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.

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.

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".

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.

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.

bigger=better
ask all my girlfriends

Is it important to have a reducer or is it ok to bolt a 3" up to the down pipe.
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I officially rendered my MBC useless. Too much flow.

Here is a link to the Enthuza site FAQ

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

It explains the difference in the exhaust sizing

a transition helps

Your girlfriends are that car savvy? Where do you find these women?

Explain please.

too much flow as it, it works badass.

http://www.boostedmiata.com/exhaust/boost_creep.jpg

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

http://www.boostedmiata.com/exhaust/...ol_exhaust.jpg

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.

brain, do you happen to log EGT's? I'd like to know how much of drop the 3" yeilds in that respect.

no. i do have an autometer EGT gauge and probe, but never installed it.

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

so you're all saying you will benefit from a 3" exhaust even if the downpipe is smaller than that?

discuss it with Corky. But I think he'll agree.

do you have the off the shelf downpipe from begi Braineack?

no, cusom built. 2.5" to 2.75"