GMMG with LT's & Catted Y-Pipe
 

Guys, I've been running a GMMG catback with the stock y-pipe and exhaust manifold for coming up to two years now, but had some American Racing LT's with a catted y-pipe fitted today. To be honest, I've not noticed any difference in response/power yet (hopefully will do after a tune), but am pretty surprised at just how loud the setup is now. At idle and low to mid throttle I'd describe the sound as being slightly louder and definitely a little deeper than before, but as soon as I hit the gas a bit heavier (and I don't even mean WOT) all hell breaks loose. The pitch changes quickly and sharply from a rumble to a very loud cackle that sounds quite metallic. Not sure if this is what is referred to as 'rasp', but whatever it's called I don't see me getting used to it anytime soon. I knew that adding LT's was going to make things louder, but I thought that it was only the ORY set-ups that would sound this wild.

You think what I'm describing is just what is to be expected or does it sound like there's a leak somewhere and one of the joints needs tightening some more? Will the sound become more dull over time as carbon deposits starts building up in the new pipework.....or is this just time to fit a Magnaflow?

Ya thats rasp. Kinda wierd its as much as your saying even with cats. Alot of guys on here add a bullet into the I-pipe. That could be your next option.

had the same sound when I matched a gmmg to the new kooks metallic spun cats. I hated the sound and I pulled the exhaust off the following week. this setup sounds the best with the ceramic cats hands down...the only problem is that the cats blow out too fast.

Yep, I had the same issue after my LT install. Cats (ceramic) helped but didn't solve it. I added a powerpack to the I-pipe and resonator tips to the GMMG. Then it sounded good, but at that point the system was so heavy that I decided to ditch it.

If you're like me, you'll be happier with the Magnaflow (just get rid of the cannon tips if you get the regular catback).

So are you saying you're now running a Magnaflow catback with your LT's? If so, how does the sound compare with your (pre-LT's) GMMG only setup?

I've heard a Magnaflow catback on an otherwise stock LS1 car and that was perhaps too quiet for my liking, but I'm thinking it might well be at about the right level when combined with LT's.

What about other catbacks, like Hooker, or is the Magnaflow the only one that will really tame the rasp?