Practically everyone knows that a catalitic converter is mandatory  nowadays and it is important to have one if you want to pass the emisson tests.

But what is it exactly and how does it work?

The defenition of a calitic converter according to the dictionary:

A catalist is something that affects the speed a a certain reaction, without being consumed itself.

The catalitic converter on a car will 'positivly influense the reaction'. This means that the process to extract certain  elements (Carbon monoxide (CO), Hydrocarbon (CnHn) and Nitrogen oxide (NOx)) out of the exhaust gasses will be accelarated.

These gasses will be converted into less harmfull elements (These would be: Carbon dioxide (CO2), Water (H2O) and Nitrogen gas (N2)).

A catalitic converter isn't much more than a hollow room, filled with Platina (pt) or Rhodium (Gh) lined dividers. Two very expensive precious metals, that's why a new catalitic converter is always so expensive. The exhaust gasses flow through the catalitic converter and react with the catalitic elements in the converter. Cleaner exhaust gasses are the result.

I hear from a lot of people that they have replaced the catalitic converter with a pipe with two flanges, with the same lenght as the original converter. This way they can just swap the cat and the replacement if they need to.

The general believe is that the catalitic converter is a restriction in the exhaust, while we actually want to get rid of those exhaust gasses as fast as we can.  Here in the Netherlands, cars that were built before the end of 1992 weren't obliged to have a catalitic converter yet.

According to Dutch law, a vehicle with the U9 indication on the registration papers, doesn't have to take a 4-gas test, but does have to meet the rule that when running in neutral, the percentage of carbon monoxide (CO) in the exhaust gasses has to stay below 0.5%.

In reality, without a catalitic converter, this is very hard to achieve on a petrol engine.

There are special high flow catalitic converters available, mostly used on racers. These cats flow much better and are much less of a restiction in the exhaust. But I am not sure yet how legal these things are for normal road use. I guess if yo comply to the regulations for exhaust gasses you'll be alright, but I am going to look a but further into this before I purchase one for the project car.