I have a 3inch catless downpipe from zzp installed on my GTP. I just got to thinking, is it really to my advantage to have a 3inch downpipe with the stock exhaust? Wouldn't the larger diameter of the 3inch dowpipe just cause even more of a problem when it is tapered back down to stock size where it connects to the cat-back? When i go WOT and then quickly let off i can hear a backfiring coming from the downpipe area, not the mufflers, but where the dp connects to the catback. What do you guys think?

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Thats why I got the 2.5" DP. Figured was pointless unless I went 3" all the way back.

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04GTP GONE! FEB = mid 90's Camaro SS or Z28 with 383 transplant shortly after. Thanks for everything GPONA even though the GP is gone I will still be on!

That 3" DP allows for a larger volume to pass through until it hits the 2.5" reducer. Having that larger volume allows for less boost stack, therefore, less KR.

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2009.5 SBM G8 GT
November 2011 Car of the Month

Exactly, but wouldn't the fact that it's being drivin into stock piping cause the pressure of the 3 inch dp to be equal to a stock sized dp?

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it is less restrictive than a full 2.5 inch

id rather have a larger collector and make it trim down in the middle than right after the manifolds

and if you are crazy about performance like i was you'll eventually get the 3" catback

mine was over killed tho

3" from the main's to the tips and i mean everything you can think of and only cost me around 150 installed

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If you are going to do a 3" catback exhaust, you may as well put in a cutout...

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2009.5 SBM G8 GT
November 2011 Car of the Month

Since we are boosted you want as much flow as possible. Is your DP catless? That could be the popping sound.

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00 GTP Daytona

Yea it's catless. I'm thinking i might not have tightened the bolts on the dp up enough which is causing some air to get sucked in and cause a backfire.

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3" is overkill for exhaust - but good for DP.

2.5" is all we need.

Our GTP's need back pressure...

3" all the way is not good.

I run 3" all the way back. But, I'm on stock manifolds. I think when that goes away, I'm gonna lose some bottom end power. Honestly, I'm fine with that. These engines make enough power downstairs that we can afford to lose some.

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One Big Ass Mistake America