I just installed a 3 inch catless downpipe on my 97 gtp. It seems to run a little better under heavy throttle. I installed new gaskets as well. There is a hissing sound that is similar to a small exhaust leak coming from the downpipe but there is definitely no exhaust leak. The more of a load there is on the engine when driving, the worse the noise (the worst is when I stop in my driveway to wait for the garage door to open then pull up my driveway and pull in) Its not that loud but it is noticeable and bothersome. After some research it seems that several people who have catless downpipes / test pipes have a similar situation and this is normal due to the increased exhaust flow. I remember 15 years ago when I had an 89 s10 pickup with a 4.3 v6 I gutted the cat and the same thing happened (it was worse) so I ended up getting a high flow cat and the sound went away. If I get a catted downpipe will this eliminate the sound? I am seriously considering re-installing my stock downpipe just so it doesn't sound like I have a leaking exhaust.
