Why Upgrade Your Midpipe on Your Subaru WRX?

The midpipe is the central section of your Subaru WRX’s exhaust system, connecting the downpipe (or the catalytic converter on certain setups) to the axle-back or cat-back portion. While often overlooked in favor of full turbo-back exhausts, the midpipe plays a pivotal role in exhaust gas velocity and overall system backpressure. The factory midpipe is engineered to meet noise regulations and emissions standards while keeping costs low—typically using a restrictive diameter, crush bends, and a thin-walled resonator. Replacing it with an aftermarket unit removes these bottlenecks, allowing the engine to exhale more freely. This translates to quicker spool, improved throttle response, and a noticeable increase in peak horsepower and torque, often in the 5–15 hp range when paired with an appropriate tune. Additionally, the acoustic character shifts: expect a deeper, more aggressive note without excessive drone if the midpipe is properly resonated.

Top Midpipe Upgrades for Subaru WRX

We’ve evaluated several popular aftermarket midpipes based on materials, fitment, power gains, sound, and street legality. Below are the standout options for the 2015–2021 WRX (FA20DIT) and older GD/GH models (EJ series). Always confirm compatibility with your specific year and model.

1. Invidia Midpipe (Catted & Catless)

Invidia is a benchmark in the Subaru community for good reason. Their midpipes are constructed from T304 stainless steel with mandrel bends to preserve exhaust flow. The catted version employs a high-flow GESI catalytic converter that keeps the check engine light off and passes OBD-II readiness in many states, while still delivering a significant flow increase over the stock piece. The catless version removes the converter entirely for unrestricted flow and a raw, aggressive tone—but it is not street legal in states with emissions testing. Invidia’s midpipe comes with a full gasket kit and bolts; installation can be done in under an hour with basic tools. On a dyno, the catted Invidia midpipe combined with a tune often yields 8–12 whp and 10–15 lb-ft over the stock midpipe.

2. GrimmSpeed Midpipe

GrimmSpeed’s midpipe is purpose-built for Subaru boxer engines. It features a 3-inch diameter (stock is 2.5-inch) with a smooth taper to the flanges, eliminating step-down restrictions. The resonator is tuned to cancel drone frequencies while preserving a sporty note. GrimmSpeed uses CNC-machined flanges and 16-gauge stainless steel for long-term durability—no cracking at welds or rust-through. Owners report a linear power delivery and a deeper rumble without rasp. This midpipe is a direct fit for 2015+ WRX and requires no cutting or welding. Expect a 6–10 whp gain on a tuned car, with improved spool by 200–400 RPM.

3. Boomba Racing Midpipe

Boomba Racing focuses on minimal backpressure while maintaining structural integrity. Their midpipe is available in 3-inch diameter with either a resonated or non-resonated option. The resonated variant uses a Helmholtz-style chamber to cancel specific drone frequencies, making it ideal for daily drivers who want performance without interior boom. The non-resonated version is lighter and louder, suited for track use. Boomba includes stainless steel hardware and a lifetime warranty on defects. On the FA20, the Boomba midpipe paired with a stage 2 tune adds around 10 whp and flattens the torque curve. It’s also compatible with most cat-back systems thanks to a standard 3-inch inlet/outlet.

4. APS Midpipe (Catted Direct Fit)

APS offers a midpipe that prioritizes emissions compliance without sacrificing flow. Their unit uses a high-density ceramic catalytic converter that exceeds OEM conversion efficiency rates, making it one of the few aftermarket catted midpipes that passes California smog (CARB EO number pending or already approved, depending on model). Construction is T304 stainless with mandrel bends and a polished finish. The resonator is a three-chamber design that reduces harshness. APS midpipes are a favorite among owners who want maximum legality with near-catless power. Gains are conservative at 5–8 whp, but the real benefit is maintaining a clean emissions profile.

