Updated: Jan 25
A de-cat exhaust is an exhaust system which has had the catalytic converter removed, hence the name "decat". Its often done to improve power output, increase exhaust sound and improve exhaust tone.
When a car is decatted, the catalytic converter is removed and replaced with a hollow metal pipe, generally the same size as the exhaust its connecting to. Other exhaust systems such as a straight pipe are also decatted.
Most modern vehicles, particularly in Europe, are equipped from the factory with one or more catalytic converters, their job is to reduce harmful emissions.
However, a catalytic converter is a restriction and can reduce exhaust gas flow and increase back pressure, therefore removing the catalytic converter will lessen restriction allowing for more power.
Table of Contents:
Why Decat a Vehicle?
Most people decat their vehicle in hopes to boost power and torque. Removing a catalytic converter will reduce back pressure and exhaust gas flow, resulting in the following effects.
Improving the exhaust scavenging effect
Reducing pumping losses
Allowing for increased boost pressures
You may improve power and torque by around 1-5% providing the correct supporting modifications are installed and the ECU is programmed to make use of the improved exhaust gas flow.
Advantages of a De-Cat Exhaust
There are some clear advantages to a de-catted exhaust system compared to an OEM exhaust with a catalytic converter.
Improved Exhaust Sound
Aside from the de-cat reducing emissions, it also behaves similarly to a muffler / silencer in that it reduces engine noise; some people think a catalytic converter makes a vehicle sound "softer" and less aggressive.
Removing the catalytic converter will allow more sound to escape the exhaust, producing a louder, deeper and more aggressive exhaust noise.
Here's a video demonstrating that.
Improved Power and Torque
When modern catalytic converters are removed, it only results in a minor increase in horsepower and torque, yet this is still an improvement.
According to a 2006 research on a 1999 Honda Civic, removing the factory catalytic converter resulted in a 3% gain in maximum horsepower.
If the Honda Civic had 160 horsepower, removing the catalytic converter pushed it to roughly 164-165 horsepower (3%).
However, modern metallic-core catalytic converters are considerably more efficient, lowering horsepower gains by just around 1% when compared to no catalytic converter.
For example, removing a current metallic-core catalytic converter from a Honda Civic with 160hp would only result in a 1-2hp boost.
Some tuners require the removal of the catalytic converter when remapping a car to allow for pops and bangs, anti-lag, and even flame maps.
The power difference between a decat exhaust and a stock exhaust with a catalytic converter is small. Although it's a small boost in power it's still a boost.
Similarly to the impact of a catalytic converter on performance, a decatted exhaust may also marginally affect efficiency.
Though this drop in efficiency is likely to be quite minor, with perhaps a few percentage points change at most.
Nonetheless, decatting a car will enhance vehicle fuel efficiency marginally, this is due to a decrease in back pressure in the exhaust system.
Disadvantages of Decatting a Car
Although there may be a small boost in power and efficiency and an improved exhaust tone, there are some serious disadvantages to a de-cat exhaust.
Causes a check engine light. Typically a catalytic converter will cause the check engine light to come on, this is due to the oxygen sensor sending a signal to the ECU that the catalytic doesn't work.
Increased noise. Some people like increased exhaust noise, other people don't. A decat exhaust can increase the loudness of the exhaust.
Cabin drone. A straight decatted exhaust system can produce vibrations and allow harsh sound waves from the exhaust to pass through the vehicle into the cabin, producing a "drone" inside the car, even while driving normally.
Increased emissions. A decat removes the catalytic converter. This will cause the vehicle to produce more emissions and fail emissions testing in most circumstances.
Illegal in most countries. Since most countries have emissions testing for road vehicles, decatting a car would likely make it illegal on the road.
May not get insured. Some car insurers won't insure a car with illegal modifications, this includes straight piped exhaust and decatted exhausts.
May be restricted from racing tracks. If the decat exhaust causes the vehicle to be over the decibel sound limit you may not be allowed to drive on the track.
Can a De-Cat Exhaust Harm Your Engine?
A decat exhaust system should not hurt or damage an engine. Although emissions control components such as the catalytic converter are required to meet legal requirements, they are not required for a vehicle to operate.
If these parts are removed, no damage should occur as long as the exhaust sensors (oxygen sensor, exhaust gas temperature sensor, etc.) remain in place.
Should You Decat Your Exhaust?
Modern catalytic converters do not substantially impede exhaust flow, therefore the potential advantages of a decat are minimal unless required for certain modifications.
In a 2006 test on a 1999 Honda Civic, replacing the original catalytic converter resulted in a 3% boost in maximum horsepower.
A modern metallic-core converter reduces the power by just 1% compared to with no catalytic converter.
It is also unlawful in many countries to use a decat exhaust on public roads. It can also cause the check engine light to activate and.
It's typically not worth installing a decat exhaust to your vehicle.
Sports Catalytic Converter
A sports catalytic converter (also known as a sports catalyst) is a less restrictive and efficient variant of a conventional catalytic converter. It typically filters exhaust gases to a lower threshold level while still meeting emissions requirements.
Generally a sports catalyst will produce very similar power increases to a decat while still meeting the emissions regulations. It also avoids the check engine light being illuminated and will usually pass emissions testing.
The sole disadvantage of using a sports catalytic converter is that you cannot use pop and bang maps, anti-lag, or other systems, these will require a decat.