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Old 12-07-2007, 12:17 PM   #1
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Default Exhaust pipe size?

I'm getting my BEGI S turbo kit very soon and I want to make a new exhaust system before tuning it with MS.
The BEGI downpipe I'll be getting is a 2.5" separated gases downpipe. My question is, should I make the rest of the exhaust system with 2.5" pipe, 2.75" or 3.00" pipes with a 2.5" to the bigger size reducer so it flows smooth. I wont be using a catalitic converter, just a muffler, I think a Magnaflow would sound good.
So, any advice on piping size and muffler size-brand would be appreciated.
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Old 12-07-2007, 12:21 PM   #2
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Keep it the same size as the downpipe. Bigger pipe isnt going to do anything for you. Imagine a small garden house attached to a big harden hose? You arent going to flow more from then big one unless you can overcome the small one therefore it doesnt matter.
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Old 12-07-2007, 12:29 PM   #3
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The larger the cross-sectional area, you proportionally reduce the exhaust velocity, which lowers the back pressure further...which is why BEGi usually steps their DP to 2.75"....make sure this isn't the case with yours, in that case, at least match the size of the DP or go up .25".
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Old 12-07-2007, 12:49 PM   #4
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A bigger exhaust will make a bigger difference the more power you plan on making (and trust me, you always want more), but in almost all cases bigger is better (I've heard of potential boost control issues with too little back pressure and an internal wastegate). I forgot how much, but IIRC, Tim was able to sustain more boost and got more power with the twins by stepping up to a 3" exhaust from a 2.5" and he's still in the under 250hp category. I only use him as an example b/c I've heard that some people say that you don't need 3" until your over 300hp, but not true.
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Old 12-07-2007, 12:56 PM   #5
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I bought a 3" for my car, and like usual made a multitude of threads. I was worried that it wouldn't make much of a power difference and I would waste money...one phone call to Bell supported what everyone here said.

My old turbo vr6 went from a 2.5" NA "straight through" exhuast to 3" home made and went from like 370whp to 418whp. Its been like 6 years, but I believe it also spooled the t04e like 400 rpm sooner.
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Old 12-07-2007, 01:43 PM   #6
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bigger=better
ask all my girlfriends
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Old 12-07-2007, 01:46 PM   #7
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Is it important to have a reducer or is it ok to bolt a 3" up to the down pipe.
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Old 12-07-2007, 01:48 PM   #8
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Quote:
Originally Posted by brgracer View Post
(I've heard of potential boost control issues with too little back pressure and an internal wastegate).

I officially rendered my MBC useless. Too much flow.
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Old 12-07-2007, 01:48 PM   #9
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Here is a link to the Enthuza site FAQ

http://www.enthuzacar.com/faq.asp

It explains the difference in the exhaust sizing
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Old 12-07-2007, 01:50 PM   #10
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Quote:
Originally Posted by levnubhin View Post
Is it important to have a reducer or is it ok to bolt a 3" up to the down pipe.
a transition helps
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Old 12-07-2007, 02:13 PM   #11
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Quote:
Originally Posted by whaaamx5 View Post
bigger=better
ask all my girlfriends
Your girlfriends are that car savvy? Where do you find these women?
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Old 12-07-2007, 02:33 PM   #12
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Quote:
Originally Posted by Braineack View Post
I officially rendered my MBC useless. Too much flow.
Explain please.
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Old 12-07-2007, 02:36 PM   #13
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too much flow as it, it works badass.



logging over 16psi with the thing backed out all the way, used to only hit 12psi with it almost completely tightened.



here's how an overlay of how it used to look, flat....


but also notice how much better spool i get. It's flat agian with my EBC. I'm sure with a better MBC it would be fine, i had trimmed my spring to dial things in exaclty had I wanted them before. Open wastegate pressure saw a rise from 7psi to 8psi only.
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Old 12-07-2007, 03:12 PM   #14
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brain, do you happen to log EGT's? I'd like to know how much of drop the 3" yeilds in that respect.
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Old 12-07-2007, 03:16 PM   #15
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no. i do have an autometer EGT gauge and probe, but never installed it.
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Old 12-07-2007, 06:25 PM   #16
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Consider this: down the line you may decide to upgrade to something that'll make you want a 3" exhaust. Why buy 2 exhausts when you could buy the 3" now and grow into it?

$0.02
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Old 12-07-2007, 08:23 PM   #17
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so you're all saying you will benefit from a 3" exhaust even if the downpipe is smaller than that?
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Old 12-07-2007, 08:31 PM   #18
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discuss it with Corky. But I think he'll agree.
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Old 12-07-2007, 08:46 PM   #19
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do you have the off the shelf downpipe from begi Braineack?
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Old 12-07-2007, 09:28 PM   #20
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no, cusom built. 2.5" to 2.75"
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