Updated: Jan 25
A back box delete is an exhaust modification that removes the back box (rear muffler / silencer) and replaces it with a typically unmuffled exhaust pipe. It's usually done to increase the loudness or improve the tone of the exhaust sound without changing the whole exhaust system.
The back box delete is also known as a muffler delete.
The rear muffler of an exhaust is not essential for the vehicle to function, it does not affect emissions and only changes the sound of the exhaust.
It typically looks like a metal box or cylinder at the rear of the vehicle. A muffler is filled with sound absorbing material like fibreglass, it also can be baffled and contain sound resonating chambers.
Table of Contents:
Is a Back Box Delete Legal?
Legality of exhaust modifications and sound limits differs in every country. As a general rule of thumb its not illegal to remove the back box of an exhaust if it does not cause the exhaust sound to go over the legal decibel limit.
A muffler delete does not effect emissions and should pass emissions testing, providing the modification has been declared to your vehicle insurance and the exhaust sound is not over the legal decibel limit, it should be road legal.
However, this varies by country. In many developed countries such as the United States, United Kingdom, Canada, and Australia, modifications to the exhaust system are highly regulated or even prohibited in some places.
Always check your local laws regarding a muffler delete exhaust modification.
You should note that some race tracks and courses also have a decibel limit, meaning a back box delete might prevent you from driving your vehicle on a race track.
How Loud Does a Muffler Delete Make the Exhaust?
The loudness of the exhaust depends on multiple factors.
Whether the engine is petrol or diesel. Petrol engines are usually louder than diesel and a back box delete will have more of an affect).
If the muffler delete is a straight pipe or a muffled version. A straight pipe has no baffles or sound absorbing material and will be louder than a muffled replacement (though a back box delete is almost always a straight pipe).
If the vehicle has other sound reducing components. This can be other mufflers / silencers, catalytic converters, particulate filters, etc. If there are other sound reducing components of the exhaust, a back box delete will have less of overall effect on exhaust sound than if it was the only muffler / silencer on the exhaust.
How big or small the original back box is. If the back box being replaced is very large, the effect on loudness will be large, if the original back box is small it'll have less of an effect.
What type of engine the vehicle has. Some engines are naturally louder or sound different than others.
An exhaust with a muffler delete will not be as loud a straight pipe exhaust system but may be considerably louder than the stock exhaust.
Does It Affect Performance?
The restriction of the exhaust gas flow that results from a muffler / silencer causes back pressure, which can reduce engine efficiency. This is due to the fact that the sound pressure that the muffler is intended to reduce must share a complicated escape route with engine exhaust.
Some aftermarket mufflers make the claim that by reducing back pressure, a back box delete can boost engine production and reduce fuel consumption.
Usually reducing back pressure causes more noise to escape the exhaust system.
Should You Install a Back Box Delete?
Installing a back box delete is a personal choice, it may slightly improve performance by a very small margin, it can also make your car sound louder.
However, it may be illegal to install this exhaust modification where you live.
Some race tracks will also ban you driving on them if your exhaust is too loud, meaning a back box delete could prevent you from attending a track day event.