# Exhaust: 2 1/2" vs. 3"



## yousaygo (Apr 3, 2006)

Hi, As usual, i have a 69 GTO with a 421, it is completely built, i can go through the specs if you want but lets say it is on the dyno for 525HP 540 ft lbs. Anyway, I need a new exhaust and me and my brother go back and forth about using a 2 1/2" vs a 3" exhaust. He thinks using a 3" is not worth it unless you have at least 600HP as the back pressure is needed. I think it is fine to use a 3" on nearly anything.
I am hoping for some real world comparison here so please feel free to take either side 
Shane


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## 66tempestGT (Nov 28, 2009)

2.5 duals are probably good for at least 800hp. good luck getting 3" over the rear axle. big free flowing exhaust benefits high rpm engines. your 421 wil probably rarely see 6000 rpm.


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## 05GTO (Oct 6, 2004)

66tempestGT said:


> 2.5 duals are probably good for at least 800hp. good luck getting 3" over the rear axle. big free flowing exhaust benefits high rpm engines. your 421 wil probably rarely see 6000 rpm.


:agree

Have you thought about running an H-Pipe with 2.5" duals?


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## yousaygo (Apr 3, 2006)

Do you guys have a preference for x-pipe or h-pipe? thanks for the comments on the exhaust...
I still cannot find a throttle cable bracket on my other post


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## Eric Animal (Oct 28, 2007)

:agree I would run 2 1/2" pipes all the way back, get some good low restriction mufflers and you will be fine!:cheers


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## Eric Animal (Oct 28, 2007)

I ran an H pipe years ago...before X pipes were around. The X pipe will give you more help at low RMPS.....thaat 421 ought to pull real good with 2 1/2'' and an X. they sound nice also.


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## 66tempestGT (Nov 28, 2009)

if i had a blower i would get some nice electric cutouts


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## Eric Animal (Oct 28, 2007)

With 3" pipes??arty:


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## 66tempestGT (Nov 28, 2009)

66tempestGT said:


> good luck getting 3" over the rear axle.


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## Eric Animal (Oct 28, 2007)

I am thinking Doug's headers w 2" primaries---3" pipes with an X---into 3" inlet x2.5" outlet mufflers----2.5 tail pipes. Gotta see the finished chassis first. I need to move a fair amount of exhaust, but that should do it.


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## 66tempestGT (Nov 28, 2009)

gonna be sweet. i still think blowers need zoomies.


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## Eric Animal (Oct 28, 2007)

Yep! but I don't want to be paying to repaint the doors of cars next to me at stop lights! arty:


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