???????

[radioheadmachine27]

Hi i was wondering if it really affects the engine/valves when u take the exhaust/mufflers off after the cats?
Is the pressure not going to be the same? it looks like my o4 gto has four cats anyways for the pressure.

[BNaylor]

Four CAT convertors? I doubt it. Probably two CATs and two resonators if it has a true dual exhaust system.

The engine needs some exhaust backpressure to operate efficiently.

Also, in the future please use a proper thread title and not just question marks.

[radioheadmachine27]

I want to talk with someone that has a gto not a novice.

[GTP Dad]

First of all you will get more responses by being nice and not insulting one of the moderators. Don't think for a minute that Bob is a novice when it comes to cars.

The 2004 does in fact have 4 convertors, two on each side, probably one to control hydrocarbons and one to control NO2. If you remove the exhaust piping before the mufflers you will get better exhaust flow and probably increased horsepower but you may also hurt performance due to the decrease in back pressure as Bob stated. The exhaust on the GTO is about as full flow as the factory will produce. If you are looking for a real performance you may want to try a set of headers and a larger exhaust as the cast iron exhaust manifolds are where the restriction in the exhaust system is actually occuring.

This url from another forum may help you in making your decision on a header installation. http://www.gmhightechperformance.com.../photo_03.html

[BNaylor]

Wow, I didn't expect that rude comment but it is no big deal.

Thanks for the clarification Lynn but it is still confusing to novice me and evidently other GTO owners. I answered based on old research when I was first looking into trading in the old GTP for a GTO. But I decided to pass on that for obvious reasons which I won't discuss in this thread. Here is an article, a GM press release that covered the exhaust configuration for a 2004 Pontiac GTO/Holden Monaro.

Click here

Now according to my buddy at the local Pontiac dealer the first CATs in each bank are full catalytic convertors monitored by the downstream 02 sensor(s). Cost is expensive at $830 each. The ones you see in between the forward CATs and the mufflers are quasi CAT convertors and a combination resonator built into one but really just classified as resonators. Hope that makes sense. Backpressure is low at 40kpa but there is room for improvement. From what I understand GTO owners are doing the resonator delete with no side effects except to sound. See the following fairly recent thread at ls1 forums.

Click here


Edit Update:

Here's another thread that answers the four CAT convertor mystery.

Click here

[GTP Dad]

I have to agree about the confusion on this particular model. I would really like to see one up close to see what that second one contains. Maybe, Rich can shed some light on this subject since he has access to the shop manuals where he works. How about it Rich! can you clear this one up for us as I think everyone including the 04 GTO owners.

[BNaylor]

Oh this gets really interesting Lynn. Someone had a muffler shop split one open a few years ago. Guess what they found inside. See link below.

Click here

[GTP Dad]

Well I guess the mystery has finally been solved. It is a resonator, go figure!
The resonator takes the prize!