Why does the engine noise increase after removing the three-way catalytic converter?

[LaLeo]

Removing the three-way catalytic converter results in increased engine noise because the converter, typically made of honeycomb ceramics or metal mesh, serves a certain level of noise reduction function. Function of the three-way catalytic converter: With increasingly stringent environmental protection requirements, more and more vehicles are equipped with exhaust catalytic converters and oxygen sensors. These devices convert harmful gases such as CO, HC, and NOx in vehicle exhaust into harmless carbon dioxide, water, and nitrogen through oxidation and reduction reactions. Since this catalytic converter can simultaneously transform various major harmful substances in the exhaust into harmless substances, it is called a three-way converter. Hazards of a damaged three-way catalytic converter: A damaged three-way catalytic converter does not affect the normal operation of the vehicle, but it can lead to difficulties in combustion, reduced driving power, engine backfiring, increased fuel consumption, incomplete combustion, carbon deposits in cylinders and spark plugs, and other phenomena. Over time, these issues can negatively impact the vehicle.

[OIsabelle]

I've been driving for over a decade and have encountered similar issues. Removing the catalytic converter does indeed make the engine sound louder, because the honeycomb ceramic structure inside the catalytic converter originally absorbed some exhaust sound waves, acting like a muffler layer for the exhaust pipe. Once removed, the exhaust flows unimpeded, allowing the sound to blast out directly. Moreover, removing the catalytic converter alters the exhaust backpressure, causing exhaust pulses to become chaotic at low RPMs. When I modified my car, I noticed particularly obvious noise below 2000 RPM, accompanied by metallic rattling sounds. What's worse, the exhaust fumes become extremely pungent, making it impossible to pass emissions testing. I strongly recommend reinstalling the proper OEM part as soon as possible.

[MacRowan]

I've seen many car owners modify their exhaust systems at repair shops. After removing the catalytic converter, the engine sound becomes louder because the entire exhaust acoustic structure is disrupted. The original exhaust pipe with a catalytic converter has a specific length and muffler chamber design, and suddenly removing it destroys this harmony. Especially when the engine runs at low speeds, it produces popping sounds like air leaks. I once worked on a Civic where the owner removed the catalytic converter, and even the middle section of the exhaust pipe started vibrating abnormally. Moreover, the joint of the removed exhaust pipe section is prone to air leaks, creating a rattling sound that can be mistaken for an engine malfunction—hardly worth the trouble.

[TheoAnn]

We discussed this before on the tuning forum. Removing the catalytic converter improves exhaust flow efficiency, but at the cost of significantly increased noise. That honeycomb ceramic layer isn't just an emissions device - it acts like an acoustic sponge. After removal, the exhaust note changes from a deep hum to a harsh, raspy roar. During my modification tests, I measured 78 dB at 3000 rpm with the cat installed, which skyrocketed to 92 dB after removal. At car meets, I've heard several decatted vehicles - their acceleration sounds like thunderclaps, loud enough to trigger nearby car alarms when idling. Most importantly, this modification is illegal. Getting caught means a 2000 RMB fine, plus another 3000-4000 RMB to restore the factory setup.