The FA20 engine has become a favorite among Subaru and Toyota enthusiasts thanks to its flat-four layout, lightweight construction, and turbocharging potential. Factory-tuned for reliability and fuel economy, its exhaust system—particularly the downpipe and headers—is one of the most restrictive areas limiting performance. Upgrading these components not only unleashes hidden horsepower but also transforms throttle response and spool characteristics. This article provides a deep dive into how downpipe and header modifications affect the FA20, covering real-world power gains, supporting mods, tuning requirements, and key considerations before you start turning wrenches.

Understanding the FA20 Engine

The FA20 family includes both naturally aspirated (FA20D) and turbocharged (FA20F) variants. The FA20D powers the Subaru BRZ and Toyota GT86, while the FA20F is found in the Subaru WRX, Forester XT, and Legacy GT. Both versions share an all-aluminum construction, dual overhead camshafts, and D-4S direct injection. However, the turbocharged FA20F uses a twin-scroll turbocharger for quicker spool and better low-end torque. Factory exhaust routing is designed for cost, noise compliance, and emissions—not performance. The stock downpipe contains a restrictive catalytic converter and narrow internal diameter, while factory headers (on turbo models) often incorporate a compact design that prioritizes packaging over flow. This is the starting point for meaningful power gains.

The Role of Downpipes in Exhaust Flow

The downpipe connects the turbocharger’s turbine outlet to the rest of the exhaust system. On the FA20F, the factory downpipe typically features a 2.5-inch diameter and a high-density ceramic cat that creates significant backpressure. Replacing it with an aftermarket unit—commonly 3 inches in diameter and optionally catless or equipped with a high-flow catalytic converter—dramatically reduces restriction. The result is faster turbine spool, lower exhaust gas temperatures, and more usable power across the RPM range.

Key performance benefits of a high-flow downpipe on the FA20F:

  • Reduced turbo lag: Improved exhaust flow allows the turbine to spin up sooner, bypassing the bottleneck created by the factory cat.
  • Increased horsepower and torque: Dyno charts from tuners such as Cobb Tuning show gains of 15–25 wheel horsepower with just a downpipe and ECU calibration.
  • Better exhaust gas scavenging: Lower backpressure helps the turbo maintain boost at higher RPM, extending the power band.

On the naturally aspirated FA20D, downpipes (often called front pipes) tie into the header collector. While gains are smaller—typically 5–10 horsepower—the improvement in throttle response and engine sound remains worthwhile for daily drivers.

Material and Design Considerations

Most aftermarket downpipes are made of 304 stainless steel or mild steel with a ceramic coating. Stainless steel offers corrosion resistance, while coated mild steel retains heat better and reduces under-hood temperatures. A catted downpipe (200 or 300 cell metallic cat) will still pass many emissions tests and prevent the check engine light if the secondary O2 sensor is properly tuned. Catless downpipes deliver the largest flow gains but risk CEL, increased exhaust odor, and legal issues in regulated regions.

Upgrading Headers: Why It Matters

Headers collect exhaust gases from each cylinder and route them to the up-pipe (on turbo cars) or directly to the exhaust system (on NA cars). On the FA20F, the factory headers are cast iron or tubular steel with a merged design that leaves room for improvement. Swapping to a set of equal-length tubular headers optimizes exhaust pulse timing, enhances turbo spool, and can yield 10–15 additional wheel horsepower when paired with a proper tune.

For the FA20D in the BRZ/86, header upgrades are one of the most effective single mods. Equal-length headers improve scavenging, flatten the torque curve, and raise peak horsepower by 15–20 at the wheels. Enthusiasts at FT86 Speedfactory report consistent gains of 10–15 ft-lb of torque from 3,000 to 6,000 RPM after installing quality headers.

Header types for the FA20:

  • Equal-length headers: Favor top-end power and a smoother torque curve, often with a more exotic exhaust note.
  • Unequal-length headers (UEL): Preserve the classic Subaru boxer rumble but may trade some peak power for a flatter torque curve at low RPM.

On turbocharged FA20 engines, unequal-length headers are less common because equal-length designs generally support better spool and cylinder-to-cylinder consistency. Some tuners also recommend coated headers to retain heat and increase exhaust velocity entering the turbo.

Quantified Power Gains: What to Expect

Combining a performance downpipe and headers yields cumulative benefits. The exact number depends on engine version, supporting modifications, and tuning quality. Here are realistic figures based on independent dyno tests:

FA20F (Turbocharged WRX / Forester XT)

  • Stock: ~210–230 whp (depending on model year)
  • Downpipe + tune: +20–30 whp, +25–35 ft-lb torque
  • Downpipe + headers + tune: +30–45 whp, +35–50 ft-lb torque

Data from IAG Performance shows that a 2015 WRX with a catless downpipe and Kelford 1.5 cams achieved 289 whp on 93 octane, with the majority of gains coming from the free-flowing exhaust and a proper dyno tune. The header added roughly 10–12 whp on top of the downpipe alone.

FA20D (Naturally Aspirated BRZ / GT86)

  • Stock: ~160–170 whp
  • Headers + tune: +15–20 whp, +10–15 ft-lb
  • Headers + front pipe (downpipe equivalent) + intake + tune: +25–30 whp

These numbers come from a NASIOC member dyno database where hundreds of logs confirm that header upgrades are the single biggest bolt-on for the FA20D. Adding a high-flow front pipe usually adds another 5 whp and improves throttle response.

