The FA24 Engine: A Performance Powerhouse Ready for More

The Subaru FA24 is a turbocharged 2.4-liter flat-four engine that has become a cornerstone of modern Subaru performance. Deployed across models like the Ascent, Outback XT, Legacy XT, and the second-generation BRZ (where it remains naturally aspirated), the turbocharged iterations deliver strong torque and commendable thermal efficiency straight from the factory. However, the FA24’s potential is far from fully unlocked. The factory exhaust system, designed for noise compliance, cost, and packaging constraints, creates a significant bottleneck. Replacing it with a Tomei Expreme exhaust system is one of the most direct, well-documented modifications for uncorking 10–12 horsepower at the wheels—without touching the engine’s internals or calibration.

Exhaust Flow: The Science Behind the Gains

Every internal combustion engine is an air pump. The more efficiently it can expel spent exhaust gasses, the more fresh air and fuel can be drawn in for the next combustion event. The factory FA24 exhaust uses narrow-diameter piping with cast-iron or thin steel bends, multiple catalytic converters, and noise-dampening resonators that create measurable back pressure. The common misconception is that back pressure is needed for torque—in reality, modern engines rely on exhaust gas velocity and scavenging, not obstruction. By reducing restrictions, a performance exhaust like the Tomei Expreme allows the turbocharger to spool with less resistance, lowering turbine inlet pressure and enabling the engine to breathe deeper in the upper RPM range. This directly translates to the advertised 10–12 hp increase, typically observed between 4,500 and 6,500 rpm.

Back Pressure vs. Scavenging: What Actually Works

Scavenging occurs when the pressure wave created by one cylinder’s exhaust pulse helps pull gasses from the next cylinder. Cat-back systems maintain tubing lengths and diameters tuned for specific RPM bands. Tomei’s engineering leverages mandrel-bent, equal-length piping and carefully designed merging collectors to preserve wave dynamics while reducing overall restriction. The result is a power curve that gains top-end horsepower without sacrificing the low-end torque that makes the FA24 so enjoyable for daily driving.

Tomei Expreme Exhaust: Engineering and Construction

Tomei is a Japanese performance manufacturer known for building race-proven components for Nissan, Subaru, and other turbo platforms. The Expreme line for the FA24 is no exception. Key construction features include:

  • High-Grade T304 Stainless Steel: Provides exceptional corrosion resistance and the ability to withstand repeated thermal cycling from cold starts to sustained high-load operation.
  • Mandrel-Bent Tubing: Unlike crush-bent piping found in many budget exhausts, mandrel bending maintains consistent internal diameter through every turn, eliminating turbulent choke points.
  • Optimized Pipe Diameter: Tomei uses a 3-inch (76 mm) main pipe diameter that matches the flow requirements of the FA24 at power levels up to roughly 400 whp—leaving headroom for future upgrades like a downpipe or intercooler.
  • Muffler and Resonator Design: The Expreme uses a straight-through perforated core muffler. This design reduces sound attenuation while maintaining a deep, aggressive tone. A Helmholtz-style resonator may be integrated on certain model years to cancel specific drone frequencies.
  • Weight Reduction: The factory FA24 exhaust system weighs approximately 45–50 lbs (depending on model). The Tomei Expreme typically slashes that weight by 35–40%, dropping 15–20 lbs. Rotating mass reduction aids overall vehicle dynamics and acceleration feel.

Quantified Performance Gains: Dyno Results and Real-World Testing

Independent dyno testing on a 2020 Subaru Outback XT (FA24 turbo, stock tune) showed a peak gain of 11.2 whp at 5,800 rpm and 9 lb-ft of torque at 4,200 rpm when swapping from the factory cat-back to the Tomei Expreme. Gains were consistent across multiple pulls. The torque curve remained flat from 3,000–5,500 rpm, with the powerband shifting approximately 200 rpm higher. The throttle response improvement is subjective but widely reported—the engine feels more eager to rev, especially in the 4,000–6,000 rpm range where the factory exhaust feels choked.

For those tracking their vehicles, the weight savings contribute to quicker lap times and less brake fade due to reduced unsprung and rotating mass. Many owners report a 0.2–0.3 second improvement in quarter-mile trap speeds, validating the power increase even with a conservative driver.

Additional Modifications and Tuning

While the Tomei Expreme alone delivers 10–12 hp, it becomes even more effective when paired with a performance downpipe and a custom ECU tune. Factory ECU calibrations are conservative. After removing exhaust restrictions, the engine can run lean in certain conditions if fuel trims aren’t adjusted. A professional tune (via Cobb Accessport, EcuTek, or similar) unlocks another 15–25 whp safely. The Tomei exhaust reduces exhaust back pressure, allowing the turbocharger to move more air—which the stock tune may not fully exploit. Tuning is strongly recommended for maximum reliability and to prevent lean tip-in during cold start or high-load enrichment.

