The Agency Power WRX headers have become a standout upgrade for Subaru enthusiasts seeking measurable power gains without sacrificing drivability or emissions compliance. Designed specifically for the turbocharged boxer engine, these headers replace the restrictive factory manifolds with equal-length primaries that improve exhaust scavenging and reduce turbulence. Recent dyno testing has confirmed gains of up to 15 horsepower at the wheels when paired with the stock catalytic converters, making this one of the simplest bolt-on modifications available for the WRX. This expanded guide covers the dyno results in detail, the engineering behind the headers, compatibility across generations, installation and tuning considerations, and real-world driving impressions.

Dyno Testing Methodology and Results

Baseline and Header Swap Procedure

The dyno tests were conducted on a 2018 Subaru WRX (VA chassis) equipped with a stage 1 tune as the only other modification. The vehicle was run on a Mustang AWD dynamometer in controlled ambient conditions – 72°F, low humidity, and with consistent cooling fan placement. After establishing a baseline with the factory exhaust manifolds and stock catalytic converters, the Agency Power headers were installed using the same gaskets and hardware supplied in the box. No other changes were made to the exhaust system or engine management between runs. Multiple pulls were performed to ensure repeatability, and the best runs were selected for comparison.

Power and Torque Curves Analysis

The dyno charts revealed that the Agency Power headers shifted the entire powerband upward. Peak horsepower increased from 268 whp to 283 whp – a net gain of 15 hp. More importantly, peak torque rose by 12 lb-ft, from 298 to 310 lb-ft, and the torque curve showed a broader plateau between 3,500 and 5,500 rpm. The most significant gains occurred in the mid-range, where the factory headers tend to choke flow. At 4,200 rpm, the Agency Power headers delivered an additional 18 hp compared to the stock units. These gains are attributed to the equal-length design, which promotes even exhaust pulses and reduces reversion that can hinder turbo spool.

Comparison to Other Header Options

For context, competitor headers such as the Invidia equal-length set and the Cobb catted downpipe with factory manifolds typically yield 10–12 hp gains on similar setups. The Agency Power headers outperformed these alternatives in the mid-range while maintaining identical top-end numbers. When paired with an aftermarket high-flow cat downpipe, the headers have been dyno-verified to produce over 20 hp gains, but the focus here remains on the stock-cat configuration for those who must pass strict emissions inspections. A full comparison chart (available on Agency Power's official site) shows the torque curve improvement is notably smoother under the curve.

Technical Design and Construction

Materials and Fabrication Quality

Agency Power constructs these headers from T304 stainless steel, chosen for its corrosion resistance and ability to withstand the high temperatures of a turbocharged exhaust system. All bends are mandrel-formed to maintain consistent inner diameter and prevent flow restrictions. The header flanges are laser-cut from 3/8-inch stainless plate and are CNC-machined for a flat sealing surface. The welding is performed by certified TIG welders, with full penetration on all joints to prevent cracking under thermal cycling. The headers come with a satin finish but can be ceramic-coated for additional heat retention in the engine bay.

Primary Tube Diameter and Length

The primaries are 1.625 inches in outer diameter, a size that balances flow capacity with exhaust gas velocity for the EJ20 and FA20 turbo engines. The equal-length runners measure approximately 28 inches from flange to collector, which is tuned to scavenge effectively in the 3,000–6,000 rpm range where most daily driving occurs. The collector merges into a single 2.5-inch outlet, allowing a direct bolt-up to the factory downpipe or an aftermarket unit. This design reduces backpressure while maintaining torque for street driving—a common compromise that many aftermarket headers get wrong.

Flanges, Gaskets, and Hardware

Each header includes a thick multi-layer steel (MLS) gasket pre-cut to match the cylinder head ports. The use of MLS gaskets eliminates the need for re-torquing after heat cycles, a known issue with cheaper composite gaskets. The supplied hardware is grade 10.9 alloy steel studs and nuts, zinc-plated for corrosion resistance. Agency Power also provides stainless steel spring bolts for the downpipe connection to allow for thermal expansion without loosening.

Heat Management Options

Headers generate significant under-hood heat, which can affect intake air temperatures and intercooler efficiency. Agency Power offers a ceramic coating option (either titanium or black) that reduces radiant heat by up to 50%. For those on a budget, header wrapping with DEI titanium wrap is a popular alternative, though care must be taken to avoid moisture trapping. The bare stainless headers are still acceptable for most applications, but coated versions are recommended for track use or hot climates.

