. Thanks

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    1. At least here in the US, they would have to prove that the aftermarket parts fitted to the car caused a problem before they could void your warranty. To make 350awhp Id recommend going with a 20g turbo setup on a conservative tune rather than straining the stock turbo or a smaller turbo. That should be plenty reliable for you.

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    2. Congratulations on the ’15 STI 😉 I’m sure you’ll enjoy it bro! I have a hatchback ’13 STI and in my opinion, You don’t need to void your warranty, the power level of the STI is enough, you JUST NEED to FEEL it. How? by changing the steering bushings, add strut bar, sway bars, chassis braces, race/street coilovers (if money not an issue). By installing all these parts, you will really FEEL the car, turn better, and put a lot of cars to shame around a corner or the track. THEN after 5 years GO CRAZY with engine mods hahaha
      Just my opinion 😀
      Your bro from KUWAIT!

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    3. Accessports V3 (The handheld ECU tuner) doesn’t leave any trace that it was connected and changed settings on the ECU. So, what I did when I had my STI was, Change the tune back to stock and put the stock intake/exhaust back on. That way they have no idea that the car has been modified. Technically the rule is that the modded part has to be at least part of the cause of the problem on the vehicle. They’re crooks and don’t want to lose money, so they will always say the part played a role in the problem. So, to summarize, if you’re just going to stage 2+ area, whenever you have a problem, throw on the stock intake/exhaust and set the tune back to default. Hope this helps! 🙂

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  3. Does this also count for older Scoobs? 90s models. I am looking to buy a car and turn it into an open track-day racer, and living in Norway, this is rather cheap as long as I don’t register it for the road, because of extreme taxation on imports (10x). So I would look to strip the interior, add racing seats and a cage, and perhaps also get rid of dead weight like the AC.

    However, to cut the costs of both building and running it, I won’t go further than Stage Two + as outlined above. That is more than enough for a beginner. On racing slicks such a car would behave very well on the track, I assume.

    This won’t happen in a couple of years, but I am looking at my options already now.

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    1. This article was written about the 2001+ turbo EJ series Subaru engine. I’m not sure exactly how it translates to older engines, but the steps are likely to be very similar.

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  4. How was the quality of the suspension, brakes, steering response and most importantly differentials (torque steer) on the earlier versions?

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  5. I got a 16 sti wanna do some mods i was thinking of a blow off valve and short ram air intake. But every part for this year car is expensive . Wanna look into stage 1 and 2 .

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    1. The stock downpipe is the biggest restriction on Subarus from the get-go. Honestly at this point I tell people go stage 2 or leave it stock. Anywhere in between is kind of a waste of money.

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  6. I know this article is old, but I must add my experience with front mount intercooler. In short, don’t do it. Mounting an intercooler up front generally requires cutting away pieces of your car. This reduces chassis stiffness and strength. Also, due to the longer piping, the amount of boost the engine actually receives is much less than it would be with a top mount, and lag is greater. Unless you are building strictly a straight line vehicle, and already have a bigger turbo pushing much higher boost, don’t mess with the intercooler. You’d be better off investing in heat shields and turbo wraps.

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    1. All this and add that a Front-Mount IC just advertises to thieves that your car is definitely worth stealing. If your car is south of 400whp, you really don’t need an FMIC.

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  7. Thank you so much fur this post really helped I’m really new to this import thingy got a 02 WRX from the auction for bout $300 automatic and traded my Astro van for my friends 04 STI kinda striped was missing the FMIC core, rims, cluster other little things that could be replaced but still had the motor trans and of course a nice size turbo Garrett M33 ar60 and a Xternal wastegate Tial brand. I’m doing a conversion on the 02 WRX to an 04 STI and I wanna run this turbo did some research on it and it’ll give 520+HP so my question is is that what are the main things I need to upgrade reading the post I’m thinking upgrading my injectors, fuel pump, and headers is there anything else I need to upgrade so I don’t blow my motor?? Plz lmk

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    1. For 520hp without blowing the motor you need to rebuild the motor with all forged internals, at least forged pistons and conn-rods.

      Running alc injection can also help keep combustion temperatures down, but with that much power the motor’s internals really need to be upgraded.

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      1. Thanks for getting back but your saying forged pistons and rods if I get those do I need to do anything to the block like shave so the Pistons to fit or it doesn’t need to be?? also if I get the rods and forged Pistons how do I know what size to buy?? Plz help

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      2. I’d recommend working directly with the shop you’ll have do this because there are many variables. Nutshell is the motor needs to be opened up. There are numerous stock-size drop-in piston options out there. This article is meant to be a basic guide for modding these cars, but there’s a reason I went pretty vague once the first few mods were covered… because the sky, and your wallet, become the only limits.

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  8. Good article. My opinion is a Subaru STI can be upgraded to around 350-360 AWHP reliably and on stock internals. My 2013 STI is just that and tuned by Cobb Performance in Plano, TX.

    In order of importance for a Subaru STI:

    1. Cat-back exhaust (Custom Aluminum Race Exhaust) this exhaust will cost you $$$$
    2. Downpipe (catless DP, Uppipe with 38mm external wastegate)
    3. EBCS ((3-port)
    4. Air intake (Cobb)
    5. Air oil separator (AIG)
    6. Turbo upgrade (20G)
    7. Injectors (1,000cc)
    8. Fuel pump (400lph)
    9. Cobb AP
    10. Protune – important

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