CAT Delete or Gutting: Pros, Cons, Performance and Cost

The catalytic converter is a system installed in the exhaust system of an automobile to reduce harmful emissions by removing gases that cause pollution such as carbon monoxide, hydrocarbons, nitrogen oxides, and soot. Catalytic converters come in many shapes and sizes, and add a restriction to the flow of gases due to which car enthusiasts feel a need for CAT delete to get maximum performance from their engine. However, you also need to about the consequences of CAT delete which I have covered in this guide so that you can make an informed decision.

CAT Delete means installing a pipe in place of where your catalytic converters go. CAT Delete significantly reduces back pressure, making the exhaust system less restrictive, and hence, they improve engine horsepower, especially on v8 engines. If your engine has primary and secondary catalytic converters, deleting only secondary CAT will give an advantage as it has no O2 sensors and the vehicle would also pass the emission test and experience a gain in horsepower. Moreover, CAT Delete exhaust systems will be a lot louder and could produce a crispier sound. However, on V8 engines, CAT Delete exhaust systems produce a nice roaring sound. However, in states of US like California with strict emission laws, CAT delete is illegal as vehicles with cat delete exhaust emit dirty smoke with excessive soot.

Here is what the user said after a secondary CAT delete:

“Removing the secondary cats def makes a big difference. My car dyno’d at 407rwhp on a mustang dyno which is a 26% drive train loss. So that puts my crank HP at around 550. That’s a 50hp gain with just a cat back exhaust(no secondary cats).”

Why Is A Catalytic Converter Important?

A catalytic converter is a device in exhaust systems that breaks down harmful emissions (CO, NO_x and HC) into harmless substances (H₂O, N₂ and CO₂). The reason why it is important is that it helps protect the engine from damage due to excessive emissions. 

Catalytic converters work by using precious metals called catalysts, which contain platinum, palladium and rhodium coated on a ceramic honeycomb substrate. These metals are used to convert toxic substances into less harmful ones. 

The metal casing of a catalytic converter is another important component surrounding the core of a catalytic converter. This metal casing directs the exhaust gas flow through the catalyst bed. The metal casing is usually made out of stainless steel.

In engines, three-way catalytic converters are used that have the benefit of performing the oxidation of carbon monoxide (CO), hydrocarbons (HC) and the reduction of nitrogen oxides (NOx) simultaneously. They are operated at elevated temperatures greater than 600°C.

In some countries, catalytic converters are required to meet certain standards. They are also mandated by law in many states and localities. Driving a vehicle without a catalytic converter will fail an emission test, making it illegal to drive. It could also cost you a heavy penalty.

“According to Alabama’s pollution laws, driving a car in the state is illegal if its catalytic converter has been removed, switched or is not functioning correctly. Anyone who violates the law faces a $10,000 fine and a year in jail.”

Environmental Protection Agency (EPA) issued the following requirements for the legal removal of catalytic converter and installation of aftermarket catalytic converters:

• The vehicle manufacturer’s warranty for the stock catalytic converter has expired. Warranties are a minimum of 8 years/80,000.
• An inspection of the vehicle has determined that the existing converter needs replacement.
• It should be installed in the same place as the stock CAT converter.
• Aftermarket catalytic converter should be CARB (California Air Resources Board) certified.
• CARB-certified catalytic converters are mandatory for 1993, 1994, or 1996 and newer vehicles. For 1992 or older vehicle models, EPA-certified aftermarket catalytic converters can be installed.
• The aftermarket catalytic converter should be of the same type as the stock catalytic converter (two-way, three-way).

Is Catalytic Converter Delete Legal?

The legality of running a vehicle without a catalytic converter is a bit of a gray area. The actual law depends on the country or state you live in.

Some countries and states do not allow you to run cars that do not meet emissions standards. For example, in California, it is illegal to drive a vehicle without the proper catalytic converter. This means that if you drive a vehicle in California without the proper emission control devices, you will have to pay a fine to the state.

It is very important that you check with your local authorities to make sure that your vehicle is roadworthy and safe before you drive it.

CAT Delete Pros

There are some great benefits of deleting catalytic convertors that excite vehicle owners. Let’s discuss what they are:

1. Increasing Horsepower

Deleting catalytic converters removes restrictions in the path of exhaust gases. It significantly reduces the frictional losses and back pressure, allowing exhaust gases to flow freely. As a result, the engine piston does not have to work hard to overcome the friction and remove exhaust gases from the engine cylinder during the exhaust stroke.

When exhaust gases could easily be removed during the exhaust stroke, the engine cylinder will be easily replenished with more fresh air for the next combustion cycle. As a result, it will increase engine efficiency and increase engine horsepower by up to 10%. You can read my guide on straight pipe exhaust systems to understand the concept of back pressure.

Back pressure is a myth and some people think higher back pressure means the engine will have better performance. But that’s not correct. So, please read the full guide I have linked above as I have explained with facts. This will help you understand what I’ve explained below.

Also read: Does straight piping increase mpg

2. Weight Reduction

Deleting the catalytic converter from the engine can result in a decrease in the weight of the vehicle, depending on its model. A catalytic converter consists of a stainless steel shell, the ceramic monolith, and a coating of catalytic materials over the ceramic monolith. Combining all those components, the weight of a catalytic converter can be around 4 to 8 kg.

Hence, by removing the catalytic converter, you can reduce the vehicle’s weight and make your vehicle accelerate more quickly.

