Three way cats have ABSOLUTELY nothing to do with how many pipes are coming/going out of it. The term "three way" is from the different oxidation "levels" to clean the exhaust.

TWC's are gonna reduce NOx (not nitrous) but oxides of nitrogen. Because the atmosphere contains around 78% nitrogen there's no way currently to get rid of this gas. Oxides of Nitrogen is how "acid rain" is formed. With combustion chamber temps approaching 2500 degrees Fahrenheit, this is when NOx forms. At lower temps, just nitrogen gas escapes, but with the high heat, it bonds to Oxygen molecules.

TWC's will also strip and store oxygen with the Cerium and Cesium elements within along with the Platinum-Palladium mesh inside the cat. If you were to split on in half you'd see the Monolithic or honeycomb design.
Carbon Monoxide is oxidized into Carbon Dioxide which is much more harmless (CO into CO2) Unburnt fuel (also known as HC's or hydrocarbons) are oxidized into Carbon Dioxide and water.

Even though water comes from comnustion that don't mean you can drink the tailpipe. There's also Hydrogen peroxide and "heavy water"- byproduct of nuclear reactors in very small quantities.

The difference between a two way and three way cat is the addition of NOx cleansing.

So the amount of chemical reactions make a Three Way Cat a three way cat...
reducing Nox
Oxidizing CO
Oxidizing unburnt fuel

Mr. Wizard take that!!!


Justin