Is a Secondary Cat Delete Worth It? The Pros and Cons of Deleting Your Catalytic Converter

What is a Secondary Cat Delete?

A secondary cat delete refers to removing the secondary catalytic converter from a vehicle’s exhaust system. The secondary catalytic converter is an emissions control device located downstream of the primary catalytic converter. Its purpose is to further reduce harmful exhaust emissions that make it past the primary cat.

According to this source, the secondary catalytic converter catalyzes any remaining exhaust compounds not processed by the primary cat, especially during cold startup when the primary cat is not yet at peak operating temperature. It provides an extra layer of emissions control.

As explained in this source, automakers install secondary cats to help meet emissions regulations. They heat up quickly and “clean up” any leftovers passing through the primary cat.

Pros of Removing the Secondary Cat

One of the main benefits of deleting the secondary catalytic converter is increased horsepower and torque. By removing restrictions in the exhaust system, more exhaust gases can flow out, reducing backpressure. This allows the engine to breathe better and produce more power. According to forums and owners who have completed the modification, secondary cat deletes typically result in modest power gains of 5-15hp.

Removing the secondary cat can also change the exhaust note, resulting in a deeper, raspy, aggressive sound that many enthusiasts prefer. The stock secondary catalytic converter dampens sound, so deleting it makes the exhaust louder and more resonant.

Eliminating the secondary cat also reduces weight. The catalytic converter assembly is heavy, so removing it slightly decreases the car’s overall weight and improves power to weight ratio. While not a dramatic change, deleting a 10-15 lb catalytic converter does incrementally improve acceleration and handling.

Cons of Removing the Secondary Cat

While removing the secondary catalytic converter may provide some benefits, there are also some significant drawbacks to consider:

Increased emissions – Removing the secondary cat eliminates one stage of the vehicle’s emissions control system. This can lead to increased exhaust emissions, including higher levels of hydrocarbons, carbon monoxide, and nitrogen oxides.1

Check engine light – Deleting the secondary cat often triggers the check engine light (CEL) due to missing emissions monitors and sensors. The CEL may be unavoidable without reprogramming the ECU.2

Failed emissions testing – With the secondary cat removed, the vehicle is much more likely to fail an emissions test. This could prevent renewing vehicle registration in areas that require emissions testing.3

Increased smell – Eliminating a stage of the exhaust treatment can lead to an increase in the smell of unburnt fuel or other emissions. This may be undesirable for many drivers.

Is it Legal to Remove the Secondary Cat?

Removing the secondary catalytic converter is illegal according to federal law. The Clean Air Act prohibits tampering with or removing any emissions control device on a motor vehicle. This includes the secondary catalytic converter, which helps reduce pollutants from the exhaust.

The secondary cat works together with the primary cat to ensure emissions remain within legal limits. Removing it will increase exhaust pollutants beyond allowable levels. All 50 states are required to follow federal emissions regulations, so it’s illegal to remove the secondary cat in any state.

California has additional statewide emissions laws beyond the federal requirements. Removing the secondary cat is explicitly illegal under California state law. Vehicles are required to keep all their emissions control devices intact in order to be road legal in California.

While enforcement varies, removing the secondary catalytic converter violates both federal and state law. Vehicles without the secondary cat installed will fail visual inspections and emissions tests required for registration renewal. Fines, failed registration, and fixes to reinstall the secondary cat can result if caught driving without this emissions control device.[1]

Effects on Engine Performance

Removing the secondary catalytic converter can provide some modest gains in engine performance on certain vehicles. According to one forum post on M5Board, deleting the secondary cat on BMW M5 models can potentially add 8-10 horsepower (source). However, other sources suggest the power gains are negligible or minimal at best. On BMW 3-series models, owners report no noticeable difference in performance after removing the secondary cat (source).

In terms of throttle response, removing the secondary cat eliminates one more restriction in the exhaust system. This allows exhaust gases to evacuate more quickly during acceleration, which can provide a slight improvement in throttle and turbo spool up response. However, the difference is generally small and likely not very noticeable in normal driving.

