For Subaru WRX enthusiasts, the exhaust system is more than just a pathway for spent gases—it’s the voice of the engine and a direct line to performance gains. The factory exhaust manifolds on the WRX are a compromise of cost, noise regulations, and thermal constraints, leaving power on the table. The HKS Legamax WRX Headers are engineered specifically to address these compromises, offering the coveted combination of a crisp, sporty exhaust note and measurable horsepower increases. Built upon decades of Japanese performance tuning expertise, the HKS Legamax series is a staple for drivers seeking a refined yet aggressive upgrade that respects engineering tolerances.

Understanding the Role of Headers in WRX Performance

In a turbocharged boxer engine like the Subaru FA20 or EJ series, the headers (or exhaust manifolds) perform a critical function beyond simple gas collection. They must manage exhaust pulses to minimize turbulence before the gases reach the turbocharger. Poorly designed manifolds create backpressure and uneven flow, leading to lag (delayed spool-up) and lost power. Aftermarket headers like the HKS Legamax improve exhaust scavenging—the process by which pressure waves help pull fresh air into the cylinders during valve overlap. This is especially important for the horizontally opposed engine layout, where equal-length runners help balance exhaust pulses across both cylinder banks. By optimizing pipe lengths and diameters, HKS engineers reduce restriction while maintaining the necessary velocity to support turbo spool.

Another key aspect is heat management. Factory manifolds are often cast iron or thin-walled steel, which soaks up heat and adds weight. The HKS Legamax headers use a smoother internal finish and thicker, heat-resistant stainless steel to keep exhaust gases hot as they exit the engine—improving turbine efficiency and reducing under-hood temperatures. The result is a system that doesn't just flow better but also preserves exhaust energy for quicker turbo response.

HKS Legamax Design Philosophy and Construction

HKS has been a leader in Japanese performance parts since the 1970s, and their Legamax line reflects a philosophy of balance over brute force. Instead of chasing the loudest possible sound or maximum peak numbers, the Legamax headers prioritize drivability, longevity, and a sophisticated exhaust note that doesn’t drone or rasp.

Premium Stainless Steel Craftsmanship

Each Legamax header is constructed from high-grade T304 stainless steel. This material offers superior corrosion resistance compared to mild steel or 409 stainless, making it ideal for daily-driven WRXs exposed to road salt and moisture. The wall thickness of approximately 1.5–2.0 mm provides a balance between weight savings and dampening of high-frequency harmonics that can produce undesirable harshness. The flanges are laser-cut and precision-milled to ensure a leak-free seal against the cylinder heads and turbo inlet. HKS also applies a brushed finish that resists discoloration better than polished surfaces, keeping the headers looking purposeful under the hood.

Optimized Tubing Diameter and Merge Collector

The primary tube diameter is carefully sized to match the displacement of the WRX engine. Typically, the Legamax headers use 1.5-inch (38 mm) primaries for the FA20 and slightly larger for EJ engines, with a smooth merge collector that transitions into a 2.5-inch (63 mm) outlet. This configuration avoids the turbulence of a sharp 4-into-1 junction while keeping gas velocity high enough to maintain low-end torque. HKS uses mandrel bends throughout to maintain consistent cross-section, eliminating the crimping seen in cheap “crush-bent” headers. The equal-length runners—a hallmark of the Legamax design—help the ECU deliver consistent fuel trims across all cylinders, which is especially valuable after an ECU reflash.

Weight Reduction and Thermal Management

Compared to the heavy cast-iron OEM manifold, the HKS Legamax headers save roughly 3–5 pounds (1.4–2.3 kg) depending on the specific model. While this may seem modest, the weight reduction is unsprung in the sense that it sits directly on the engine, improving overall mass distribution. The thin-wall stainless also radiates heat faster than cast iron, but HKS includes thermal coating options or recommends wrapping for track use. For daily driving, the natural heat dissipation is sufficient to keep engine bay temperatures within safe limits without requiring heat wrap.

