Please read I'm really trying to figure this out. For reference: 2012 GT manual with: ported '18 manifold, Airaid CAI, crankcase breathers, Lund Tune. Exhaust described below.

I installed ARH 1-7/8" Long tube headers this past weekend. Due to my inexperience and the hours I was able to work on it at the shop I was renting a lift in I ended up having to run open headers to be able to leave in time. I also had to leave a top bolt out of the driver side header and the two middle top bolts out of the passenger. I know that's dumb but I was struggling and knew how much time I had and wanted to get my car out of the shop in running condition even if it wasn't fully complete. I also didn't install the upstream O2 sensors because I wasn't able to remove them from the stock headers. Needless to say the exhaust was insanely loud. But I kind of enjoyed it, not going to lie. It was a bit easier to stall but seemed to rev a little faster.

Yesterday I went in to tighten the steering shaft because it had some wobble and I went ahead and put the upstream O2 sensors in because I'd received my new ones in the mail. Decided to wait to reinstall the rest of the exhaust. The installation of the O2 sensors seemed to mess with the performance of the car. It also made a very disconcerting noise when engine braking. I know it was running super fuel rich before I put them in so maybe that's why it was acting up. It still ran rich after the O2 sensors but probably not as much so. It stalled much easier. It stalled easier on open headers to begin with but it became worse. Sound didn't change.

And finally, today I installed the rest of the exhaust and managed to put the bolt in the drivers header and one of the missing ones in the passenger(the other eludes me still, cant find a good position above or below to get at it). The headers came with a catted 3" H-pipe, which I installed and connected to my stock diameter straight piped catback(2.75" LTH resonator delete over axle pipe and 2.5" SLP Loudmouth). I figured with straight pipes aside from the catalytic converters it would still be loud, just not as ridiculous as the open headers. I was EXTREMELY surprised and disappointed when I turned the car on to discover my car was now quieter than it was with stock headers and cats. It is nearly silent, with or without the windows down. No matter the RPM, it is quieter than it was with stock headers. I don't want to be obnoxiously loud but I definitely want to hear something. It also feels slower. Slower than before the manifold I can't say for certain but definitely slower than fuel rich open headers.

I'm at a loss here. I understand I won't have the performance I had with open headers but I didn't expect a silent exhaust. I simply can't believe that the "high-flow" cats silenced that much noise.

 

Mufflers are what control the sound for the most part. Did you use your stock mufflers?

 

This is kinda hard to unpack and sort out, but I agree it does seem strange that headers would be quieter than the stock manifolds, all else being equal . . . so I suspect something else is not equal.

Did you have cats on the car before? I would not expect the new high flow cats to be quieter than the factory cats; unless you had no cats before.

About the O2 sensors -- they are important to the way the car runs, and I'm pretty sure your tune will need to be adjusted due to the change of position of the O2 sensors. That is needed to get the car running right. I also wonder if that might have something to do with it being quiet; so that is the first thing that I would correct -- Lund should be able to update the tune for you.

 

Sorry for the text wall. I was trying to give as much info as possible to help try to diagnose this.

Before this I had stock headers, cats, and H pipe with my straight piped catback.

Lund told me when I bought the tune for my intake manifold that the car would adapt to the increased exhaust flow after a couple run cycles. I'm still going to datalog and try to get a revision but that's what they told me.

 

My car has zero mufflers or resonators. Only thing restricting exhaust flow right now is high-flow cats.

 

Update:
Disconnected the battery for 10 minutes as per Lund's recommendation. I honestly forgot to do that after installing the exhaust. Also reflashed the tune to be safe.

Seems to have helped but not as big a difference as I'd hoped for. I can at least hear my exhaust now. I'm going to let it go for a couple drive cycles and then get a datalog to send to Lund. Pretty sure it's running lean now from what I've seen monitoring AFR on the nGauge.