2018-2021 WRX STI Cat-Back Exhausts

A cat-back exhaust replaces everything from the catalytic converter rearward on your 2018-2021 WRX STI — mid-pipe, resonator (if equipped), and mufflers. It is the single most popular first performance modification for a reason: meaningful sound transformation, a small but real power gain once tuned, and zero emissions or inspection concerns since all factory catalytic converters stay in place.

Start with tubing diameter. For a stock-turbo WRX STI, 3-inch systems are the standard and support everything up to moderate turbo upgrades. Larger diameters only help once you are making serious power; on a stock car they cost low-end response and add drone for no benefit. Material matters for longevity: T304 stainless steel is the baseline you should expect at this price point, while titanium saves 20-plus pounds but at a steep premium.

Sound level deserves honest thought about how you actually drive. "Mild" systems stay near stock at cruise with a deeper tone under load — the right pick for a daily driver. "Aggressive" systems are genuinely loud and will drone on the highway; great for weekends, tiring every day. Resonated mid-pipes tame rasp and drone without killing character, and they are worth the money on long-tube or equal-length header setups.

Tip finish — polished, burnt blue, matte black, or carbon fiber — is personal taste, but confirm tip size clears your bumper. Filter this collection by material, sound level, and resonated options to find the right system for your 2018-2021 WRX STI.