HKS LEGAMAX Premium Exhaust Review: Honda Civic Sedan FC

If you drive a 10th-gen Honda Civic Sedan with the 1.5L turbo and you have spent any time in the FC owner forums, you have already heard the whispers about the HKS LEGAMAX Premium exhaust. It is the priciest cat-back in the segment, the one with the burnt-titanium quad tips, and the one that JDM purists keep insisting is worth twice the price of a Borla. At our Tampa shop we have bolted it onto enough 2016-2021 Civic Sedans to have a strong opinion, so this review answers the only questions that matter: does it actually fit your car, what does it sound like at 70 mph, and is $1,500 the right number for a refined street exhaust that does not need a tune?

Will the HKS LEGAMAX Premium Fit Your Civic FC?

Before anything else, this. The HKS LEGAMAX Premium part number 31021-BH004 fits a very specific slice of the 10th-generation Civic Sedan family, and ordering the wrong year or trim is the single most common mistake we see at NLP Performance. Read this section even if you think you already know what your car is.

The 31021-BH004 LEGAMAX Premium fits the 2016-2021 Honda Civic Sedan with the 1.5L turbocharged L15B7 engine, in Non-Si trims only. That includes EX-T (2016-2018), EX-L, Touring, and Sport 1.5T. The easy visual confirmation is to walk around to your rear bumper: if you have two exit cutouts — one tip on each side — you have the dual-exit 1.5T car and BH004 is your part. If your bumper has a single center exit, you have the 2.0L i-VTEC non-turbo, and BH004 is the wrong part.

It does not fit:

• The 2.0L LX, EX, or Sport sedan (single-tip non-turbo — different muffler geometry, no aftermarket cat-back maker offers a real upgrade for that trim).
• The Civic Si Sedan (FC3) — the Si gets its own HKS part number (we cover it below).
• Any Civic Hatchback (FK7) — the hatch uses part number 31021-AH004 with a 97mm tip and JASMA certification.
• The Civic Type R (FK8 or FL5) — bigger piping, different mid-section.

What Makes the LEGAMAX Premium Different?

HKS has been building exhausts in Japan since 1973, and the LEGAMAX Premium line sits at the top of their street catalog. Every system in the LEGAMAX Premium family is full SUS304 stainless, mandrel-bent, and built with a low-restriction silencer specifically tuned to suppress unwanted resonance. The Civic Sedan version, 31021-BH004, uses a 60mm center pipe (with a 50mm reduction section at the cat-back joint) feeding into a quad-tip layout — two 75mm burnt-titanium polished tips on each side of the bumper. That quad-exit look alone separates it visually from every dual-tip option in the segment.

On the engineering side, HKS publishes a roughly 50% reduction in back-pressure versus the OEM cat-back, which is significant on paper but easy to misread. The 1.5L turbo Civic still has a stock catalytic converter and a stock front pipe sitting upstream of where the LEGAMAX bolts on, and those two pieces are the real restriction. The LEGAMAX gives you headroom; a downpipe is what lets the engine actually use it.

How Does the HKS LEGAMAX Premium Sound on a Civic Sedan?

This is where the LEGAMAX Premium splits the room. HKS publishes 68 dB at idle and 79 dB at peak (close-by) on the Civic Sedan, against an OEM baseline of roughly 58 dB at idle and 68 dB at peak. That is a clean +10 dB across the board — clearly louder than stock, but not in the range where you have to apologize to neighbors at 6 a.m.

In the real world, what owners notice first is the character change rather than raw volume. The tone is deep and crisp at idle, smooth on a cruise, and rich under throttle. There is no rasp, no popcorn, no aftermarket buzz at the resonator. Inside the cabin at 70 mph on flat highway, with the windows up and the radio off, most owners report drone is minimal-to-none. That is unusual for a 1.5T sedan exhaust; the Borla S-Type, by comparison, droning between 2,200 and 2,600 RPM, is well documented on the same chassis.

There is a recurring honest-feedback theme in the forums you should hear before you spend $1,500: on a fully stock 1.5T car, the LEGAMAX Premium can feel too subtle. Owners chasing a more aggressive volume frequently say they wished they had paired it with a catted downpipe out of the gate. If you want louder, the LEGAMAX is not actually your problem — the stock catalytic converter is. We will cover that pairing below.

