Had my HKS exhaust for about a year now. When i first installed that, it was a huge difference in sound. But after a week of driving, sound died down but was still a noticeable difference.

2 weeks ago, I installed HFC's and it sounded awesome at low revs and high revs. The sound that everyone wants, the aggressive purring and resonating sound. A few days later, all that aggressive purring sound is gone until I go past 5000RPM, whereas, the purring sound came in at 2500RPM. I also noticed more droning noise when going uphill at ~1800-2200 RPM.

So is it normal for catback exhausts and HFCs to have their exhaust sound die down after a couple of days by a drastic amount?

Also, when I start the car, the garage fills up with gas fumes.

 

I don't know why it would quiet down as much as you claim. You may be getting used to it?

Since you have HFC's you will notice more fumes from the exhaust since the OEM cats are very restrictive. Also, since you added HFC's you will definitely fill the garage up with exhaust fumes since you are running the car with the door closed.

 

Maybe the HFC's has broken in? Or you're just used to it.

 

Also, I do have the HKS exhaust and I did run HFC's as well for a while. I did notice it was louder at first and had more of a throaty tone.

 

Which HFC's are you running? I'm considering one to pair with my hks as well.

 

I had Crawford HFC's.

 

The sound has definitely changed, I thought I got used to too but it wouldn't make sense that the car is quieter after HFC's were installed.

I usually get the growly throaty sound when I reach 3k RPM any where I go without the HFC, I got this sound around 2.5k RIGHT after the HFC's were installed. Now, I literally have to go past 4.5k RPM to get the growl.

Did an ECU reset and the idle air reset as well, but no luck. I know it changed the exhaust note by a small amount in the past before the HFCs were installed, but it remained the same now.

I am thinking it might be a leak somewhere? Any thing I can do to check if it's an exhaust leak?

Some symptoms I have right now that weren't there before:
- @ ~4.5k RPM, there is a light ringing noise like when your ears are ringing
- The famous hissing sound ( i can live with this one, happens to majority of people who changes their cats, doesn't sound bad either)
- A burning rubber smell (could be my brake pads that needs to be replaced ASAP?) after going to 80KM/H then to a complete stop (can only smell it when I come to a complete stop). Literally smells like those pink erasers for erasing pencils

 

An exhaust leak will make the exhaust louder. Its normal for the exhaust to become slightly quieter, its just the ECU adjusting to the changes.