Car Culture, Featured, Modified & Tuning

Subaru WRX STI Modification Guide

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 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

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

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.

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

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.


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.


Car Technology, Featured, Modified & Tuning

Five Steps to Making More Power in Your Subaru WRX

More Mods For Our Project WRX

The Subaru Impreza WRX is a performance bargain: A quarter-mile time of 13.5 seconds at 101 mph is as quick as any WRX STI we’ve tested, and the standard WRX model is about 10 grand cheaper. Being power hungry, we have investigated a number of claims that even more output—over the stock 265 horsepower and 244 pound-feet of torque—is available with a few simple modifications. As we found, your results may vary. Here’s how we got that extra power:

STEP 1:The Intake Trick 

Getting more air into the engine’s combustion chambers produces more power, right? Right. After a simple installation, the low-restriction conical air filter and straight intake tube of the K&N Typhoon kit gave us a 17-hp increase at the wheels and a new jolt of torque worth 21 pound-feet. The intake carries its own million-mile warranty and doesn’t void the factory warranty. The downside: It doesn’t meet California emissions regs, and the increased intake noise was hated by the old folks in our office.

RELATED: Subaru’s 15 Fastest Cars of All Time


STEP 2: The Exhaust Tactic 

We were feeling pretty smart after installing a Magnaflow exhaust. All it took was a few bolts and clamps, and the WRX was putting out a pleasant burble at idle without being too loud. Power was unchanged, but torque increased by 10 pound-feet. Like the intake, the stainless-steel exhaust is guaranteed for life and won’t void the Subaru warranty.


STEP 3: But They Don’t Add Up

We put our project car on a dynamometer with the K&N Typhoon intake and the Magnaflow exhaust (the numbers mentioned earlier were achieved with each performance piece installed solo on the WRX). With both installed, horsepower was up by 11 over stock and torque was up 20 pound-feet—increases that are less than those produced by the modified intake alone. The extra airflow on both ends was apparently too much for the factory ECU to cope with, and it compensated by cutting power right when the turbo reached maximum boost. Either the intake or exhaust works fine alone, but ECU modifications are required to get them to work in tandem.

RELATED: 10 of The Rarest Subarus Ever


STEP 4: Let’s Get Serious

So, to get even more power, we had the engine reprogrammed. Remember, that voids your powertrain warranty. We called Cobb Tuning for its AccessPort, which can completely revise the engine software. And to further free up the exhaust, we threw in a Cobb downpipe with a high-flow catalytic converter. The AccessPort connects to the OBD II plug and automatically saves the factory engine programming. It also has preset engine maps for various states of tune, but none that accounts for an aftermarket intake. Cobb says there are too many variables to have a single program, so we got a $460 custom engine tune. After that, wheel horsepower was up 57 to 266, torque increased a staggering 68 pound-feet to 293, and now the engine sounds meaner than Sasquatch’s wife.


BONUS STEP: Getting a Grip

We installed a set of higher-performance Bridgestone Potenza RE-11 tires, swapping out the stock Dunlop SP Sport 01s. Skidpad grip increased from 0.84 g to 0.87 g, with a noticeable reduction in the understeer that plagues the stock WRX.


On The Dyno: Stock vs. Tuned

The dyno tests, done at Speed Industry in Troy, Michigan  report horsepower and torque measured at the wheels. Correcting for driveline losses, a conservative estimate is that crank horsepower has been increased by 65, over the stock figure of 265, for a total of 330. The power boost shows up most noticeably at higher speeds. Our souped-up WRX was 0.3 second quicker to 60, at 4.4 seconds, and 0.4 second swifter through the quarter-mile, at 13.1. But the advantage at 100 mph is 1.3 seconds, and the gap gets wider all the way to the 142-mph top speed. So for less than $4000, high-speed acceleration has improved enough to shame any stock WRX STI.


Waiter, Check Please!

K&N Typhoon kit………………………………………………………… $322*
Magnaflow exhaust…………………………………………………… $774*
Cobb Tuning AccessPort…………………………………………… $695
Cobb Tuning downpipe……………………………………………… $595
Custom engine tune…………………………………………………… $460
Bridgestone Potenza RE-11 tires………………………………… $732
Total…………………………………………………………………….. $3578
*Suggested price, actual retail price may differ.