I dont know if you are aware but the clio cat im told is a 200 cell (dunno wtf that means) sports cat anyway and is actually very good as standard, and is factory fitted and passes MOTS no probs
They are 200 cell (means less resistance to airflow (more cell, more restriction)).
For a very concise explanation you could go to the Cats-Direct website here
http://www.cats-direct-shop.co.uk/technical-information.php A quotation from their website is here:
"How Does It Work? The catalytic converter usually consists of a honeycomb structured ceramic monolith, which is surrounded by matting. The matting holds the monolith securely in place and provides a cushion against road shock and vibration. This is all encased in a steel shell to provide further protection. Another option is a steel monolith, which is more durable. The steel monolith we supply has a texture similar to cotton wool. The catalytic converter is situated in the exhaust pipe, usually close to the manifold. This is because the catalyst needs to reach a certain temperature before it starts to operate properly, so the closer it is to the engine the quicker it reaches operating temperature. The monolith is coated with platinum, palladium and rhodium. These substances cause a chemical reaction to take place as the exhaust fumes pass through the monolith. The carbon monoxide is converted to carbon dioxide, the hydrocarbons are converted into water and carbon dioxide, and nitrous oxide is converted into nitrogen." In order to see the explanatory diagram, you really need to click on the link.
For a longer, but more chatty explanation, you could go to The Naked Scientists website here
http://www.thenakedscientists.com/HTML/content/interviews/interview/569/ The explanation is too long to copy and paste here, but it is well worth clicking on as it makes entertaining reading.
