Catted vs Catless Evo X Downpipe: Which Should You Buy for Street Use?

The Evo X Stock Downpipe Is the Real Problem

Most Evo X owners who start digging into exhaust mods quickly realize the bottleneck isn’t the cat-back — it’s the factory downpipe. The OEM unit measures just 2.5 inches and exits through a cast iron O2 housing that was never designed with performance in mind. Upgrading to a full 3-inch aftermarket downpipe is one of the most impactful bolt-on moves you can make on the 4B11T, and the gains show up immediately on a dyno — typically in the range of 15–20 whp on a stock turbo before any tune.

But once you decide to upgrade, you hit a fork: catted or catless? The answer depends almost entirely on how you use the car. For a dedicated track build that gets trailered to events, catless is a reasonable option. For anything that sees daily miles, commutes, or state emissions testing, the calculus changes fast.

This article breaks down both options across the variables that actually matter for street-driven Evo X builds: power output, CEL risk, emissions compliance, drivability, and long-term ownership headaches.

Power Gains: How Big Is the Gap, Really?

The catless camp often promises big numbers, and there’s some truth to it — but the real-world gap between a quality catted and catless downpipe is smaller than the forums suggest.

For context, the stock Evo X O2 housing and downpipe are genuinely restrictive. One vendor notes that the OEM turbine outlet has only a 2.25-inch exit, and even the stock downpipe itself is 2.5 inches. Any quality 3-inch aftermarket downpipe — catted or catless — addresses that restriction immediately. A catless setup does flow more freely through the cat section, and bench flow testing does show airflow advantages for catless pipes. But on a street-tuned car with the stock or mildly upgraded turbo, the power difference between a quality high-flow catted downpipe and a catless unit tends to land somewhere between 5–10 whp at most. Some back-to-back dyno comparisons across platforms have shown the gap to be statistically negligible at stock turbo power levels.

Where catless does pull ahead more noticeably is on bigger turbos at higher power targets. A larger turbocharger moving more exhaust mass will see a more pronounced benefit from eliminating the cat entirely. But for the majority of street Evo X builds running the stock 4B11T turbo or a modest upgrade, a high-flow catted downpipe will deliver nearly identical real-world performance — and the trade-offs of going catless are substantial.

The Invidia catted downpipe for the Evo X, for example, uses 76mm (approximately 3-inch) polished 304 stainless steel with smooth mandrel bends and two O2 sensor bungs — a setup that flows well and supports meaningful power gains. STM Tuned carries this option alongside their own USA-made Evo X exhaust components, including the STM Evo X Recirculated O2 Housing Downpipe, which is hand-fabricated, TIG-welded, and back-purged in Webster, NY.

CEL Risk and the Evo X O2 Sensor Setup

This is where catless setups cause the most friction for street drivers. The Evo X runs two oxygen sensors — the front sensor sits in the downpipe, and the rear sensor is located in the catalytic converter section. Cobb’s own Evo X diagnostic documentation is specific about what happens without a cat: codes P0139 (rear O2 slow response) and P0140 (no activity detected) are both commonly triggered on vehicles running catless downpipes. The recommended fix, according to Cobb’s documentation, is to install a catted downpipe.

A catless setup will almost certainly throw a CEL without a tune that disables the rear O2 monitor. Even with a tune, some owners report intermittent codes depending on driving conditions. A high-quality catted downpipe with a sufficient cell-count catalytic converter — typically 200 cells or higher — gives the rear O2 sensor enough catalyst activity to stay quiet, especially when paired with a proper ECU calibration.

For street use, a persistent CEL is more than an annoyance. In states that run OBD-II emissions testing, a car with active codes will fail outright regardless of tailpipe output. That alone makes catless a non-starter for many owners who want to keep the car legal.

Emissions Legality and Street Drivability

Removing a catalytic converter from a federally-certified vehicle creates real legal exposure. The EPA’s Clean Air Act prohibits tampering with emissions control systems on street-driven vehicles, and while enforcement against individual owners is rare, it does create complications at state-level inspections — particularly in states with visual or OBD-II checks. Without a catalytic converter, a car is likely to fail its emissions test due to increased pollution output, and the absence of the converter may trigger the CEL because the ECU detects an issue with the emissions control system.

Beyond legality, the drivability difference is something most street owners underestimate. A catless downpipe tends to produce a sulfur smell under hard acceleration, especially at lower speeds or in traffic. The exhaust note also gets significantly louder and harsher — which some owners enjoy on track days but find fatiguing on a daily commute. Open dump-style catless setups are even more extreme; venting the wastegate directly to atmosphere is not legal for use on public roads and produces an aggressive sound that draws attention in all the wrong ways.

A catted downpipe with a high-flow converter hits a practical middle ground: meaningfully better flow than stock, no rotten-egg smell, manageable sound levels, and the ability to pass inspection. For owners who want to push the build further, pairing a catted downpipe with a quality high-flow cat in the mid-pipe position — like the STM Evo X GESi EPA Stainless High Flow Cat — recovers additional flow without the emissions and drivability penalties of going fully catless. The STM unit uses a 300-cell GESi Gen 1 EPA-compliant converter rated to support up to 850 HP, bolts directly to the stock downpipe location, and includes both O2 bungs for factory sensor and wideband use.

For owners running a catted downpipe and looking to complete the exhaust system, the STM Evo X Stainless Cat-Back Single Exit Exhaust weighs just 18.2 pounds versus the 43.35-pound factory unit — a 25-pound weight savings — and bolts directly to stock or aftermarket 3-inch cats with all gaskets and hardware included.

Which Should You Buy?

For a street-driven Evo X — one that commutes, sees highway miles, goes through annual inspection, or gets driven to the track rather than trailered — a catted downpipe is the right choice. The power gap between a quality catted unit and a catless pipe is small enough that most drivers won’t feel it. The CEL headaches, emissions exposure, and daily drivability compromises that come with going catless are real costs that add up over time.

Catless makes sense in a narrow set of circumstances: dedicated track cars that are never driven on public roads, builds targeting maximum power on large turbos where every cfm matters, or owners in states with no emissions testing who have a full tune and are comfortable managing the O2 monitor.

For everyone else, a catted downpipe paired with a high-flow cat section delivers 90%+ of the performance upside with none of the street-use headaches. STM Tuned stocks a range of catted Evo X downpipe options — including the Invidia HFC downpipe and their own USA-made recirculated O2 housing downpipe — and over 90% of orders ship same-day from their facility in Webster, NY. If you’re building an Evo X that needs to be fast and livable, that’s the combination worth buying.

Quick Reference: Catted vs Catless for Street Use