HFC's
 

May be a stupid question. sorry if it is, but what are HFC's? what does it stand for, what do they do etc..

High flow cats.

..cats being catalytic converters. Catalytic converters catalyze air, converting pollution to less-pollution. They are usually a honeycomb structure, which means they often decrease your car's horsepower and torque by slowing the exhaust flow.

High flow cats are designed to catalyze while allowing better exhaust flow. The result is usually more noise and more power (on rare occasions, decreasing exhaust back pressure can actually lower your cars power output).