5. Cobb Tuning Midpipe (Part of their Turbo-Back System)

While Cobb sells their midpipe as part of a full turbo-back exhaust, the midpipe section is also available separately. It features a 3-inch diameter with a high-flow metal-core catalytic converter (for catted models) and a dual-chamber resonator. Cobb’s reputation for precision fitment and tune-reliability is well-deserved. The midpipe includes a larger-diameter oxygen sensor bung to prevent air leaks and comes with Cobb’s specialized gaskets that seal better than OEM. On an Accessport stage 2 tune, the Cobb catted midpipe contributes to a 10–15% increase in airflow and a smoother powerband. Note that Cobb recently discontinued some of their non-GESI converter products, so check their latest catalog for your WRX year.

Understanding Midpipe Configurations: Catted vs. Catless

The choice between a catted and catless midpipe is the single most important decision you will make during your WRX exhaust upgrade. Here is a structured breakdown:

  • Catted Midpipe: Incorporates a high-flow catalytic converter. Benefits include street legality in most regions (check local laws), reduced exhaust smell, and negligible effect on OBD-II readiness. Modern catted midpipes use GESI or metal-core converters that flow as well as some catless designs, costing you only 2–5 hp compared to a fully open pipe. Recommended for daily-driven cars.
  • Catless Midpipe: No catalytic converter. Advantages: maximum possible flow, loudest sound, and a slight weight reduction. Disadvantages: produces a strong fuel odor (especially on cold starts), illegal on public roads in most countries, and will trigger a check engine light unless you install a defouler or tune to disable the rear O2 sensor. May also attract attention from law enforcement at inspection.

We strongly advise using a catted midpipe unless your WRX is used exclusively on track. The performance difference is narrowing every year as converter technology improves.

Other Considerations for Your Midpipe Upgrade

Compatibility with Your Model Year and Variant

Subaru WRX generations differ in flange angles and length. For example, 2015–2021 WRX (VA chassis) use a 2.5-inch stock midpipe with oval flanges; 2008–2014 WRX (GR/GV) use a slightly different layout; and the 2002–2007 bug/blob/hawk eyes have their own unique fitment. Always verify that the midpipe you are buying is listed for your exact vehicle. Most manufacturers provide detailed fit notes—ignore them at your own risk. Additionally, if your car has a non-stock downpipe (especially a catless downpipe), you may need an adapter or a specific midpipe version that matches the downpipe’s outlet diameter and flange pattern.

Pairing with Downpipe and Catback

A midpipe is part of a system. For best results, match your midpipe diameter to your downpipe and cat-back. Mixing a 3-inch downpipe with a 2.5-inch midpipe creates a restriction that nullifies flow benefits. Conversely, using a 3-inch midpipe with a stock downpipe (which necks down to 2.0 inches near the turbo) improves only the exit side. The ideal scenario is a 3-inch downpipe + 3-inch midpipe + 3-inch cat-back. If you are on a budget, you can upgrade just the midpipe and cat-back while keeping the stock downpipe, but the biggest gains come from addressing the downpipe as well. A midpipe alone typically adds 5–8 whp; combined with a downpipe, that jumps to 15–25 whp with a proper tune.

Material and Build Quality

Stainless steel (T304 or T409) is the standard. T304 is more corrosion-resistant and easier to polish, while T409 is slightly less expensive and still durable. Some brands (e.g., Boomba Racing) use 16-gauge tubing for stiffness. Avoid cheap aluminized steel—it will rust within two years in salt-belt climates. Welds should be smooth and continuous; flanges must be thick (1/4-inch or more) to prevent warping. Check that included gaskets are the correct material (graphite or multi-layer steel) and that bolts are stainless or zinc-plated to avoid seizing.

Installation Guide for Your Subaru WRX Midpipe

Replacing the midpipe is one of the easiest exhaust modifications you can do. Allow 45–90 minutes with basic hand tools.

