Downpipe noise

**War97gtp** · 11-10-2014

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.

**War97gtp** · 11-12-2014

I noticed on zzp's site their downpipe comes with interlock flex instead of single wall flex which they say causes a "hissing" noise. I have a ebay "speed daddy" downpipe which probably has the single wall flex could this be the issue? Also I used the new "crunch style" gaskets that came with my downpipe but I noticed the original header to downpipe gasket is a thin metal type. Will the crunch style gasket be ok? (I saved the original one just incase)

**jimzssei** · 11-13-2014

It's the flex ... I worked in an exhaust shop, and would have to explain to customers about the sounds if they were getting a new convertor... But yep, that's what you're hearing!

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**BillBoost37** · 11-13-2014

There are a few things.

Like mentioned it might be the flex, but some DP's come with dual wall flex and those usually don't make much noise.
Stock Dp's are dual wall a certain amount of the way down to quiet the exhaust right out of the manifold.
And then.. there's the cat.

**SnowTh1ef** · 11-13-2014

Needs more raceker mods so you don't hear it.

**War97gtp** · 11-14-2014

Ok so if i get the 3 inch offroad zzp downpipe with the better flex most likely the hissing will quiet down? or should i get the catted 3 inch zzp downpipe to make sure that its not because the cats gone. I also noticed a tiny hole on the bottom of the back side of the resonator and after the down pipe installation it seems that its hissing as well (im guessing because of the extra exhuast flow) it looks like this hole is supposed to be there for some kind of vent? Can i plug it up with an exhuast sealer paste so i dont hear the hissing from that? My guess is that hole is a drain for the bit of water the cat produces and since i dont have a cat anymore it wont matter anyway.

**BillBoost37** · 11-14-2014

How about you get under and find out if there are any leaks. Fix them..then ...you might also be able to tell if the flex is loud.

**War97gtp** · 11-14-2014

I did go under and there doesn't seem to be any leaks and I didn't expect there would be everything lined up and bolted in nice and tight with new gaskets. The only place there is a "leak" is that tiny vent hole on the bottom rear side (toward the back of the car) of the resonator, that hole is supposed to be there because there are 2 identical ones on the bottom back side (toward the front of the car) of the mufflers but those ones don't seem to make any noise. Im going to plug the resonator vent hole with jb weld tomorrow that will get rid of half of the problem, and a better downpipe with higher quality flex will fix the other half. I just have to decide if I want catted or not.

**Scottydoggs** · 11-14-2014

header and after market down pipes are thinner, i think thats what your hearing. headers made my car louder under the hood for the most part.

and id not plug that hole in the res. its to let water drain out so it dont rust out. aka condensation.