Exhaust 3"------->2.5"

My92cashtrap

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May 11, 2019
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I have mac LT headers with 3" collectors. Would i take a performance hit if i used a 3" to 2.5" collector and ran 2.5" for an X-pipe and exhaust? I'm thinking yes...

Engine is a 347.
 
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Gs87GT

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Sep 25, 2019
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Penn's Woods
I agree with the General on this one....

Along these lines but different, can you run MACs 3" exhaust (cat back system) with a 2.5" H-pipe?

I'm thinking about what exhaust to use with my GT this coming summer hopefully

Thanks
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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This is how I would run the 3" into 2 .5" exhaust , I would make the merge gradually maybe over a length of 4-6" , if running cats the merge would be after the cats.
This is not based on any actual flow data, just a cartoon in my mind.
 

Gs87GT

Active Member
Sep 25, 2019
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If you're still running cats, you will need hi-flow ones.
As for the cat back exhaust, I still like the sound of the Flowmaster Force II.
I have a bbk off road H pipe to install, no cats. Was just wondering if the mac 3" cat back would mate with the 2.5"bbk off road H pipe or if the flange size is different or not.

Sorry to take this off the OPs origional question.
 

Noobz347

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This is how I would run the 3" into 2 .5" exhaust , I would make the merge gradually maybe over a length of 4-6" , if running cats the merge would be after the cats.
This is not based on any actual flow data, just a cartoon in my mind.
Yep... Give those cats as much flow as possible. Gasses will do a great deal of cooling in the converters and be more dense going out than it did coming in. This is a good place to begin reduction and maximize exhaust scavenging.

you will need hi-flow ones.
Yep!
 

7991LXnSHO

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3” pipes the whole way will add weight without adding much benefit for most 347’s. Even flowmaster and Hot Rod’s chart shows you need to be making at least 400 hp at that engine size to use 3” duals.