devonmikeyboy wrote
The CAT is designed to remove particles not noise.
No, the 2-way catalyst on a diesel engine catlyses the oxidation of carbon monoxide to carbon dioxide and of hydrocarbons to water and carbon dioxide. The hydrocarbons are from unburned/partly burned fuel and while it is true that soot particles are also from partly burned fuel, as solid particles they are not in a form that the catalyser can
help to oxidise. Particulate traps are fitted to some newer diesels.
The catalyst will provide some back pressure, but nowhere near as much as the turbine. Turbocharged engines are already extracting a lot of energy from the exhaust, providing significant silencing as a byproduct. Removing the catalyst and/or silencer boxes will affect the noise made by the car a lot more than the power it produces.
Any mods to the engine's gas flow, from the air filter inlet trunking to the end of the exhaust pipe, should be accompanied by a remap to make sure that positive benefits are gained.