So I have a 04 5at coupe with Invidia Gemini cat back, which drones a lot at like 1.5-2.5k RPM. Will HFC reduce the drone? Also anything else I can do to reduce it? (Shifting manual kinda helps, but still not that great)

 

HFCs will probably amplify the sound.

 

No, they will not. Reducing airflow at the intake and/or exhaust will help. But I don't know of any combinations of breather mods that will eradicate any rasp or drone.

 

Sound resonance is just a factor of pipe length x RPM. The Invidia pipe length was tuned for sound & power at high RPMs. Driving it at low RPMs is going to drone. You're right in the fact that the AT likes to drop RPMs down to the 1500 range when you're cruising, and Invidia did not tune their pipes for this low of an exhaust flow.

You could have a muffler shop add resonators into the mid pipes to help with dampening but the difference would be minimal. There's not much you can do here except live with it or sell the exhaust and get one designed for sound throughout the rev range, like the Nismo.

 

Alright, thank you for clarifying. Im actually getting used to the drone and just shift manually when it gets annoying. I am going to try HFC anyway, since I want it a bit louder, if drone gets too unbarable, I'll recell them I guess