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2000-2005 Discuss your 2000-2005 Bonneville SE, SLE, and SSEi Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 09-29-2004, 01:40 PM   #1
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Hey P-dad,

Exhaust! I am considering doing something with my exhaust, maybe. What is the most restrictive part of my exhaust? Is it the manifold or the mufflers or what?

I don't plan on doing any more mods outside of a little on the exhaust.

Also, do you think the stock plugs are okay with my set up and possibly a bit of exhaust improvement?

Thanks!
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Old 09-29-2004, 02:18 PM   #2
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Your plugs should be fine with your mods, inspect them in 1500 miles for any deposits or dulling.
I'm not aware of anyone swapping out the cat only for a high flow one. This would be inexpensive compared to headers or ported manifolds. The rear manifold has the most lumps inside and could be the cause of most of the restriction?
The stock resonator and mufflers are actually very good, I would not consider changing out the mufflers untill mine are shot, over 120,000 miles now and no signs of decay?

If you do go with a high flow cat be sure and get the rear O2 spoof to avoid a nuisance ses light, no effect on the pcm or performance but the pcm will throw a code.
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Old 09-29-2004, 02:25 PM   #3
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Quote:
Originally Posted by PontiacDad
Your plugs should be fine with your mods, inspect them in 1500 miles for any deposits or dulling.
I'm not aware of anyone swapping out the cat only for a high flow one. This would be inexpensive compared to headers or ported manifolds. The rear manifold has the most lumps inside and could be the cause of most of the restriction?
The stock resonator and mufflers are actually very good, I would not consider changing out the mufflers untill mine are shot, over 120,000 miles now and no signs of decay?

If you do go with a high flow cat be sure and get the rear O2 spoof to avoid a nuisance ses light, no effect on the pcm or performance but the pcm will throw a code.
So the rear manifold and the cat are the most restrictive?
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Old 09-29-2004, 02:30 PM   #4
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I ran my headers with cat for about 6 months and noticed an improvement when I finally upgraded the cat, so I know it was restrictive.
The rear manifold is only my guess from looking at both the stock front rear manifolds, I have not seen any flow tests of either.
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Old 09-29-2004, 02:42 PM   #5
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I would say the downpipe is the most restrictive part. If you were to look inside the flex coupling you'd see the pipe is a maxiimum of 2.25", if not smaller, replacing this with a 3" part would help a lot.

If the newer style catalytic converters are like mine, then you have a 3" entry and 2.5" exit - that would fit nicely with the larger downpipe.

Cheers,
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Old 09-29-2004, 02:48 PM   #6
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Quote:
Originally Posted by PontiacDad
I ran my headers with cat for about 6 months and noticed an improvement when I finally upgraded the cat, so I know it was restrictive.
The rear manifold is only my guess from looking at both the stock front rear manifolds, I have not seen any flow tests of either.
Maybe someone who reads this will share flow tests, if any on this. I have read through a lot of the posts on the ported manifolds. I was told it would add around 10 HP and about that in torque. Moreover, I would breathe out a little better.

This bit on the cat is new to me. Thanks for sharing it. Did you get your high flow cat from intense as well?

Now I know that with your set up and the headers that the stock cat would be hard pressed to keep the flow going good, but on my mostly stock getup, will it, in your opinion, help me out to go with the high flow cat?

Depending on what I find out, I will probably be doing one or both of these mods.

Thanks P-Dad for the time and info!
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Old 09-29-2004, 02:51 PM   #7
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Quote:
Originally Posted by Foghorn
I would say the downpipe is the most restrictive part. If you were to look inside the flex coupling you'd see the pipe is a maxiimum of 2.25", if not smaller, replacing this with a 3" part would help a lot.

If the newer style catalytic converters are like mine, then you have a 3" entry and 2.5" exit - that would fit nicely with the larger downpipe.

Cheers,
Wow, So I may not need the manifolds replaced with ported ones, just the downpipe? What about the cat with a high flow cat?

Can one just replace these parts or would I have to change more?
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Old 09-29-2004, 03:09 PM   #8
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I know I am talking about a different car here, but my stock downpipe was very restricting ... being only 2.25" in some places. I had a custom one made with a high flow cat, and the results were very nice. It was well worth the $$$ spent.
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Old 09-29-2004, 03:25 PM   #9
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Quote:
Originally Posted by kevo
Quote:
Originally Posted by Foghorn
I would say the downpipe is the most restrictive part. If you were to look inside the flex coupling you'd see the pipe is a maxiimum of 2.25", if not smaller, replacing this with a 3" part would help a lot.

If the newer style catalytic converters are like mine, then you have a 3" entry and 2.5" exit - that would fit nicely with the larger downpipe.

Cheers,
Wow, So I may not need the manifolds replaced with ported ones, just the downpipe? What about the cat with a high flow cat?

Can one just replace these parts or would I have to change more?
The more you change the better...but how fast do you want to go and how much do you want to spend? In my opinion, between the manifolds, downpipe and cat - I would do the downpipe if I was only going to do one.

If you have quite a few miles, like PDad does, then maybe the cat would be the next step and lastly the manifilds. I think a 10hp gain from ported manifolds may be a little ambitious.

You could do the downpipe, new flex coupler and cat all at once which would be the best bang for the buck.

You could then add an aftermarket PCM for a few more horsepower, that would fit alll your parts together nicely.

Cheers,
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Old 09-29-2004, 03:45 PM   #10
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Quote:
You could do the downpipe, new flex coupler and cat all at once which would be the best bang for the buck.
This sounds like a plan. My car only has 47K or so on it. Should I do the cat or not, IYO?
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