A catless (race) downpipe removes the catalytic converter from the first section of exhaust behind the turbo, while a catted downpipe keeps a high-flow converter in the same pipe. Catless flows the most and is the loudest, but it is built for off-road and competition use. A catted downpipe gives up a little flow in exchange for less smell, less rasp and a better chance of keeping the car's emissions monitors happy. The right pick depends on how and where the car is driven.
This guide explains what a downpipe does, what the cell numbers mean, and how to choose between a race pipe, a 200-cell pipe and a 400-cell pipe for a tuned turbo BMW, Audi, Mercedes-AMG or supercar.
What a Downpipe Does on a Turbo Car
On a turbocharged engine, exhaust gas leaves the cylinders, spins the turbine and then exits through the downpipe. The downpipe is the first section after the turbo, and on most modern cars it also houses the factory catalytic converter. Because it sits right behind the turbine, any restriction there raises the pressure the turbo has to push against. Lowering that backpressure lets the turbo spool more easily, which is why the downpipe is usually one of the first exhaust parts owners change.
Factory converters use a dense substrate so they can clean the exhaust quickly from a cold start. That density is exactly what makes them restrictive. Aftermarket downpipes either remove the converter or swap it for a less dense, high-flow core.
Catless, 200-Cell, 400-Cell and Factory-Style Catted Explained
The cell number describes how many channels per square inch the converter core has. Fewer cells means larger passages, less restriction and less surface area for the catalyst to work on.
| Type | What's inside | Flow | Sound and smell | Typical use |
|---|---|---|---|---|
| Catless / race | No converter | Highest | Loudest, most rasp, strongest exhaust smell | Track and competition builds |
| 200-cell | Open high-flow core | Very high | Loud, deeper tone, less smell than catless | Aggressive builds that still want a converter |
| 300/400-cell | Denser high-flow core | High | More refined, closest to stock manners | Daily drivers that want a freer-flowing exhaust |
| Factory-style catted replacement | Factory-spec converter | Stock | Stock | Replacing a damaged original without changing the car |
Some downpipes also include resonators. The IRL resonated S58 pipes, for example, are listed as cutting rasp for a deeper, smoother note while keeping a race-style flow path.
Power Difference in Practice
Every listing on our store describes the same basic result: less backpressure, quicker spool and sharper throttle response. How much that turns into measurable power depends on the engine, the turbo, the fuel and above all the tune. A catless pipe removes the most restriction, a 200-cell pipe sits very close behind it, and a 400-cell pipe trades a bit more flow for refinement. On a mildly tuned car the gap between them is usually smaller than the gap between stock and any of them. If you are chasing every last bit of flow on a big-turbo build, the race pipe is the logical choice. If you want most of the response with fewer compromises, a high-flow catted pipe makes sense.
Smell, Sound and Drone
A converter does more than clean emissions; it also quiets the exhaust and removes much of the raw fuel smell. Without one, cold starts and idling in a closed garage smell noticeably stronger, and passengers in following cars will notice too. Catless pipes also add rasp, a harsh higher-pitched edge in the tone. A 200-cell core takes some of that edge off, and a 300 or 400-cell core takes off more. Drone, the steady booming at highway cruise, is shaped more by the rest of the exhaust (mid-pipe, resonators, mufflers and valves) than by the downpipe alone, but a louder downpipe gives the rest of the system more to deal with.
Check-Engine Lights and Tuning
Modern cars use an oxygen sensor after the converter to judge whether the converter is still working. Remove the converter, or fit a less dense one, and that sensor sees a different signal. Many listings say plainly that a check-engine light may occur. The usual fix is a tune that accounts for the new hardware, and several listings recommend a supporting tune to get the most from the pipe. Talk to your tuner before you order so the software and the hardware match.
Most downpipes reuse the factory oxygen sensors, so have the right socket on hand before you start. Seized sensors are common on older exhausts.
Emissions and Off-Road-Use Rules
Catless and high-flow downpipes and other emissions-related parts are intended for off-road and competition use and may not be legal for road use where you live. Check your local laws and inspection rules before you buy or install one. This applies to catted high-flow pipes too: a high-flow converter is not the same as a certified replacement. Our catless parts are marked for off-road use in their listings, and many state the same thing in their "Good to know" notes.
Which One to Choose
- Track or competition car: a catless race pipe gives the most flow. Plan the tune and expect more noise and smell.
- Aggressive weekend car: a 200-cell pipe keeps a converter in place with flow close to a race pipe.
- Daily driver: a 300 or 400-cell pipe is the most refined option and is the easiest to live with every day.
- Show car or damaged original: a factory-style catted replacement keeps the car as it was.
Parts and Tools
- IRL S58 200-Cell Race Downpipes — 200-cell metallic converters for the G80 M3, G82 M4 and G87 M2
- IRL S58 400-Cell Catted Downpipes — the more refined high-flow option for the same cars
- STRATEC S58 High Flow Catted Downpipes — keeps high-flow converters, listed as suiting daily drivers
- Spool B58 5 Inch 200-Cell Catted Downpipe — catted version of Spool's 5 inch B58 pipe
- Spool B58 5 Inch Race Downpipe — the catless version of the same design
- IRL F97 X3M / F98 X4M Resonated Downpipes — resonators added to reduce rasp
- STRATEC Lamborghini Urus Factory Catted Replacement Downpipe — factory-spec converter for a like-for-like replacement
- O2 Sensor Socket & Thread Chaser Set — for removing and refitting oxygen sensors and cleaning their threads
Browse every option in Exhaust & Downpipes, or narrow it down by make in BMW Performance Parts and Supercar & Premium Exhausts. If you need to read or clear codes after the install, our OBD2 scanner basics guide covers the essentials.
Shop: Catted & High-Flow Cat Downpipes and Catless Race Downpipes.
Install Safety
Only work on a cold exhaust; downpipes sit next to the turbo and stay hot long after the engine is off. Support the car on proper jack stands or a lift, never on a jack alone. Penetrating oil on the flange nuts and sensor threads the night before saves a lot of trouble. Professional installation is recommended in many listings, and it is worth it if you do not have a lift.
Frequently Asked Questions
Is a catted downpipe quieter than a catless one?
Yes. The converter core absorbs some of the sound and takes the harsh edge off the tone. A 400-cell pipe is generally quieter than a 200-cell pipe, and both are quieter than a race pipe.
Do I need a tune for a downpipe?
Many listings recommend a supporting tune, and a tune is the usual way to manage the oxygen-sensor readings that can set a check-engine light. Check the listing for your part and speak with your tuner before installing.
Will a catted downpipe pass inspection?
Do not assume it will. High-flow converters are not the same as certified replacement parts. Check your local laws and inspection rules first.
Do downpipes reuse the factory oxygen sensors?
Most do. Check the listing for your specific part; some include sensor bungs, extensions or heat shields that change how the sensors are fitted.