I know this has probably been answered before, but there were too many threads listed in search results. What happens when they clog? They can't be flushed or boiled out can they? What makes a high-flow cat flow higher?

 

There's a big chunk of ceramic material with a zillion little holes thru it. The actual catalyst material is a thin coating on the ceramic.

Soot or crud can plug up the holes, or if you run too rich, it might even get hot enough to melt parts of it. If you hit something, or bang the cat hard with a hammer, the ceramic can break; then you have a pile of rubble so it doesn't flow so good.

I guess a high-flow cat has a bigger brick with more holes and/or bigger holes. Then the exhaust flows through with less resistance.

 

Convertors contain a mixture of precious metals, most notably platinum as the major component. The inside materials are shaped into various forms depending on the way the cat is supposed to be used. Some are a 'bed', others are a matrix.

Once the convertors removal efficiency gets below a certain point its ability to 'convert' is reduced, and tail pipe emissions increase.

Most 'clogged' convertors are a result of over rich fuel mixtures which actually melt the inside of the convertor restricting the airflow.

 

my carb cleaner can says its safe for cat cleaning. havent tried it yet though.