  • Tools Needed: 12mm, 14mm, and 17mm sockets and wrenches; penetrating oil (e.g., PB Blaster); jack and jack stands or ramps; exhaust hanger removal tool (or large pliers); anti-seize compound.
  • Steps:
    1. Lift the car safely on level ground. Support the exhaust with a jack stand under the center section.
    2. Spray all bolts and hanger rubber insulators with penetrating oil and let soak 10 minutes.
    3. Unbolt the midpipe from the downpipe (or catalytic converter connection) using a 14mm socket. If the joint is stubborn, tap the flanges with a hammer while applying turning pressure.
    4. Remove the two hangers: one at the rear of the midpipe and one near the resonator (if equipped). A hanger removal tool makes this easier; plain pliers will work but may tear the rubber.
    5. Slide the midpipe out backward over the axle. On some 2015+ models, you may need to lower the rear subframe bracket slightly—check instructions for your specific brand.
    6. Compare the new midpipe to the old one: confirm gasket orientation, and transfer the oxygen sensor (if your old setup had one on the midpipe). Apply a thin coat of anti-seize to the sensor threads.
    7. Install the new midpipe by reversing removal: loosely attach the hangers first, then tighten all flange bolts to factory torque spec (typically 30–35 ft-lbs for 12mm bolts; 40–50 ft-lbs for 14mm). Do not overtighten; gaskets compress.
    8. Start the engine and check for leaks—listen for hissing or feel for exhaust pulses near flanges. Retighten if needed.

If you are not comfortable working under a car, a professional installation costs $100–$200 and is worth the peace of mind, especially if you are also performing the downpipe or cat-back simultaneously.

Post-Installation: Tuning and Learning

Even a mild midpipe change can affect air/fuel ratios slightly. We strongly recommend an Accessport or Ecutek tune to optimize the new flow. A base tune from a reputable vendor (e.g., COBB OTS stage 1+ if you kept the stock downpipe, or stage 2+ for full turbo-back) will yield safe gains. Without a tune, you may see minimal improvement and risk running slightly lean under heavy load. Also, the ECU may take a few drive cycles to adapt trims—do not panic if you see a small drop in short-term fuel trim initially. Reset the ECU by disconnecting the battery for 10 minutes after installation.

Performance Results: Dyno and Driving Impressions

Based on independent testing from forums like NASIOC and IWSTI, a quality catted midpipe on a 2015+ WRX with an intake and stage 1 tune typically adds 6–10 whp and 8–12 lb-ft torque. The power difference is most noticeable in the midrange (3,000–5,000 RPM), where the engine feels eager to rev. On a full stage 2 setup (downpipe, midpipe, cat-back, tune), the midpipe contributes roughly 10% of the total gain but smoothens the power delivery substantially. Sound-wise, the midpipe deepens the exhaust note and adds a bass layer that complements the boxer rumble. Resonance (drone) is well-controlled on resonated versions. For EJ engines, the midpipe has a similar effect but the tone becomes more guttural and old-school Subaru-like.

Conclusion

Choosing the right midpipe for your Subaru WRX boils down to your performance goals, legal requirements, and sound preferences. For most daily drivers who want a meaningful power increase and a sportier exhaust note without legal headaches, a catted midpipe from Invidia, GrimmSpeed, or APS is the ideal balance. Enthusiasts chasing every last horsepower on a track-only car may opt for a catless alternative from Boomba or Invidia and complement it with a tune. Whichever you pick, ensure compatibility with your year and other mods, and don’t skip the tune. The midpipe is a small but mighty upgrade that transforms how your WRX drives and sounds—it’s one of the best bangs for the buck in the Subaru aftermarket world.

For more data and community feedback, we recommend visiting dedicated Subaru forums like NASIOC and IWSTI, or checking part reviews at MAP Dyno Pro and SubiSpeed. Always cross-reference current part numbers with your exact vehicle VIN before purchasing.