Install Notes: 45 Minutes on a Lift

The LEGAMAX Premium is a true bolt-on. It reuses every OEM hanger location, comes with the gaskets you need, and slides into place without welding, cutting, or any factory exhaust modification. On a two-post lift at our Tampa shop, a clean install takes about 45 minutes. In a driveway on jack stands, plan for 90 minutes to two hours if it is your first exhaust job.

The only step that catches first-time installers is removing the OEM rubber hanger isolators from the factory exhaust — they grip the steel rod tight after a few years of heat cycles. A spray of soapy water on the rubber and a steady pull-and-twist is the trick. Once the OEM cat-back is on the ground, the LEGAMAX hangs into place using the same isolators (or new ones if yours are cracked). Torque the slip joint, double-check the rear hanger alignment so the burnt-Ti tips center in the bumper cutouts, and you are done.

How It Compares: Civic Sedan 1.5T Cat-Back Showdown

The LEGAMAX Premium does not lack competition on the 1.5T Civic Sedan platform. Here is how it lines up against the two cat-backs we sell most often at NLP Performance, plus the popular Borla S-Type as a reference point.

If you are deciding strictly on dollars-per-dB, the Invidia N1 is the price-to-volume winner — sportier than the HKS, well under $900, single 101mm titanium tips per side, and an authentic Invidia growl that scales well with downpipes. If you just want a quiet, OE-style replacement (the stock cat-back is rusting out and you do not care about volume), the Magnaflow BRExhaust slots in for similar money and stays civilian-quiet. The LEGAMAX Premium is the choice when you care about everything — sound character, drone control, finish, and the quad-tip look — and you are willing to pay the premium for HKS to nail all of it.

The Real Upgrade Path: Pair the LEGAMAX with a Downpipe

The honest truth from every shop that has tuned the 1.5T Civic Sedan: the LEGAMAX Premium cat-back alone, on a stock downpipe and stock tune, will give you maybe a couple of wheel horsepower at most. That is not the LEGAMAX’s fault. The factory catalytic converter on the L15B7 is the real bottleneck. Once you swap that with a high-flow catted 70mm downpipe, two things happen at once: the LEGAMAX wakes up acoustically (you get a noticeably more aggressive tone under throttle) and the engine finally has room to actually use that 50% lower back-pressure.

Our recommended stage progression for a 2016-2021 Civic Sedan 1.5T:

• Stage 1 — Bolt-on cat-back only: HKS LEGAMAX Premium for the look, sound character, and drone-free street manners. No tune. Marginal HP.
• Stage 2 — Cat-back + catted downpipe: Add the Invidia 70mm catted DP. Real torque gain in the midrange, more aggressive WOT tone, still no required tune for most setups.
• Stage 3 — Cat-back + catted DP + tune: Pair with a Hondata FlashPro reflash and you are firmly in 20+ wheel-HP territory on 91 octane.

Other HKS Civic FC Exhaust Options (Si, Hatchback, Type R)

If you came to this article because you Google’d “HKS Civic FC exhaust” and you are not actually shopping for the Non-Si Sedan version, here are the three siblings you probably want instead. Every one of them is in stock at NLP Performance.

HKS Hi-Power Muffler — Civic Si Sedan FC (2017+)

The Civic Si Sedan FC3 gets its own dedicated HKS part number, the Hi-Power Muffler. It is a slightly more aggressive tune than the LEGAMAX Premium, designed specifically for the Si’s 1.5T (K20C2 in some references, 1.5L turbo across the FC3 Si). Quad-tip rear bumper layout, full bolt-on, no tune required, and currently the only HKS solution for the Si Sedan.

HKS LEGAMAX Premium — Civic Hatchback FK7

The hatch gets its own LEGAMAX Premium part number, AH004. The biggest visual difference is a single oversized 97mm titanium tip per side instead of the sedan’s 75mm quad-tip layout, and the hatch version is JASMA-listed. Pricing is $1,700 — a touch above the sedan part — and it ships with the same SUS304 construction, drone-free silencer, and no-tune-required spec.

HKS LEGAMAX Premium — Civic Type R FK8

FK8 Type R owners get the BH003 LEGAMAX Premium — larger 3-inch piping, a center-exit triple-tip arrangement to match the OEM Type R bumper, and a louder published dB rating than either the Sedan or Hatch versions. This is the version HKS tests at high boost, and yes, the dyno numbers you read on JDM forums (in the 540 PS neighborhood) belong to this part and not BH004. Apples to oranges.

Frequently Asked Questions