Performance Gains and Driveability Improvements

Measurable gains from the Legamax headers are most noticeable in the mid-range, where the combination of improved flow and better exhaust scavenging produces a broader torque curve. Independent dyno tests on a 2015+ WRX (FA20) show gains of 15–20 horsepower and 18–22 lb-ft of torque at the wheels when paired with a quality cat-back exhaust and a conservative ECU tune. On a mostly stock car, the gains are typically 10–12 horsepower, but the real-world improvement in throttle response is what drivers feel first.

Dyno-Proven Horsepower and Torque Curves

The horsepower curve with the Legamax headers is not just higher but also flatter, with less drop-off near redline. The torque peak shifts slightly earlier in the RPM range, improving in-gear acceleration for passing and highway merging. A popular third-party review from a tuner (link below) documented a WRX that gained 18 whp and 20 lb-ft with only the headers and a pro-tune, all without any other exhaust modifications. This demonstrates that the headers alone are a worthwhile upgrade even before considering a downpipe or intake.

Throttle Response and Spool Characteristics

One of the most immediate benefits is the change in turbo spool behavior. The reduced backpressure allows the turbo to spin up approximately 200–300 RPM earlier, bringing boost on sooner. This makes part-throttle driving much more responsive—no more waiting for the boost to build when exiting a corner or merging into traffic. For daily drivers, this translates into a car that feels lighter and more eager without requiring a heavy foot. However, HKS cautions that if the car is not tuned, the ECU may compensate by pulling timing or enriching the mixture on wide-open throttle, which can diminish gains. A tune is strongly recommended to fully realize the potential.

Sound Signature – The Crisp Note Explained

The “crisp” exhaust note that HKS promotes comes from the equal-length runner design and the specific collector geometry. Unlike unequal-length headers that produce the classic Subaru “boxer rumble” due to uneven timing of exhaust pulses, the HKS Legamax equal-length design creates a smoother, more refined tone that is often described as “European” or “exotic.” It eliminates the off-beat throb in favor of a higher-pitched, rasp-free growl that becomes more aggressive under full throttle without being obtrusive at cruising speeds. This is particularly appealing to owners who want a sporty sound without the drone and vibration associated with cheaper aftermarket headers. The sound pairs exceptionally well with a resonated mid-pipe or a quality cat-back exhaust like the HKS Legamax catback itself.

Installation Guide and Compatibility

Installing the HKS Legamax headers is a moderate-level job requiring basic mechanical skill and a socket set. The direct-fit design means no cutting, welding, or modifications are needed—provided the car is not equipped with aftermarket engine mounts or extremely low clearance. Most installations can be completed in 3–5 hours on a lift, or 5–7 hours on jack stands.

Required Tools and Preparation

  • Metric socket set (10 mm, 12 mm, 14 mm, 17 mm)
  • Torque wrench (in-lb and ft-lb ranges)
  • Penetrating oil (for exhaust manifold bolts)
  • New exhaust gaskets (HKS includes gaskets, but some prefer OEM Subaru gaskets for the flange)
  • Anti-seize compound for bolts
  • Jack stands and a trolley jack
  • Optional: O₂ sensor socket (if sensors need to be removed)

Before starting, spray the stock manifold bolts with penetrating oil and allow 15 minutes to soak. The bolts on WRX headers are notoriously tight due to heat cycling and corrosion, especially in the 2015–2021 models. A breaker bar or impact wrench is recommended.

