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Discussion Starter #1
i have 02 wrx
i have as follows,
GUTTED AIR BOX 4-1 header, with uppipe, 4inch to 3in bellmouth downpipe, apexi N1 cat back boost controller set at 14.8psi fully catless

I have a 2.5inch diameter flex pipe thats about 5 inches long this connects the downpipe to the cat back. i was told that since the apexi is designed to be used with the factory cat, its 2.5 inches itself, then it goes to 3inch later down the exhaust.

QUESTION IS ?>>>>> by me having a 2.5 inch flex section do you think i am losing POWER, is that big enough or do i need 3inch? would there be any hp increase by me cutting out that section of pipe and putting a 3inch flex section in place?
 

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It really depends on how the entire set up works, theres many factors that could change the amount of power your particular set up would yield from a 2.5 to a 3 from the downpipe to the cat back. But if you want me to be honest, if you're catless and a flex pipe from the downpipe to the catback is 2.5 compared to 3, the HP differences are not going to be noticable AT ALL. Just my 2 cents. :wiggles:
 

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I was wondering the same thing. isn't the exhaust only as good as the smallest diameter pipe? if you have a 3" dp with a 2.5 section on it wouldn't it really only flow as good as a 2.5" pipe? you guys know what i'm saying?
 

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In no way will a 2.5 section to a 3 create a choke powerful enough to cause backpressure that would disrupt airflow and negating in power loss. Measure half an inch and just imagine the difference.
 

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I think your both rite, The exhaust will only flow what ever the smallest pipe will allow. Past the 2.5" section is only going to flow 2.5" worth of air. But on the other hand the turbo itself is the largest air restriction, so past that I do not think it really matters, thats still a pretty big pipe for that engine unless you get into some more serious mods? Just my two cents??
 

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Discussion Starter #7
so should i ????

should i rip that off go 3inch flex section. rip it all off and go with straight pipe. whole exhaust welded together. probably going to break something if i do that. or leave what i got?
 

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If you have money to invest elsewhere, do it there first. This isn't exactly what you call a dire problem.
 

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Like was said, it really doesn't matter much what you do.

To address some other thoughts in the thread, dyno tests in Hot Rod and Car Craft have shown that changes wherever in an exhaust system have an effect on the power. I too thought that exhaust would only flow as well as the smallest diameter, but the dyno shows that numbers change based on the setup of the entire system. So in theory, chaning from 6 inches of 2.5 inch diameter pipe to 12 inches of 2.5 inch diameter pipe somewhere in the system would effect the power output. Weird I know, but the numbers told the story.

I kinda saw this played-out when I changed my cat-back. Even though I still had the stock dp and third cat, changing to a larger catback and better flowing muffler made a difference.
 

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sorry this is a bit off topic but i didn't want to watse a whole thread with one question. i was wondering, i have the apexi system and since it is 2.5" would i still be able to bolt a 3" downpipe to it without problems?
 
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