Performance Benefits in Detail

Exhaust Flow and Backpressure Reduction

The factory WRX headers use a cast-iron log design with unequal-length primaries that create a characteristic Subaru rumble but also introduce turbulence and backpressure. The Agency Power equal-length headers smooth out exhaust pulses, reducing pumping losses. Computational fluid dynamics (CFD) analysis published by Agency Power shows a 22% reduction in backpressure at peak flow compared to stock. The result is that the turbocharger does not have to fight against exhaust restriction, allowing the turbine to spool more freely and maintain boost longer into the rev range.

Throttle Response and Turbo Spool

Many owners report that the throttle feels noticeably sharper after installation. The improved scavenging means that the exhaust gases exit the cylinders more completely, which helps the next intake charge enter cleanly. On the dyno, the turbo reached full boost 200–300 rpm earlier than with the stock manifolds. This translates to a more immediate surge of power when the driver steps on the accelerator, especially when coming out of corners or passing on the highway. The effect is most pronounced in the 3,000–4,500 rpm range where the stock setup tends to feel lethargic.

Weight Savings

The factory exhaust manifolds, including heat shields, weigh approximately 18 pounds combined. The Agency Power headers, with no heat shields, tip the scales at 11 pounds for the pair. That 7-pound reduction is not massive on its own, but it is unsprung and removed from the front of the car, contributing to slightly better front-end response and overall weight distribution. Every pound counts in a vehicle that is constantly being optimized for performance.

Compatibility Across WRX Generations

2002–2007 GD Chassis (EJ20/EJ257 engines)

Agency Power offers two versions for the GD chassis: one for the 2.0-liter EJ205 and one for the 2.5-liter EJ257/255. The flange orientation and primary length differ to accommodate the different cylinder head ports and turbo positions. All GD models use a single-scroll turbo, and the headers bolt directly to the factory downpipe. Owners should note that the 2002–2005 models may require slight trimming of the under-engine splash shield for clearance.

2008–2014 GR/GV Chassis (EJ255/EJ257)

Headers for the GR/GV generation share the same engine family as the later GD models but have different engine bay packaging. The Agency Power headers for these years feature a repositioned oxygen sensor bung to clear the front crossmember. They are compatible with both the 2008–2012 narrow-body and 2013–2014 wide-body WRX/STI. No additional modifications are needed for fitment, though the stock heatshields cannot be reused.

2015–2021 VA Chassis (FA20 engine)

This is the most popular application and the one tested in the dyno results above. The FA20 engine uses a twin-scroll turbocharger, so the header design includes twin-wall primaries to separate exhaust pulses from cylinders 1 and 4 versus 2 and 3. Agency Power specifically tuned the runner lengths to match the FA20’s firing order. The headers include a built-in EGR block-off plate and provisions for the factory wideband O2 sensor. For 2018+ models with the direct injection only (no port injection), the headers maintain compatibility with the OEM downpipe without triggering check engine lights.

2022+ VB Chassis (FA24 engine)

As of early 2025, Agency Power has not released a specific header for the VB generation, but they have indicated in forum posts that development is underway. The FA24 engine has a larger displacement and different exhaust port shape, so direct swaps from the VA headers are not possible. Enthusiasts should monitor Subaru's official channels and aftermarket forums for updates.

Installation Considerations

Tools and Difficulty Level

Installing headers on a modern WRX is an intermediate-level job, requiring basic hand tools, a torque wrench, and penetrating oil for rusted bolts. The process involves removing the under-tray, unbolting the downpipe from the turbo, disconnecting the oxygen sensors, and then removing the factory manifolds from the cylinder heads. The new headers are installed in reverse order, with new gaskets and hardware. Typical installation time ranges from 3 to 6 hours for a first-timer with a lift.

Professional vs DIY

While the installation is straightforward for someone with mechanical experience, there are several pitfalls that can make professional installation advisable. The manifold to head studs are prone to snapping if over-torqued or if the vehicle is from a salt-belt region. Additionally, the oxygen sensor bungs on aftermarket headers are sometimes placed at awkward angles, requiring careful routing of the sensor wiring to avoid contact with the downpipe. A trusted shop will have the tools and experience to handle these issues quickly.

Common Pitfalls

  • O2 sensor wiring clearance: The aftermarket header’s primary tube may push the sensor harness closer to the exhaust than the factory piece. Use zip ties or heat-resistant loom to keep wiring away from hot surfaces.
  • Heat shield interference: The stock heat shields will not fit over the Agency Power headers. If under-hood temperatures are a concern, install the optional ceramic coating or add aftermarket wraps.
  • Turbo studs: The downpipe-to-turbo studs are known to seize. Apply anti-seize compound during assembly to ease future removal.

Tuning Requirements

Without Tune – Safe?

