The BMW B58 engine is one of the most powerful and tuner-friendly inline-six engines produced in the modern performance era. Found in vehicles like the BMW M240i, 340i, 440i, 540i, M340i, Toyota Supra, and more, the B58 responds incredibly well to bolt-on modifications. Among the most popular upgrades is the B58 catless downpipe, known for delivering the highest exhaust flow, fastest turbo response, and maximum performance gains compared to catted alternatives.

We break down everything you need to know about installing and using a B58 catless downpipe in 2025—benefits, sound improvements, tuning essentials, drawbacks, costs, and more.

What Is a B58 Catless Downpipe?

The downpipe is the section of exhaust that connects directly to the turbocharger. On the B58, the factory downpipe includes a restrictive catalytic converter that reduces exhaust flow to meet emissions requirements.

A B58 catless downpipe completely removes the catalytic converter, replacing it with a free-flowing pipe. This eliminates exhaust restriction, allowing the turbo to spin more efficiently and push out exhaust gases with minimal resistance.

The result? Faster spool, more horsepower, and a dramatically more aggressive sound.

Performance Benefits of a B58 Catless Downpipe

Upgrading to a catless system brings the strongest gains available from any downpipe option. Here’s what you can expect:

1. Maximum Horsepower Gains

With a tune, a B58 catless downpipe can add:

• 25–45 HP

• 30–70 lb-ft of torque

These gains come from improved exhaust flow and optimized turbo efficiency.

2. Faster Turbo Spool

With no catalytic converter blocking flow, the turbo reaches boost sooner. Drivers experience:

• Sharper throttle response

• Reduced turbo lag

• Stronger mid-range power

This makes the car feel significantly quicker even before tuning.

3. Louder, More Aggressive Exhaust Sound

A B58 catless downpipe dramatically changes exhaust tone. Expect:

• Deep, throaty idle

• Loud, aggressive acceleration

• Sharper turbo whistle

• More pronounced pops and burbles (with tune)

This is the go-to choice for enthusiasts wanting the most raw and sporty sound from their B58.

4. Lower Exhaust Temperature & Backpressure

With less restriction, temperatures drop around the turbo area. This can:

• Reduce heat soak

• Improve long-term turbo reliability (when tuned properly)

Drawbacks of a B58 Catless Downpipe

While performance is unmatched, a catless setup also comes with considerations.

1. Not Emissions Compliant

A B58 catless downpipe is not legal for street use in many regions. It will fail emissions testing and may cause compliance issues depending on local laws.

2. Strong Exhaust Smell

With no catalytic converter, raw exhaust fumes are more noticeable—especially on cold starts or in enclosed spaces.

3. Check Engine Light Likely

Removing the catalytic converter triggers an O2 sensor fault. A tune is required to prevent a check engine light.

4. May Be Too Loud for Some Drivers

The sound increase is significant. Daily drivers who prefer quieter operation may find it excessively aggressive.

Do You Need a Tune for a B58 Catless Downpipe?

Yes. Tuning is highly recommended and considered mandatory.

A Stage 2 tune will:

• Remove the catalyst efficiency CEL

• Adjust air-fuel ratios

• Control boost safely

• Maximize horsepower gains

• Improve drivability

Running a B58 catless downpipe without tuning can lead to improper fueling and reduced performance.

Installation Tips

Installing a downpipe on the B58 engine requires basic mechanical knowledge; most shops complete the job in 1–2 hours. Here’s what you should know:

1. Choose Good Quality Steel

304 or 316 stainless steel offers maximum durability and heat resistance.

2. Look for Smooth Welds & Mandrel Bends

These ensure the highest possible flow and reduce turbulence.

3. Heat Shielding Matters

Because catless setups run hotter, choose a version with:

• Thermal wrap

• Built-in heat shielding

• Cerakote or ceramic coating

This protects engine components and wiring.

4. Pair With a Midpipe or Catback for Best Sound

A B58 catless downpipe transforms the exhaust tone, but combining it with a performance midpipe gives the deepest, cleanest sound.

Cost of a B58 Catless Downpipe

As of 2025, typical pricing includes:

• B58 catless downpipe: $200–$450

• Premium or brand-name options: $500–$900

• Installation labor: $100–$250

This makes it one of the best value-per-horsepower upgrades for the B58.

Is a B58 Catless Downpipe Worth It?

For enthusiasts seeking maximum performance, raw sound, and the most responsive turbo behavior, the B58 catless downpipe is unquestionably worth it. When paired with a proper Stage 2 tune, the B58 platform becomes dramatically faster, more exciting, and significantly more capable on both street and track.