BMW B58 downpipe guide: gains, fitment and catless vs 200-cell
August 26, 2026 · OKTAN
The B58 is one of the best straight-sixes BMW has ever built — and it responds beautifully to a freer-flowing exhaust. If you're chasing quicker spool, a stronger note and real headroom for a tune, the downpipe is where you start.
What a downpipe actually does
The factory downpipe houses a restrictive catalytic converter that chokes exhaust flow right where it matters most — at the turbo outlet. Replacing it drops back-pressure, letting the turbo spool sooner and breathe at the top end. On a supporting tune it's commonly the single most effective bolt-on you can fit.
Catless vs 200-cell high-flow
Catless gives maximum flow and the most aggressive sound, but is intended for off-road / competition use only and will usually need a tune to manage the check-engine light. A 200-cell high-flow catalyst keeps things far more street-friendly and emissions-conscious while still flowing dramatically better than stock. Choose based on where and how you drive.
What fits
Our B58 downpipe covers the M340i, M440i, M240i and Z4 M40i (G20, G22, G42, G29). It's a true bolt-on to the OEM mid-pipe — no cutting or welding — and retains your OEM sensor positions.