HKS 31021-BH004 Legamax Premium Cat-Back Exhaust for 2016-2021 Honda Civic Sedan

Vehicle Fitment

• 2016-2021 Honda Civic Sedan (FC1/FC2 platform - L15B 1.5L turbocharged, FWD - LX, Sport, EX, EX-T, EX-L, Touring trims)

Fitment Notes

• Fits the Honda Civic Sedan (FC1/FC2) only - does NOT fit the Civic Coupe (FC3), Civic Hatchback (FK7), or Civic Type R (FK8), all of which use separate HKS applications despite sharing the same 10th-generation platform
• Compatible with all L15B 1.5L turbocharged Civic Sedan trims - fitment does not vary by trim level for this exhaust
• The Sport trim Civic Sedan uses the same L15B 1.5T drivetrain and exhaust outlet location as other trims and is confirmed compatible
• Emissions equipment is maintained completely intact - no modifications to catalytic converter, oxygen sensors, or other emissions-related components; suitable for emissions-compliant street use
• Direct bolt-on installation using OEM exhaust hanger and connection points - no cutting or welding required
• For the Civic Coupe (FC3) with L15B, see HKS 31021-BH001; for the Civic Hatchback (FK7), see HKS 31021-AH004; for the Civic Type R (FK8), see HKS 31021-BH003

Features

• 60mm (2.36") large-diameter center pipe with full straight-through internal silencer structure reduces exhaust back-pressure by 50% compared to the stock exhaust - allows the L15B turbocharger to spool more freely from low RPM and improves power delivery across the entire rev range
• Legamax Premium vehicle-specific tip design - HKS studies the FC Sedan's rear fascia geometry from multiple design perspectives to select a tip configuration that becomes a visual focal point while remaining proportionally balanced with the car's styling
• SUS304 grade stainless steel construction throughout - provides excellent corrosion resistance, heat tolerance, and a high-lustre polished finish that resists discoloration and pitting in all conditions
• Advantex glass wool muffler packing - chosen for superior heat and moisture resistance compared to standard steel wool, maintaining consistent sound character and eliminating the tonal drift that occurs as lower-quality packing compacts over time
• JASMA certified - confirms compliance with Japan Automobile Sports Muffler Association standards for sound level and exhaust performance; tuned for a sporty, deep note without excessive harshness or resonance
• Designed for zero drone - the combination of dual muffler design, resonator, and full straight-through structure delivers a deep, throaty exhaust note under acceleration with minimal cabin resonance during highway cruising
• Legamax Premium mufflers combine sports muffler performance characteristics with the finish quality, sound refinement, and visual presentation expected of a premium-tier exhaust system
• Engineered and manufactured in Japan by HKS

Specifications

• Part Number: 31021-BH004
• Series: Legamax Premium
• Exhaust Type: Cat-back
• Compatible Platform: Honda Civic Sedan FC1/FC2
• Compatible Engine: L15B 1.5L turbocharged inline-4
• Main Material: SUS304 stainless steel
• Center Pipe Diameter: 60mm (2.36")
• Exhaust Pressure Reduction: 50% vs. stock
• Internal Structure: Full straight-through
• Packing Material: Advantex glass wool
• Sound Certification: JASMA certified
• Emissions: All emissions equipment maintained intact
• Country of Manufacture: Japan
• UPC: 4957266923726

Included Components

• 1x HKS Legamax Premium cat-back exhaust assembly (pipes, muffler, tips)
• Mounting hardware and gaskets

Product Description

The HKS 31021-BH004 Legamax Premium is a vehicle-specific cat-back exhaust system for the 2016-2021 Honda Civic Sedan (FC1/FC2) with the L15B 1.5L turbocharged engine. As part of HKS's Legamax Premium line - which pairs sports muffler performance with premium-grade materials and application-specific aesthetics - this system is engineered around the FC Sedan's specific exhaust routing, rear fascia geometry, and acoustic targets rather than adapted from a generic platform solution. The 60mm center pipe diameter and full straight-through internal structure reduce exhaust back-pressure by 50% compared to the restrictive OEM system, allowing the L15B's turbocharger to build boost pressure more quickly from lower in the RPM range and improving throttle response and power delivery throughout the powerband.

The tip specification for the FC Sedan is selected through HKS's vehicle-specific design process - engineers study the rear bumper design from multiple viewing angles to choose a tip configuration that creates visual impact as a focal point without appearing out of scale or mismatched with the sedan's overall proportioning. SUS304 stainless steel construction throughout provides a high-lustre polished finish with strong resistance to corrosion and heat discoloration, while Advantex glass wool packing inside the muffler body maintains consistent, deep sound quality over the system's service life by resisting the compaction and heat degradation that gradually degrade the acoustic character of lower-quality muffler packing materials.

The system is JASMA certified for sound level compliance and maintains all OEM emissions equipment intact - catalytic converter, oxygen sensors, and related hardware are completely unmodified, making the 31021-BH004 suitable for street use in emissions-tested markets. This part is designed specifically for the four-door sedan body style; owners with the Civic Coupe (FC3), Hatchback (FK7), or Type R (FK8) should use their respective HKS applications, as each body style has a distinct exhaust exit location, hanger position, and tip design. The FC Sedan was produced through the 2021 model year before Honda transitioned to the 11th-generation Civic platform.