Updated: Oct 16, 2021
Replacing the catalytic converter with a less restrictive metal pipe is known as a decat, it is a common modification done on high-performance tuner cars and track-based racing cars.
For most people, it is not worth decatting your car. Installing a decat is illegal in most countries and only provides a small performance increase when compared with a less restrictive sports catalytic converter.
In this article I will outline the benefits and drawbacks of a decat.
Table of Contents:
Benefits of a Decat
The catalytic convertor is a piece of equipment fitted to a vehicles exhaust on essentially every modern car, it is deigned to reduce exhaust emissions and toxic fumes.
A decat involves removing the catalytic converter and replacing it with a hollow metal pipe, usually the same size as the exhaust.
Although a decat is illegal and increases the output of toxic gasses, contributing to negative health effects and air pollution, it does have some benefits for performance, as listed below.
Enhanced Exhaust Sound
A decat acts similarly to a muffler, it removes some of the sound from the engine, some people say it makes a car sound "softer".
The video below demonstrates this.
The sound with a decat is slightly louder with a deeper tone, it doesn't sound as muffled or "soft" as the standard catalyst or the sports catalyst.
You can hear more sound from the turbo and backfiring from the engine.
Slightly Improved Performance Potential
Modern catalytic converters are very efficient and only result in small improvements in horsepower and efficiency when removed, though it is still an improvement.
One 2006 study on a 1999 Honda Civic revealed that removing the stock catalytic converter resulted in only a 3% increase in maximum horsepower.
If the Honda Civic had 160hp, removing the catalytic converter increased the horsepower to around 164-165hp (3%).
Though, newer metallic-core catalytic converters have even higher efficiency, only reducing horsepower around 1% compared with no catalytic converter.
For example; the Honda Civic with 160hp would only gain around 1-2hp if you removed a modern metallic-core catalytic converter.
Although the differences get higher with more horsepower, it would still be a very insignificant difference in power.
Still, some tuners require the catalytic converter to be removed if they're remapping a vehicle to accommodate for pops and bangs, anti-lag, and even flame maps.
The power difference between a car with a decat and a stock cat is very minimal and probably unnoticeable, though power is still increased and so is tuning potential.
Slightly Improved Efficiency
Similarly to a catalytic converters effect on performance, it can also slightly reduce efficiency.
Though this reduction in efficiency is probably also very minimal and just a few percentage points difference at the most.
Still, decatting a car will very slightly improve vehicle efficiency due to a reduction in back pressure in the exhaust system.
Drawbacks of a Decat
Installing a decat to a vehicle is illegal in many countries and regions.
It increases the exhausts output of toxic fumes and gasses, contributing to negative health effects and air pollution.
The following are all of the drawbacks to decatting a vehicle.
Installing a decat onto a vehicle will remove its ability of emission reduction, a catalytic converter will usually reduce unburned hydrocarbons (CnHn) and nitrogen oxides (NOx).
Removing a catalytic converter will result in increased output of unburned hydrocarbons and nitrogen oxides.
Will Fail Its Mot
This increase in emissions and toxic gasses will cause a vehicle to fail its MOT.
Installing a high-flow sports cat is a good midpoint for a modified or tuned car, improving performance, sound and efficiency slightly more than a stock catalytic converter while retaining its ability to reduce emissions and pass an MOT.
Illegal in Some Countries
Since most vehicles need a catalytic converter to satisfy emission standards, driving without one on public highways is generally prohibited and illegal.
Some countries have no laws prohibiting the usage of a decat, but most modern countries require a vehicle to have one fitted.
In the UK, It is illegal to drive a vehicle that has been changed such that it no longer meets the vehicle emissions regulations it was intended to fulfil.
If a car is stopped and emission tested in which it fails, the potential penalty for a car is £1,000, while the potential penalty for a van, truck, or bus is £2,500, in the UK.
Could Cause a "Check Engine" Light
Most of the time, a car equipped with a decat will illuminate the "check engine" light because the sensors detect a value that is outside of the target range.
Typically, this is due to an oxygen sensor in the exhaust detecting that the catalytic converter efficiency is less than its target.
This, of course, would occur in the absence of a catalytic converter.
Some laws in specific countries have exhaust decibel limits, sometimes fitting a decat may exceed these noise regulations.
Other than the law surrounding exhaust noise, some people may find the sound of a decatted vehicle annoying or obnoxious.
Though, for other people this is a benefit.
Should You Install a Decat?
Modern catalytic converters do not significantly restrict exhaust flow, meaning the potential benefits of a decat are minimal unless they're specifically needed for specific modifications.
A 2006 test on a 1999 Honda Civic revealed that replacing the original catalytic converter resulted in only a 3% increase in maximum horsepower.
Whereas a modern metallic-core converter cost the vehicle only 1% horsepower more than leaving the stock catalytic converter on.
It is also illegal in many countries to install and drive a vehicle with a decat on public roads.
Other drawbacks exist too, such as causing the "check engine" light to illuminate and, for some people, the increased noise is a drawback.
Therefore, you probably shouldn't fit a decat unless you have a specific requirement for one, and even then, considering to fit a sports cat may be a better option.
What About a Sports Cat?
A sports cat is a less restrictive and more efficient version of a standard catalytic converter.
It usually filters the exhaust gasses to a lower minimal level but still to a level required by emission standards.
A sports cat will improve performance and tuning potential similarly to a decat without many of the drawbacks that come with a decat.
Usually, a sports cat avoids the "check engine" light illumination as well as being slightly quieter than the decat, a sports cat will also pass an MOT almost every time.
Therefore, a sports cat is the best option for modified and tuner cars.