The BMW M3 and M4 are designed for performance, but the catalytic converter downpipes are a roadblock for achieving it. It is like strapping a horse to a mobility scooter. Switching to catless downpipes removes any and all restrictions, allowing your S58 engine to breathe like a dragon.
The gains? You get a turbo that is rougher than a bear, an exhaust note with an aggressive snarl and crackle, and real attainable horsepower. Adding an S58 catless downpipe is not just an upgrade; it is a revolution. It turns a refined beast into an untamed monster. This change goes beyond raw numbers, it is about the performance of an uninhibited vehicle. So, are you ready to unleash the beast?
Understanding Catless Downpipes
Your BMW’s factory downpipes are designed to keep things quiet and clean. But if you’re after maximum performance, they’re just holding you back. Catless downpipes strip out the restrictive catalytic converters, unlocking faster turbo response, big power gains, and a brutal exhaust note that turns heads.
You may wonder, “Why Go Catless?” Well, the straightforward answer is:
- Power Unleashed: Less backpressure means real horsepower gains (especially when tuned).
- Turbo Symphony: Hear every whistle, spool, and aggressive crackle.
- Throttle Response: Your turbos breathe easier, reducing lag.
What are The Performance Gains?
Now let’s talk about numbers. For all car enthusiasts, the easiest way of understanding is to talk about the gain in numbers. Here are the power improvements you will get:
- Increased 25-30 wheel horsepower (depending on tuning)
- Increased 30-60 wheel torque (depending on tuning)
- Peak RPM gain between 4,000-6,500 RPM
- Up to 8% reduced lag in turbo
But what about the gains in acceleration? Here are the real-world acceleration benefits:
| Setup | 0-60 MPH | 60-130 MPH |
| Stock | 3.8s | ~10.5s |
| Catless + Tune | 3.4-3.6s | ~9.2s |
Is that all? Of course not! Let’s have a look at the technical advantages of installing a catless downpipe into your S58 engine:
- 17-22% reduction in exhaust backpressure (measured via pre/post dyno)
- Exhaust gas temps drop 50-75°F, reducing heat soak during hard driving
- 5-2.5 psi boost increase from improved flow (logged data)
That sounds impressive, doesn’t it? However, to unlock the full potential of your catless downpipes, you need to take care of some supporting upgrades too.
- ECU tuning to unleash the peak potential
- Intercooler upgrade to ensure consistent power
- Upgrade to a high-flow intake to balance out the airflow
Here is a pro tip: The sweet spot of a catless upgrade is Catless downpopes + Stage 2 tune. It will get you 550-600 WHP on pump gas!
Legal and Environmental Considerations
Catless downpipes exist in a regulatory gray area that every performance enthusiast should understand:
Legal Considerations
Here are the legal considerations regarding catless downpipes:
Federal Law Violation (U.S.)
- Removing catalytic converters is technically illegal under the Clean Air Act
- EPA can impose fines up to $4,819 per violation (though rarely enforced on individuals)
State Emissions Testing
- Instant Fail States: CA, NY, NJ, MA (visual + OBD inspection)
- Possible Pass States: TX, FL (OBD-only if tuned to suppress CEL)
- No-Test States: Still federally non-compliant
Environmental Considerations
While the performance gains are undeniable, there are ecological trade-offs:
Emissions Increase
- 5-8 times more nitrogen oxides (NOx)
- 10 times more carbon monoxide (CO)
- Particulate matter rises significantly
Sound Pollution
- Often exceeds 95dB at wide-open throttle
- Many municipalities enforce 80-85dB noise ordinances
Making It Work Legally
Some alternatives can make your car legal for street drives.
- High-flow catted alternatives (200-300 cells) offer 90% of the performance
- Track-use exemption (with proper documentation)
- Swap-back solutions for annual inspections
Tuning Considerations
Going catless isn’t plug-and-play – your ECU needs recalibration to maximize gains and avoid issues:
Check Engine Light Prevention
- Catless setups trigger P0420/P0430 catalyst efficiency codes
- Proper tuning disables these codes while maintaining full OBD-II functionality
Performance Optimization
- Adjusts air/fuel ratios for the increased exhaust flow
- Eliminates factory torque limits in lower gears
- Can add 30- 50 WHP over catless pipes alone
Tuning Solutions Compared
| Tune Type | Power Gain | CEL Fix |
| Off-the-Shelf (OTS) | 20-30 WHP gain | Yes |
| Custom Dyno Tune | 30-50 WHP gain | Yes |
| Piggyback | 15-20 WHP gain | No |
Pro Tuning Tips
- Always flash before driving – the stock tune can cause lean conditions
- Monitor AFRs – catless setups run richer, requiring careful tuning
- Consider flex fuel – unlocks another 50- 80 WHP when paired with catless
Potential Issues
- Overboost situations if not properly tuned
- Warranty implications: flashes are detectable
- Smog check readiness: some tunes fail OBD monitors
However, there are some smart steps you can take to avoid such issues.
- Install catless pipes
- Immediately flash Stage 2 tune
- Log data to verify safe operation
Conclusion
Catless downpipes unleash the S58’s full potential with explosive power gains and intoxicating sound, but come with real trade-offs in emissions compliance and noise. The ultimate upgrade for track-focused builds, yet street-driven M3/M4 owners may prefer high-flow catted alternatives for balanced performance without the legal headaches. Choose wisely based on your priorities and local regulations.
