Milltek Downpipe With HJS High Flow Sports Cat | Honda Civic Type R | FK2 2.0T K20C1 | 2015-2016Downpipe with HJS 200 Cell High Flow Cat Pipe : 3. 00" (76. 20mm) 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 which will be fully integrated into a large bore downpipe, has only 100 200 cpsi depending on application. These cats have an open area of between 75 85%,