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Discussion Starter #1 (Edited)
Ok guys im going to get a longtube header 304 stainless for my 3.5 engine swap. Have a few ?s regarding the sizing for the primaries on the headers.
My options:
1.625" or 1.75" primaries
If I pick the 1.625" I will be running a 2.5" outlet followed by a 2.5" midpipe and borla exhaust. If I pick the 1.75" I will be running a 3" outlet followed by a 3" midpipe and borla exhaust. Either option will be completely catless. :)

My question is which will provide more power? What are the cons of a full 3" exhaust on a na 3.5 I5? I like the idea but would like your opinion.
 

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maniac mechanic
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5,779 Posts
DO NOT run a 3" exhaust. Completely unnecessary and ruins the exhaust note. I originally bought a 1.625 long tube into a full 3" exhuast and a magnaflow muffler and it was awful even with forced induction I lost all low end power, one of the main regrets decision wise on my truck.
 

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maniac mechanic
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5,779 Posts
I think the 1.625" is plenty for these motors and the primaries are long. You are adding a ton more flow already over stock plus good exhaust scavenging and some other benefits. IMO the 1.625" back to a 2.5" and just a muffler would give you the best results without leaning the powerband to far in either direction just a nice overall increase.
 

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I'd suggest a highflow cat also. I have a header with stock size outlet. I ran catless for about 1 month and went and ordered a magnaflow cat. Anything over 3k rpms and it was so obnoxiously loud I couldn't stand it anymore. Stock piping to a flowmaster and 2.5" pipe. If I'd sit at a long traffic light I could smell my exhaust.
 

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[MTS] Who is this guy?
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3,779 Posts
So where did you order a long tube for our trucks that you were able to choose the primary and collector size?
 
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