Overall, deleting the secondary catalytic converter may provide a very small bump in peak horsepower and torque on some vehicles. However, the gains are typically minimal and often not noticeable during normal driving. More significant exhaust modifications are required to achieve truly meaningful power gains.

Effects on Fuel Efficiency

Removing the secondary catalytic converter can potentially lead to a small decrease in fuel efficiency, though the effects vary. According to discussions on car forums like Reddit and MBWorld, most people find they lose 0-3 MPG after deleting their secondary cats [1] [2]. This small MPG drop seems to occur because removing the secondary cat changes the backpressure and airflow in the exhaust system, which can disrupt the engine’s finely tuned fuel mapping. Re-tuning the engine after deleting the secondary cat may help regain some lost fuel economy, but often not all of it. Overall, deleting the secondary cat should not be seen as a reliable way to gain MPG, and may in fact cause a slight decrease.

Effects on Engine Health

Removing the secondary catalytic converter can potentially have negative effects on overall engine health and longevity. Without the secondary cat, more contaminants and particulate matter can reach the engine ([1] These include carbon deposits, unburnt fuel, and other engine exhaust particulates that would normally be filtered out by the secondary cat. Over time, this can lead to increased build up and fouling of internal engine components.

Additionally, the increased contaminants can accelerate wear on key engine parts like the pistons, cylinder walls, valves, and rings ([2] This potentially faster wear can lead to loss of compression, oil burning, and decreased engine performance. Most experts recommend continuing to run the secondary catalytic converter to protect overall engine health, especially for those who plan to keep their vehicle long-term.

Installation Process

Installing a secondary cat delete requires some mechanical skills and specialized tools. Here’s an overview of what’s needed:

Tools Needed:

  • Jack and jack stands
  • Basic hand tools – sockets, ratchet, wrenches, etc.
  • Cutting tools like a reciprocating saw to remove the secondary cat
  • Welder to weld in a straight pipe in place of the secondary cat

Estimated Time: 3-5 hours

The installation time can vary depending on experience level. Plan for 3-5 hours for the entire job. Removing the secondary cat and welding in the straight pipe replacement will be the most time consuming steps.

Cost: $200-$400

The parts needed like piping and clamps will be around $50-100. If you don’t have the tools, renting or buying them can add $100-200. Paying a shop for labor will add $200 or more. So total costs can range from $200 for a DIY with tools up to $400+ to have a shop do everything.

Maintenance Considerations

Removing the secondary catalytic converter does require some additional maintenance considerations. Two key areas to focus on are:

Replacing oxygen sensors – The oxygen sensors located before and after the secondary catalytic converter will need to be replaced when removing it. This is because they are designed to work with the backpressure created by the secondary cat. Without it, the sensors will get faulty readings and cause issues like check engine lights. Plan to replace the pre and post secondary cat oxygen sensors when deleting it (source:

Cleaning exhaust valves – Without the secondary cat helping burn off contaminants, more carbon buildup is likely to occur on exhaust valves. This can hamper performance over time. Plan to periodically clean the exhaust valves to remove deposits. Some recommend cleaning every 20,000-30,000 miles after deleting the secondary cat (source:

Is it Worth Deleting the Secondary Cat?

When weighing the pros and cons, deleting the secondary catalytic converter may be worth it for some users but not for others. According to one forum, removing the secondary cats provides only a slight increase in sound with little to no performance gains that you can feel ( The main benefits seem to be a slightly louder exhaust note and the ability to pass emissions testing in areas that do not do visual inspections. However, deleting the secondary cat is illegal in many places and can result in fines if caught.

For users who are simply looking to increase the aggressiveness of their exhaust sound, a secondary cat delete may be worthwhile. According to users on Camaro6, it will provide a moderate increase in volume without being excessive ( This change may be desirable for driving enjoyment. However, users looking solely for performance gains are unlikely to notice any improvement. Therefore, secondary cat deletes are probably not worth it for those users.

In summary, secondary cat deletes make the most sense for individuals who want a louder exhaust note for the driving experience and are willing to take on the legal risks. For most others, the minimal benefits likely do not justify removing the secondary catalytic converter.

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top