So about two weeks ago I installed my GMPP manifold and ZZP downpipe. Immediately after installing I noticed a hissing sound when the car is under boost. Driving normal and not in boost it doesn't hiss. I found the downpipe to catback leaking so I installed a remflex gasket and fixed that leak however the sound is still there. Any ideas? I have checked the torque on everything twice. I'm thinking maybe the flex pipe but it's a new part so it shouldn't be. Also all new gaskets were put in during install and I have a ZZP catback that has been on for a little over a year.

 

Catted? I remember reading on here that the zzp high flow cats often make a hissing sound that can be confused for a leak.

Edit: https://www.cobaltss.net/forums/show...43#post7563643

 

Yeah 400 cell cat

 

You will hear a hiss sound with a high flow cat downpipe. It has alot to do with the harmonics of the exhaust gases flowing through the metal matrix cat. You will get used to it.

 

Come to think of it I do hear a little hiss type sound from the cat when I was underneath the car. If that's how it is supposed to be that's fine.

 

Thanks man, it is what it is lol.

 

Yup... totally normal.