Step-by-Step Removal and Installation

  1. Disconnect the battery (negative terminal) and allow the engine to cool completely—exhaust components can exceed 400 °F. Working on a hot engine risks burns and can warp new gaskets.
  2. Remove the heat shields from the stock manifold (two or three shields held by 10 mm nuts).
  3. Unbolt the downpipe from the turbo outlet to create clearance. You do not need to fully remove the downpipe—just loosen it enough to move it aside.
  4. Spray the oxygen sensor threads with penetrating oil if they are located in the manifold. Use a sensor socket to loosen them carefully to avoid damaging the wiring.
  5. Remove the manifold-to-cylinder head bolts (14 mm, typically 8 bolts). Work from the outside in to prevent warping. Once free, lift the old manifold out from the bottom or the side depending on clearance.
  6. Dry-fit the HKS Legamax headers to ensure the flange holes align with the studs. Apply a thin layer of anti-seize to the studs. Use new gaskets—HKS includes them, but double-check orientation.
  7. Tighten the manifold bolts in a crisscross pattern to the manufacturer’s torque spec (typically 30–35 ft-lb for EJ engines, 28–32 ft-lb for FA20). Do not overtighten as stainless steel flanges can distort.
  8. Reinstall the O₂ sensor if removed, using anti-seize on the threads (be careful not to contaminate the sensor tip). Torque to 25–30 ft-lb.
  9. Reattach the downpipe and torque the fasteners to 35–40 ft-lb. Reinstall any heat shields that came with the HKS kit (the Legamax often includes a small shield around the collector).
  10. Double-check all bolts for tightness, then reconnect the battery. Start the engine and listen for leaks. If any hissing is heard, shut off and retighten the affected flange.

Torque Specifications and Gasket Care

Do not reuse old exhaust gaskets; even if they appear intact, they will not compress correctly on a new flange surface. HKS supplies new gaskets, but quality aftermarket ones like GrimmSpeed can further reduce leak risk. Always follow a torque sequence and re-check after the first heat cycle. A common mistake is tightening the collector flange to the turbo inlet without checking alignment, which puts stress on the turbo housing. Loosen and re-align if needed before final torque.

ECU Tuning Recommendations

While the car will run on the OEM tune, the new headers will shift the exhaust flow characteristics enough that the ECU’s fuel trims may become erratic at high loads. To maximize the horsepower gains and protect the engine, an aftermarket tune is essential. Options include a Cobb Accessport with a stage 2+ OTS map (designed for a cat-back and headers) or a professional dyno tune from a shop that specializes in Subaru. HKS advises that with a proper tune, the Legamax headers can push the WRX to the 300–320 whp range when combined with a downpipe and intercooler upgrade. Without a tune, expect only a fraction of the dyno gains and potentially lean conditions under boost.

In the United States, replacing the factory catalytic converter-equipped manifold (which is part of the exhaust system) with a header that deletes or relocates the catalyst may violate EPA tampering laws. The HKS Legamax headers are designed as a race-only part and do not include a catalytic converter. For street-driven cars in states with emissions testing, this header will cause a check engine light (CEL) due to missing or relocated oxygen sensors. HKS offers a version with a high-flow catalytic converter built into the collector for street-legal applications in some markets, but it is less common. Owners should check local regulations and consider using a spacer or defouler on the O₂ sensor—though this is not a guaranteed fix. Warranty-wise, installing headers can void the engine and emissions warranty on the modified components, but the Magnuson-Moss Act protects against blanket denials unless the part directly caused the failure.

Comparing HKS Legamax to Other WRX Header Options

The WRX header market includes options from Perrin, GrimmSpeed, Cobb, and Tomei, ranging from equal-length to unequal-length designs. HKS stands out for its build quality and sound refinement. While equal-length headers from other brands can deliver similar peak power, the HKS Legamax is the only one that includes a matched thermal coating option and a reputation for minimal drone. The price point ($800–$1,200 depending on the model) sits in the mid-range; cheaper headers often require modifications to fit or suffer from cracking at the welds. For the daily driver who values a “set and forget” upgrade with a proven track record in Japanese tuning culture, the HKS Legamax is a clear winner.

Conclusion

The HKS Legamax WRX Headers represent a carefully engineered upgrade that delivers on its promises: a crisp, refined exhaust note, measurable horsepower gains, and improved throttle response without sacrificing reliability. For the enthusiast willing to invest in a quality tune and accept the emissions trade-offs, these headers transform the driving character of the WRX from a competent turbo commuter into a responsive, engaging machine. Whether carving back roads or merging on the highway, the Legamax headers reward the driver with a more immediate connection to the engine—and a sound that turns heads without turning deaf.

For further reading, visit the official product page on HKS Japan, check a comprehensive install guide on SubiSpeed, see real-world dyno results from Driven Performance, or join the discussion on Subaru Forester Forum for community feedback. If you prefer a video walkthrough, Import Image Racing offers a detailed installation tutorial.