Modifying Stock GT catalysts - Pontiac G8 Forum: G8 Forums - G8Board.com
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post #1 of 3 (permalink) Old 08-01-2017, 05:35 PM Thread Starter
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Modifying Stock GT catalysts

I seem to remember a thread about removing the built-in restriction in the GT catalysts. I have searched the Board 9 ways from Sunday and have not found a thing. Anyone know what I am talking about? Seems to me that it involved cutting out the existing downpipe and flex joint, then replacing with straight pipe. Am I crazy?

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post #2 of 3 (permalink) Old 08-01-2017, 05:47 PM
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Quote:
Originally Posted by PSM09 View Post
I seem to remember a thread about removing the built-in restriction in the GT catalysts. I have searched the Board 9 ways from Sunday and have not found a thing. Anyone know what I am talking about? Seems to me that it involved cutting out the existing downpipe and flex joint, then replacing with straight pipe. Am I crazy?

I did some cutting and welding a few years back, basically the torque ring near the flange is supposed to be there, the secondary pipe to the cat is correct diameter near stock tune, but the angle into the cat substrate is accute.
So if you want to retain the stock Euro 4 ceramic cat there needs to be a 90° bend to flow horizontally into the substrate.
As background I got rid of the flex joint, torque ring, upped the secondary pipe from flange to cat to 3”, back down to 2.5” then 2”. Swapped the cats for the HSV version, replaced the entry with 2”, swapped headers for HSV tri-y mid length.
Then came up with a solution that did work! HSV mid throw tri-y primaries, 2.5” mandrel secondaries to 200 cpsi barrel cats positioned as far back as possible.
This was all about HP and rpm - midrange to top.
Have pics and vid avail.
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post #3 of 3 (permalink) Old 08-01-2017, 05:51 PM
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Member is JCMG8. He has photos and posted them a few places on here where he essentially cut out the flex and tried to improve the inlet diameter all the way to the cat.
I can't find his original post, and all the more recent ones show up in a google image search but photo****it has turned all of these threads into advertisements for their $400 P500 service so they're all worthless anyway.
Maybe JCMG8 can repost some here to answer your question.

Also if you get a few people contacting you with those GXP cats, let me know...

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