3. Lesser Overheating Of Your Engine

CAT delete also results in overheating of the engine. Since the engine has to do less work to push exhaust gases through a catalytic converter, the engine will remain cooler. Moreover, if a catalytic converter is clogged with carbon and ash deposits, it will block the natural flow of exhaust gases, which will cause the exhaust manifold to heat up. 

4. Smooth Throttle

By deleting the catalytic converter, you will experience a smooth throttle when you push a gas pedal. 

When the car starts moving, the catalytic converter gets heated up by the exhaust gases that go through it. Once the engine is warmed up, the catalytic converter starts working efficiently.

However, when a catalytic converter’s honeycomb substrate is clogged, it will severely restrict your engine’s ability to exhale. If the engine can’t exhale, it will not be able to inhale fresh air. An engine that can’t breathe well will stall and you will not experience a smooth throttle.

5. Amazing Deep Exhaust Sound

In V8 engines, deleting the catalytic converter results in an amazing deep roaring sound of exhaust gases that car enthusiasts like to hear when they push the accelerator pedal.

However, for some vehicles, CAT Delete causes a crispy, rattling or popping sound which can irritate the people in the surroundings.

One user says this:

“I replaced the cat with a straight pipe on my old ’95 3.4L and did not like the rasp at all! Other than that, no negatives.”

CAT Delete Cons

Here are the cons of deleting catalytic converter:

Air Pollution

Catalytic converter reduces the amount of pollutants coming from the vehicle and makes the exhaust system cleaner, improving the quality of the air that comes out of the tailpipe. After removing a catalytic converter, a vehicle will not be able to pass the emissions test. Your vehicle will emit more pollutants than before.

Toxic pollutants, such as carbon monoxide, nitrogen oxides, and unburned hydrocarbons will be released into the atmosphere from your vehicle, resulting in smog which will cause several lungs related issues.

However, if your vehicle has two catalytic converters, removing the secondary catalytic converters could still pass you the emission tests while freeing up some horsepower at higher RPMs.

Voiding Vehicle’s Warranty

Any modifications that you make in the engine, such as CAT Delete, can void its warranty. It means that if you face any problems in your engine after removing the catalytic converter, you will not be able to fix it for free.

Requires ECU Tuning

CAT Delete also requires ECU tuning. As you are messing with the exhaust system, the ECU detects changes, throws a warning code, and can turn on the check engine light. 

There are two oxygen sensors in the engine. One is before and another is after the catalytic converter. The first oxygen sensor measures oxygen in the exhaust gases so that the air/fuel mixture can be adjusted. The oxygen sensor after the catalytic converter measures its efficiency. 

After a CAT delete, the O2 sensor, which is downstream of the catalytic converter, will record high oxygen levels in exhaust gases, due to which the ECU (Engine Control Unit) will think that the catalytic system is not functioning efficiently. As a result, it will show the P0450 code, turn on the check engine light and also put your vehicle into reduced power mode.

You can install a small O2 sensor adapter into the exhaust system that will move the second O2 sensor out of the exhaust stream, due to which the ECU will think that the catalytic converter is still present and is cleaning the exhaust gases. 

Decreased Resale Price Of Vehicle

After CAT Delete and installation of aftermarket exhaust systems, the resale price of a vehicle is reduced. Customers do not trust the engine’s performance after CAT delete, and they also don’t want to get into trouble to buy vehicles that are not passing emission tests after CAT delete.

Gutted CAT vs CAT Delete (Straight Pipe)

If you’re confused about whether gutted (hollowed) CAT or CAT delete is better, I would recommend CAT Delete by replacing it with a straight pipe. The reason is simple i.e. lesser turbulence with a straight pipe compared to the hollowed catalytic converter.

A catalytic converter consists of a bunch of honeycomb-patterned catalyst bed that offers restrictions to the exhaust gases.

As you know that a catalytic converter has a larger size compared to the piping to which it is welded. If the catalytic converter is gutted, the absence of any restriction will dramatically reduce the flow velocity of exhaust gases. In engineering/physics, the larger the pipe dia, the lesser the will be flow velocity. Moreover, if gases flow from a smaller area to a larger area, their pressure increases in a region with a larger area which will try to push back the flow.

Due to the lack of restriction in a gutted catalytic converter, it will act as a pipe twice the size of straight piping. For exhaust gases to flow easily, they should have higher velocity, which is only possible if some restriction is present or the diameter of the pipe is reduced. So, a flow of gases from a 2.5″ exhaust header to a 4″ hollowed catalytic converter will cause reversion and the exhaust flow will reduce dramatically.

Moreover, gutted CAT resonates more compared to the CAT Delete system. So, there is another drawback as resonation also impedes the gas flow.

Here is what a user said:

“I tried gutted stock cat. I tried a test pipe. I am running 2.5 gutted cat now. Test pipe performs better than a gutted cat.”

Catalytic Converter Delete Cost

The cost of CAT delete depends on whether you’re doing it yourself or asking a professional mechanic to do this job. If you decide to do it yourself, you can expect to spend $70 to $200 to buy piping and some necessary tools to cut the pipe before and after the catalytic converter and weld aftermarket piping. If you engage the service of a professional auto mechanic, the cost of CAT delete is about $350-$650.

A DIY delete is an economical way to remove your catalytic converter. However, if you are not sure of what to do or you are not comfortable doing it, it is recommended that you seek the services of a professional auto mechanic.

CAT Delete: FAQs