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Old 06-04-2009, 09:20 AM   #1
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Default Making your own downpipe; sizes?

I would like to get your opinions on what size you'd go with if you had the tools to make your own DP (because I do).

My current is a 2.5" v-band and full 2.5" exhaust. It'll probably stay that way for a while, but I was wondering...

I plan on installing a cutout in my exhaust, since I bought the car w/ most of the 2.5" intact, I figure it's a cheap way to make the 2.5" more efficient, as if to flow like a 3" exhaust would. (yes, it will be loud, I know this)

Now I'm wondering if I make my downpipe any size, and install a cutout of the SAME SIZE, then leave the 2.5" behind the driver connected, would that be recommended? I would prefer this method because it'll save money, save lots of time, and WTF would I do w/ half of a 2.5" exhaust system for a Miata once I take it off? Might as well leave it. Oh, and I can always cap the cutout and be stealth, which comes in handy for interstate driving.

So basically, would you recommend a 3" downpipe and cutout reducing to a 2.5" halfway under the car? 3.5" downpipe and cutout? Would a 4" downpipe even fit between the firewall and transmission/would it be worth it since the turbo exhaust housing is only 2.5"?

I would just need to buy another v-band and probably $80 worth of exhaust tubing to make another downpipe and midpipe (running no cat) of any size.

*Biggest reason I was considering this because the newb who made the DP for the car (before I bought it) welded the v-band to exhaust tubing about halfway through the arch of a 90* bend AT AN ANGLE, and essentially the DP is now the biggest restrictor in my exhaust.
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Old 06-04-2009, 05:22 PM   #2
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FWIW, most of your restriction is from the cat and muffler.
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Old 06-04-2009, 08:22 PM   #3
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Regarding the downpipe design: Most important is nice smooth flow, no sharp corners. If your turbine exit is larger than 2.5" or 3" I'd add a reducer to the start, before you turn down. 2.5" is a good choice, larger and you'll have a problem missing the frame.

Advantage of 3" over 2.5" due to flow restriction for that short section is pretty much nothing.

A cutout at WOT and boost will be f'ing loud, unpleasant loud.
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Old 06-04-2009, 11:31 PM   #4
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3" downpipe would probably be the max i would make for my the miata as we wont be running any GT35 monster turbos on our 1.6L I recently made a 2 1/2" downpipe and swaged to a 3" after the flex pipe. Pretty easy. The key to fabbing the least restrictive exhaust is starting out with the right manifold that sets the turbo lower than say the greddy manifold. Begi/FM cast manifolds would be perfect for this. Although i did build my full exhaust for less than the price of their turbo manifolds.

Do what you feel is right.
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Old 06-05-2009, 09:18 AM   #5
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My ebay turbo is a T04E w/ a .63 A/R. I figure its going to want to breath.

Wish I had a pic of the DP, you guys would probably scream in terror.

I think I'm going to re-do the DP w/ 2.5 stepped up to 3" in the shortest step possible, turn down snake between the tranny and frame, go back to the bend just in front of the diff housing and put a 3" cutout and leave the 2.5 behind the diff for when I need to take interstate trips.

2.5" exhaust tubing is HEAVY. 3" will obviously be heavier, I wonder how much weight is saved w/ just a DP, resonator, and turndown? Maybe I shouldn't worry too much about that since my car has some weight removed already in way of A/C and PS, and I have the hollow spoke 14" wheels that are like 10 lbs each.
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Old 06-05-2009, 10:19 AM   #6
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Since you're starting w/ 2.5", I'd stick w/ that. I doubt you're going to flow enough through it to see the difference.

I'd use 16ga. stainless, do it right the first time.
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Old 06-05-2009, 10:39 AM   #7
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My dp is 3" and I had a hell of a time trying to get it between the engine/trans and the steering shaft. When the engine is torqued over it pushes the dp into the steering shaft and sends vibrations and noises threw the car. In order for me to fit it in I had to disconnect the motor mounts and shimmy the engine over and shove it through (got it)...but I have a 3 bolt style flange. If you vband it you should have far less trouble.

Last edited by astroboy; 06-05-2009 at 12:10 PM.
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Old 06-05-2009, 10:44 AM   #8
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through
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Old 06-05-2009, 12:04 PM   #9
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Here's how I did my DP, I have my turbo sitting pretty far forward and there wasn't much room to make the down turn. This it 2.5"

I'm looking for a pic of it installed so you can see what I mean...

Can you tell which welds were mine, and which were done by my pro welding buddy?
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Old 06-05-2009, 12:15 PM   #10
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Here's my 3"



not pretty but it get's the job done
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Old 06-05-2009, 02:15 PM   #11
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I made my own SGDP. It started off at 2" from the flange with 1" on the waste gate. After the 90* I went to a 45* then transitioned to 2.5". I introduced the SG pipe just before the last 45*.

yes, yes I know shitty welding
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Old 06-05-2009, 02:17 PM   #12
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I guess now I need to post pics of mine...

Tomorrow.
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Old 06-05-2009, 03:01 PM   #13
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Yep, let's see it.
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Old 06-05-2009, 03:04 PM   #14
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You guys have some nice lookin dp's!
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Old 06-06-2009, 11:24 PM   #15
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it's an interesting idea to do a cut out off the dp i would run an electric exhaust valve that you could open and close from the cockpick. Then maybe a side pipe or something.
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Old 06-07-2009, 05:14 PM   #16
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Updated the cardomain (no I'm not serious about cardomain, I just use it becaues it resizes the pics and keeps me from messing with it)

Pics of the DP on page 2

http://www.cardomain.com/ride/3245769/2
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Old 06-07-2009, 05:41 PM   #17
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Quote:
Originally Posted by Project84 View Post
Updated the cardomain (no I'm not serious about cardomain, I just use it becaues it resizes the pics and keeps me from messing with it)

Pics of the DP on page 2

http://www.cardomain.com/ride/3245769/2
I would certainly remake that DP, go 2.5", turn down right away and notch the frame out as necessary.
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Old 06-07-2009, 10:11 PM   #18
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Yeah, that's what I thought... I knew it would hurt flow being made that way. Glad I can escape the blame on that one.

My welding skills are better than what you can see there too... that's welding at the storage unit w/ the use of a 5,000 W generator that kept popping fuses after more than 10 seconds of welding. :(

It'll all work for now. Mainly just need to be able to transport the car safely about 4 miles to my house when I close on it in a few weeks. Once there, it'll get put on the to-do list along w/ a million things the house needs.
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