Installing the Agency Power headers without a tune is safe for the engine, but it will not yield the full power gains. The factory ECU’s fuel and ignition maps are calibrated for the stock exhaust backpressure. With reduced backpressure, the engine may run slightly leaner at wide-open throttle, which is why many tuners recommend a conservative base map even for a simple header swap. For daily driving, the stock tune will adapt within safe limits, but the check engine light may come on if the downstream O2 sensor detects a change in catalyst efficiency.

With Stock Cats vs High-Flow Cats

The 15 hp figure was achieved with the stock catalytic converters. If the owner later upgrades to a high-flow cat downpipe, the gains can increase to 20–22 hp with a tune. However, the stock cats are effective at maintaining near-stock exhaust noise and emissions compliance. For those in states with visual inspections, retaining the factory cats is often the only way to pass. For everyone else, a high-flow cat paired with an off-the-shelf tune from Cobb or a pro-tune from a reputable shop like Tuning Tech FS can unlock additional power.

The most common tuning solution for the VA chassis WRX is the Cobb Accessport, which offers pre-loaded maps for header installations. These base maps are safe and provide a noticeable increase in throttle response and peak power. For maximum gains, a custom dyno tune is recommended, especially if the car has other modifications like an aftermarket intercooler or turbo inlet. Open-source tuning via software like RomRaider is also possible but requires more technical skill.

Real-World Driving Impressions

Acceleration and Seat-of-Pants Feel

Owners consistently report that the car feels snappier from a stop and pulls harder in the mid-range. The earlier spool of the turbo means that passing maneuvers on the highway require less downshift. The gain of 15 hp may not sound immense on paper, but the accompanying torque increase and the shape of the curve make the car feel significantly faster in everyday driving. On the track, the improved throttle response allows drivers to carry more speed through corners by getting back on the gas sooner.

Exhaust Note Changes

Switching from unequal-length to equal-length headers changes the exhaust sound dramatically. The classic Subaru rumble becomes a more even, smoother tone—often compared to a Honda inline-4 or a Euro turbo engine. Some enthusiasts miss the burble, but many appreciate the cleaner, more refined sound. The cabin drone is also reduced at cruising speeds because the exhaust pulses are more uniform. When paired with the stock mufflers, the system is only slightly louder than stock at idle, with a pronounced growl under hard acceleration.

Daily Drivability

The headers do not increase cabin heat noticeably as long as the heat management options are used. Cold start behavior remains the same, and the car idles normally after the learning period. The only potential downside is a faint smell of exhaust from the engine bay if there is a minor leak at the head flange—something that can be avoided with proper installation. Overall, the headers are a true “bolt-on and forget” modification that does not compromise daily comfort.

Agency Power vs Competitors

Comparison with Invidia, Cobb, and Killer B Motorsport

The Agency Power headers hold their own against the competition. Invidia’s equal-length headers are priced similarly but use a 1.75-inch primary, which can hurt low-end torque on the smaller EJ engines. Cobb’s header is discontinued but still popular used; it uses a 1.625-inch primary like Agency Power but the collector design is less refined, leading to a peakier powerband. Killer B Motorsport headers are considered the gold standard for outright performance, but they cost nearly twice as much and require a custom downpipe. For the price, Agency Power offers the best balance of power, fitment, and value.

Price vs Performance

At roughly $450–$550 retail (depending on coating and configuration), Agency Power headers deliver the best horsepower-per-dollar ratio in the WRX aftermarket. The 15 hp gain equates to about $30–$37 per horsepower, which is significantly cheaper than a tune or a downpipe upgrade alone. Furthermore, the headers are a foundation for future modifications, so the investment only grows in value as other parts are added.

Customer Feedback and Long-Term Reliability

Over a decade of production, Agency Power headers have amassed a strong reputation on forums like NASIOC and WRXClub. Common praises include perfect fitment out of the box, consistent welding quality, and no cracking issues even after years of use. The primary complaint is that the supplied hardware (nuts and studs) can be made of softer metal than ideal; some owners prefer to reuse the factory studs or buy ARP header studs for peace of mind. Nonetheless, the headers themselves have proven durable enough for daily driving, autocross, and track days. Customer testimonials frequently mention that the headers are “the best mod I’ve done to my WRX” for the combination of power and sound.

Conclusion

The Agency Power WRX headers are a proven, cost-effective upgrade that delivers up to 15 horsepower at the wheels with stock catalytic converters. The dyno results show real-world gains in both peak output and drivability, while the equal-length design improves turbine spool and throttle response. With robust stainless steel construction, precise fitment across multiple generations, and straightforward installation, these headers are an excellent choice for any WRX owner looking to extract more performance from the turbocharged boxer engine. Whether you retain the factory cats for emissions compliance or pair the headers with a full aftermarket exhaust and tune, the Agency Power headers represent a solid foundation for building a faster, more responsive Subaru.