Decat/Catless Downpipe vs Catted

Decat/Catless Downpipe vs Catted

A catless downpipe replaces the factory catalytic converter, allowing maximum exhaust flow which increases hosepower and makes turbo reach boost faster. However, after removing the catalyst, the second O2 sensor detects no change in CO emissions before and after the catalyst so it sends signals to the ECU, triggering fault codes like P0420 (Catalyst Efficiency Low) and turning the check engine light. On the other hand, a catted downpipe retains a high-flow catalytic converter, balancing performance with emissions compliance. While it doesn’t provide as much power as a catless downpipe, it reduces exhaust restrictions while keeping emissions in check.

In this article, we'll compare Decat and catted downpipes, breaking down the pros, cons, and which option is best based on your performance goals, local regulations, and daily driving needs.

Catless Downpipe Pros & Cons

Pros:

1. Increased Horsepower & Torque
2. Faster Turbo Spool
3. Aggressive Exhaust Sound
4. Lower EGT
5. Cost Effective

Cons:

1. Check Engine Light
2. Fails Emission Tests
3. Environmental Impact
4. Unpleasant Exhaust Smell
5. Voids Warranty

Problems after installing Decat:

After decat you should remap the ECU and remove second 02 Sensor from the ECU to prevent any check engine light turning on. Schiller tuning can help you with services like DTC OFF, 02 Off, etc. If you’re a tuner you can read the original ECU file using ECU TUNING TOOLS and if you’re a car owner who wants to decat you can reach to the nearest dealership in your area and send us your ECU file through our Tuning File Service, our experts will send you the remapped file in a short time.

What does a high flow catted downpipe do?

The catted downpipe is in the middle of the spectrum, a balance between power and legality. It improves the exhaust flow, reduces backpressure, improves turbo spool time, reduces the CO smell and it’s legal. This allows for better performance and while still reducing emissions. 

After installing a catted downpipe, the second O2 sensor doesn’t trigger any DTCs so there is no need to DTC remove.

How does a catted downpipe affect the sound of your car?

A catted downpipe deepens and enhances the exhaust note, making it slightly louder and more aggressive than stock, but quieter than a catless downpipe. It strikes a balance between performance and noise, giving a refined growl without excessive drone or rasp.

How does downpipe increase HP?

It is a saying that the engine breathes better after installing a downpipe, and that is true it does! By replacing restrictive factory catalytic converter with a high flow or catless downpipe the backpressure is decreased and turbo spools faster. This results in an increase in both torque and horsepower.

Is a catted downpipe legal?

90% of the time the catted downpipe is legal compared to the no cat one. The catted downpipe reduces emissions and can pass emission test so its legal, but in some regions and states like California for example they might not meet the OEM standard. So you should always check local laws before installing one.

Is it safe to run a downpipe without a tune?

It is no recommended to setup a downpipe without proper ECU tuning. The check engine light is turned on if you decat. The AFR becomes lean or rich, reducing power power gain. Most of the time the AFR tends to become leaner after decat and lean AFR can cause excess heat in both the cylinder and the exhaust. EGT might become so high that it can destroy the turbo and downpipe overtime. An optimized remap can unlock horsepower safely and ensuring everything runs smoothly.

Do I need to remap after downpipe? 

Yes, it is essential to remap the ECU in order to recalibrate the stock maps. The stock ECU file is set up for the factory catalyst, this makes that after installing the downpipe you might not even feel a thing. That’s because there are certain maps that limit torque, AFR, EGT, etc. These limiters won’t allow the downpipe to show its true power, therefore it’s important to have professionals remap the ECU to reach the best optimization.

Schiller tuning, with over a decade of experience in ECU tuning, offers expert remapping services for petrol and diesel engines using professional software like WinOLS. Whether you're a tuner or car owner, simply send us your ECU file through our Tuning File Service, and our experts will provide you with the best remap to unlock your vehicle’s full potential.

Can I run Stage 2 without downpipe?

It is not ideal. In stage 2 the boost pressure is increased we need to have better exhaust flow and less backpressure. With the stock catalytic converter, the boost pressure is limited to a certain threshold. Without the downpipe, you might experience higher EGT, less efficient turbo spool, and reduced overall performance. For optimal results, it's highly recommended to include a downpipe as part of a Stage 2 setup.

Are downpipes only for turbos?

Downpipes are most commonly associated with turbocharged vehicles, but they can also be used on naturally aspirated engines, though their benefits are more pronounced on turbo setups. It allows the exhaust gases to exit the turbo more efficiently, which is essential for better performance, especially in modified setups like Stage 2 or 3 tunes. In NA engines, the benefits of a downpipe are less significant since there is no turbo to feed. You can always install headers for your naturally aspirated engine and gain a lot more. If you want to learn more about headers you can check out THIS ARTICLE.

Is a catless downpipe safe?

It is safe for the engine if the ECU is tuned properly but dangerous for lungs. A catless downpipe offers performance gains, especially in turbocharged vehicles, but may cause emissions issues and legal concerns. Be sure to check local laws, and if installing one, remap your ECU to avoid check engine lights and optimize performance.

Do catless exhausts smell?

Yes they do. There is nothing there to filter out the CO, HC and NOx emissions so there is always some odors. These emissions are highly harmful for lungs and can damage environment.