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.

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.

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