If you're looking to squeeze more power out of your FA20 engine, upgrading your 15 wrx headers is one of the most effective ways to do it. When the 2015 model year first hit the streets, it was a massive departure from the old EJ-powered cars we'd all grown used to. One of the biggest changes was the move to a front-mounted, twin-scroll turbocharger, which completely changed how the exhaust manifold—or headers—needed to function.
Honestly, the stock exhaust manifold on the 2015-2021 WRX isn't terrible, but it's definitely built for mass production and cost-saving rather than peak performance. It's heavy, a bit restrictive, and doesn't exactly let the engine breathe the way it wants to. If you're chasing faster spool times and a broader power band, the headers are usually the first place you should look after you've already handled the intake and downpipe (or J-pipe, in our case).
Why Upgrade Your Headers Anyway?
You might be wondering if it's actually worth the money to swap out the factory pieces. After all, a set of quality 15 wrx headers isn't exactly cheap. But here's the thing: the factory manifold is made of thick, cast components that hold a lot of heat and have somewhat turbulent internal transitions.
When you switch to a high-quality aftermarket header, you're usually getting smooth, mandrel-bent stainless steel tubing. This reduces backpressure significantly. Because the 2015 WRX uses a twin-scroll turbo, the header's job is to keep exhaust pulses separate until they hit the turbo housing. A well-designed aftermarket unit does this much more efficiently than the stock part, leading to better "scavenging." In plain English, that just means it pulls the exhaust out of the cylinder faster, which lets more fresh air and fuel in for the next combustion cycle.
Most people notice that their turbo starts making boost a few hundred RPM earlier. On a car that already has a pretty responsive turbo, that extra "oomph" down low makes a huge difference in daily driving. You aren't just gaining peak horsepower at redline; you're gaining usable torque everywhere.
The Big Debate: Equal Length vs. Unequal Length
If you've spent any time on the forums, you know this is the hill some Subaru fans are willing to die on. The "Subie Rumble" that everyone loves is a product of unequal length (UEL) headers. However, from the factory, the 15 wrx headers are equal length (EL). This was a deliberate choice by Subaru to optimize the twin-scroll turbo setup.
Now, some companies do sell UEL headers for the 2015+ WRX. If you absolutely must have that old-school growl, you can get it. But you have to be aware of the trade-offs. Using UEL headers on an FA20 engine usually results in a loss of efficiency. You might see slower spool times and slightly higher exhaust gas temperatures (EGTs) in certain cylinders.
Most performance-oriented owners stick with equal length. You still get a much deeper, more aggressive tone than stock, but you keep the performance benefits that the FA20 was designed for. It sounds more like a high-strung four-cylinder race car and less like a tractor, which some people actually prefer once they hear it in person.
Material Choice and Durability
When you're shopping for 15 wrx headers, you'll mostly see 304 stainless steel. This is the gold standard for a reason. It handles heat cycles well and resists corrosion, which is a big deal if you live somewhere where they salt the roads in the winter.
Some cheaper options use 409 stainless, which is okay, but it'll develop surface rust pretty quickly. It's not necessarily going to fail, but it won't look pretty for long. Then you have the high-end stuff made from thick-wall tubing or even Inconel, though that's usually overkill unless you're building a dedicated track monster.
One thing to look for is the thickness of the flanges. Thin flanges tend to warp under the intense heat of the turbo, which leads to exhaust leaks. And if there's one thing you don't want, it's an exhaust leak right before the turbo. It'll kill your performance and make the car sound like a broken lawnmower. Look for headers with at least 1/2-inch thick flanges to ensure a solid seal.
Heat Management: Wraps and Coatings
Since the 15 wrx headers sit right at the bottom of the engine, they're close to a lot of sensitive components. They also hold a ton of heat. Many enthusiasts choose to either ceramic coat their headers or wrap them in exhaust tape.
Ceramic coating is the "cleaner" way to do it. It looks professional and does a great job of keeping the heat inside the tubes. This keeps the engine bay cooler and keeps the exhaust gas velocity high. Heat-wrapping is often cheaper and very effective, but it can be a mess if you have an oil leak. Oil-soaked wrap is a fire hazard, so if you go that route, make sure your engine is bone-dry.
Keeping that heat in isn't just about protecting your plastic splash guards; it actually helps the turbo work better. Hotter air moves faster, and faster air means a faster-spinning turbine.
Installation Realities
I'm not going to sugarcoat it: installing 15 wrx headers can be a bit of a pain if you're doing it on jack stands in your driveway. It's not that the process is overly complicated, but space is tight. You're going to be reaching for bolts you can barely see, and you'll likely lose a little skin on your knuckles.
One of the biggest tips I can give is to buy brand-new OEM gaskets. Don't try to reuse the old ones, and don't always trust the cheap gaskets that sometimes come in the box with aftermarket headers. Subaru's multi-layer steel gaskets are top-tier and almost never leak if the surfaces are clean.
Also, be prepared for the "stud" situation. Sometimes the studs that hold the header to the engine block or the turbo come out with the nut, or worse, they snap. Having a stud removal tool and a few spare Subaru studs on hand before you start will save you a frantic trip to the dealership in a friend's car mid-install.
Supporting Mods and the All-Important Tune
You can't just bolt on a set of 15 wrx headers and call it a day. Well, you can, but you're leaving a lot on the table and potentially risking your engine's health. Any time you significantly change how the engine breathes, you need to update the ECU.
A proper tune will account for the increased airflow and the change in backpressure. Without it, you might run lean or experience "overboost" issues because the turbo is suddenly much more efficient than the factory computer expects. Most people combine a header upgrade with a J-pipe and a professional dyno tune (or a solid e-tune). This is where you see those 20-30 horsepower gains that make the upgrade feel worth every penny.
Final Thoughts on the Investment
At the end of the day, upgrading your 15 wrx headers is about more than just a number on a dyno sheet. It's about how the car feels when you're pulling out of a corner or merging onto the highway. The FA20 is a great engine, but it feels a little "stifled" from the factory. Opening up the exhaust path with a quality set of headers lets the car wake up.
Whether you're going for the loudest setup possible or just want a smooth, reliable daily driver with a bit more kick, the headers are a foundational part of the build. Just do yourself a favor: do the research, buy a reputable brand with thick flanges, and for the love of all things boost-related, get a tune once they're installed. Your WRX will definitely thank you for it.