just replaced the headers in my 98 civic to an after market part, don't know what kind of headers because i bought them with the car, and afterwards i noticed it didn't sound nearly as muffled as it was. the car has a modified exhaust (straight piping and exhaust tips)

i was just curious to know how much muffling the converter actually provided seeing that it increased in volume after i removed the stock headers which has that part in it.

 

The stock cast-iron manifold & cat muffle a lot of the sound.

 

Cats really dont "muffle" sound since the substrate runs parallel with flow. If they muffled the sound, manufacturers wouldn't use mufflers and resonators in the exhaust system.Changing the factory manifold over to a good header though will increase sound just from the increase in gas velocity.

 

have you ever put a test pipe on a car? the noise difference is ridiculous, if a cat doesnt muffle noise where does that change in db's come from?

 

the cat plays a pretty big role in quieting the exhaust, just from experience

 

I've heard cars with no cats and just manifolds/headers and cars with manifolds and cats, with no exhaust system on both, and i'll be damned if you could say one is quieter than the other. Just from experience.