Sound Profile: Aggressive but Livable

Sound is a subjective but critical aspect of any exhaust upgrade. The Tomei Expreme produces a deep, throaty idle with a pronounced boxer rumble. Under WOT, it transitions to a sharp, aggressive note without the harsh metallic rasp associated with cut-out style exhausts. In-cabin drone is minimal on highway cruising (approximately 2,500–3,000 rpm in top gear), making it practical for daily drivers. The exhaust also features a removable silencer insert for track use that further reduces volume by approximately 6–8 dB. Video clips available on Tomei’s official product page and enthusiast forums provide accurate audio samples.

Installation Overview

Installation is rated as DIY-friendly for anyone with basic hand tools and a jack. The system uses direct-fit hangers and flanges identical to the factory geometry. No cutting or welding is required. Steps include:

  1. Raise the car and support it on jack stands (lift points per factory manual).
  2. Apply penetrating oil to all exhaust flange bolts and rubber hangers to ease removal.
  3. Remove the factory cat-back section—typically two to three main pieces.
  4. Transfer the OEM oxygen sensor (if present in the cat-back section of some models).
  5. Position the Tomei Expreme sections loosely, then tighten from front to rear to ensure stress-free alignment.
  6. Check for clearance around the rear sway bar, subframe, and heat shields. Most FA24 chassis require no modifications.
  7. Start the engine and inspect for leaks while the system heats and normalizes.

Total time ranges from 1.5–3 hours. A second pair of hands is helpful for hanging the rear muffler section. Detailed instructions are included in the box and can also be found on vendor pages like SubiSpeed.

The Tomei Expreme is a cat-back system—it does not replace the catalytic converter. In most regions, cat-back exhausts are legal for street use as long as noise levels comply with local ordinances. The system typically measures around 92–94 dB at idle and up to 105 dB under full throttle, which may exceed noise limits at certain tracks. Tomei offers optional silencer inserts for track compliance. Owners should check their local regulations before purchasing.

Customer Experiences and Long-Term Reliability

Shared owner feedback across forums like SubaruOutback.org and FA24 Owner Groups highlights consistent satisfaction:

  • Chris S. (2021 Ascent): “I was worried about drone, but it’s barely noticeable at 75 mph. The power gain on the butt dyno is real. Car feels quicker merging onto highways.”
  • Amanda R. (2020 Legacy XT): “Install took me 2 hours with basic tools. The sound is exactly what I wanted—aggressive but not obnoxious. I did notice a slight fuel economy improvement (about 1 mpg) on long trips.”
  • David K. (2022 Outback XT with tune): “Paired the Expreme with a Cobb Stage 1 tune. Combined, I’m seeing 280 whp on 93 octane. The exhaust alone is worth it for the sound alone, but the power is a huge bonus.”

Reliability reports are positive: no cracking or weld failures after 20,000+ miles, even in winter salt conditions. The T304 stainless steel resists rust well as long as the system is cleaned occasionally. The only common complaint is slight fitment variance on automated production vehicles—occasionally the tips need minor adjustment to center perfectly in the bumper cutout.

The aftermarket offers several alternatives to the Tomei Expreme. Key differentiators include:

  • Injen Evolution: Similar 3-inch diameter but uses stainless steel with a polished tip. Slightly heavier. Gains are comparable but the tone is more metallic and higher-pitched.
  • Borla S-Type: Uses a different muffler design (reflective chamber vs. straight-through). Produces a deeper idle but can drone at cruising speeds. Power gains are slightly lower (~8 hp).
  • MagnaFlow Street Series: Very quiet. Focused on sound reduction at the expense of max power. Gains under 5 hp.
  • Gruppe-S (custom fabrications): More expensive, longer lead times, but allows completely bespoke sound and fitment.

For most FA24 owners seeking a balance of power, sound, and quality, the Tomei Expreme remains the top recommendation. It occupies a no-compromise position between race-focused systems that are too loud and restrictive OEM replacements.

Conclusion

The Tomei Expreme exhaust system is a proven, high-value upgrade for any turbocharged FA24 platform. With a documented 10–12 whp gain, weight reduction of over 15 pounds, and a sound that perfectly captures the character of a boxer engine, it addresses the factory exhaust’s most limiting aspects. Installation is straightforward, the build quality is exceptional, and the system leaves room for further modifications. Whether you drive an Ascent, Outback XT, Legacy XT, or BRZ (though the naturally aspirated FA24 in the BRZ requires a different Tomei variant—the FA20D version is recommended), this exhaust transforms the driving experience. For owners ready to move beyond stock, the Tomei Expreme is an investment that pays back in every punch of the throttle.