Your original catalytic converter will typically have 400 or 600 cells per square inch (cpsi), these original cats are restrictive in their flow, typically allowing an open area of around 55-65%.

A Milltek Sport Hi-flow Sports Cat, has only 100-200 cpsi depending on application. Milltek Sport Hi-Flow Cats have an open area of between 75-85%, thus reducing back pressure substantially and increasing flow rates, all very desirable for extracting more performance, especially on turbocharged engines. Emission levels are maintained at legal limits by careful selection of the volume of the metallic substrate. Oxygen Sensor positions remain unchanged from the original equipment.