Supporting Modifications for Maximum Gains

To fully exploit the increased exhaust flow, several supporting mods are strongly recommended:

Intake Upgrades

A less restrictive intake allows the engine to breathe in more fresh air, matching the improved outflow from the downpipe and headers. On the FA20F, a cold air intake (CAI) can add 5–10 wheel horsepower when paired with a tune. On the FA20D, a simple drop-in high-flow panel filter or CAI also yields small but measurable gains.

Intercooler (FA20F)

Turbocharged FA20 engines are prone to heat soak with factory top-mount intercoolers (TMIC). A larger front-mount intercooler (FMIC) or upgraded Spearco/Srock TMIC lowers intake air temperatures, allowing more timing advance and boost without detonation. This is especially beneficial if you increase boost pressure via the downpipe and tune.

Fuel System and E85

Ethanol (E85) provides a significant knock resistance increase. With a downpipe, headers, and a proper Flex Fuel tune, the FA20F can safely run higher boost and earlier timing. Gains of 50–70 wheel horsepower over a standard 93 octane tune are not uncommon. The FA20D does not benefit as much from E85 due to is direct injection limitation, but a flex fuel sensor can still allow a small bump in timing.

Exhaust System (Catback)

While the downpipe and headers are the primary power producers, a free-flowing catback exhaust eliminates any remaining restriction after the downpipe. Many tuners recommend a 3-inch catback for turbo cars and a 2.5- to 3-inch system for NA engines to maintain velocity without choking flow.

Tuning Requirements

No exhaust upgrade on the FA20 will reach its full potential without recalibrating the engine control unit (ECU). The factory tune is programmed for stock airflow and backpressure; installing a downpipe and headers will push air-fuel ratios and ignition timing outside safe parameters. Here are the most common tuning paths:

  • Cobb Accessport: A user-friendly handheld tuner that allows you to flash preloaded maps (e.g., Stage 2 for downpipe) or custom calibrations from a professional tuner.
  • Open-source tuning (RomRaider / Tactrix cable): More advanced, often used for fine-tuning boost, fuel, and timing tables.
  • Dyno tuning: The gold standard—perform actual pulls on a dynamometer while an experienced tuner adjusts the ECU in real time.

Without a tune, adding a downpipe or headers can cause lean conditions, knock, and eventually engine damage. Most tuners recommend at least a base map before driving the car aggressively after installation.

Upgrading these exhaust components can run afoul of environmental regulations in many regions. Key points to consider:

  • Catted vs. catless: Catless downpipes and headers produce a strong fuel odor and are illegal for road use in most states/countries without a catalytic converter. High-flow catted options are usually legal but may still fail an OBD-II readiness check.
  • Check engine light: Without tuning, a catless downpipe triggers a P0420 code due to insufficient catalyst activity. Tuning can disable the code or adjust thresholds, but this may still cause issues during inspection.
  • Noise laws: Headers combined with a catless front pipe and aftermarket exhaust can exceed local noise ordinances. Check your jurisdiction’s decibel limits before installation.

It is your responsibility to understand applicable laws. Many performance shops offer catted downpipes and headers that comply with emissions standards while delivering a majority of the power gains.

Installation Considerations

Replacing the downpipe and headers on the FA20 is a moderately difficult DIY job, but it does require mechanical skill, proper tools, and patience. Here is what to expect:

  • Access to the downpipe requires removal of heat shields, the intercooler (on turbo models), and often the front crossmember. Some bolts are located in tight spaces; a swivel socket and extensions are essential.
  • Stock fasteners on Subaru engines tend to corrode. Penetrating oil, heat (with a torch), and careful hammering may be needed to break them loose. Replace with new nuts, bolts, and gaskets during reinstallation.
  • Jack stands or a lift are highly recommended. The work is done from both above and below the car.
  • If you are installing headers on an FA20D, you may need to disconnect the steering intermediate shaft to maneuver the header past the oil pan.
  • After assembly, check for exhaust leaks (they will cause a sound change and possible O2 sensor error) and re-torque bolts after a heat cycle.

For owners not comfortable with mid-level mechanical work, professional installation typically costs $300–$600 depending on the shop and vehicle.

Conclusion

Upgrading the downpipe and headers on an FA20 engine is one of the most effective ways to unlock usable power. On the turbocharged FA20F, the gains are immediate and significant—20–30 wheel horsepower from a downpipe alone, with an additional 10–15 from upgraded headers, all while improving spool and driveability. For the naturally aspirated FA20D in the BRZ/86, headers deliver the biggest single-pony increase, adding 15–20 wheel horsepower and a much stronger mid-range. In every case, a proper tune is non-negotiable to ensure reliability and maximize results.

Before opening the wallet, consider your local emissions rules, the need for supporting mods (intake, intercooler, catback), and the cost of tuning. When done right, these modifications elevate the FA20 from a pedestrian commuter engine to a responsive, exciting powerplant that rewards every press of the throttle. For the enthusiast seeking a balanced blend of performance and practicality, a well-chosen downpipe and header setup offers the best return on investment for the FA20 platform.