Both. but mainly all the weld on the inside of the tube -- that DP will not promote flow.

Why not smooth that bend out? It wouldn't have cost you much more time to have not made it so sharp.

Could you just take a ubend and cut right in the middle of the radius and rotate the pipes 90° to make that transition?

3" (76.2mm) O.D. Tight Radius 180 degree U-Bend - VIBRANT



What kind of space are you fitting such an aggressive snake elbow S into?

Braineack, the weld seams inside the tube are actually flat and smooth, the discoloration and photo lighting make them look raised up.

And psyber, I didn't know about that tight radius until you just posted it. I bought a normal radius Vibrant U bend off of Amazon and thats all I had to use.
I will definitely remake the downpipe sometime in the future, but I think the correct solution to this problem is to build a low mount tubular manifold. I'll probably do that sometime this winter if I feel like spending more money.

That tight radius is nowhere near as tight as what you need. its a 5" centerline radius.

Good work OP for a first time. Are there any outstanding questions after having tried it now? I like helping out when I can with tips for people. Theres a lot of crap advice on that first page, I stopped reading the tips not long after they started.

A few things to note....

1. use stainless filler. for mild to stainless use 409L.
2. don't ever use tape on a weld joint. you'll thank me later. chances are you can hold a section or two in line and mark it with a sharpie, pull the assembly out and tack it. time consuming but so is trying to remove melted tape residue that's now in your weld joint. (protip: use acetone to remove your sharpie BEFORE you weld)
3. fitup is critical to preventing warpage. Ideally, a belt sander will get you there but you can get creative. you shouldn't be able to see light through the joint your about to weld.
4. now that your fitup is spot on, do something like what brain said and use another vband clamped to keep your welded flange flat. using a previously welded flange isn't a great idea though because its likely warped too.
5. if you do things this way, you really only need 3 tacks to hold it together. with your perfect fit up, you don't even need filler wire for these tacks. in fact, I recommend you DONT use filler for your tacks... a tack with filler will show through the final weld if your into weld aesthetics, and we all obviously are. this is mostly for thin wall tube, I use a small dab of filler in my pipe manifolds but its not anywhere near the profile that the weld will have once the completed weld is done.
6. start welding between your tack welds, nowhere near them. the heat will contract your tack weld too.
7. regardless of what your wearing, wear a welding respirator. that smoke is low calorie but its definitely not good for you. For coated metals, take proper precautions.


I don't see anything wrong with what you made besides the obvious tight angle you put on it. the weld coming through the inside is actually not a bad thing. What I would say though is that the o2 sensor doesn't look like an optimal position but I cant see what angle it will sit in the car and it could be a packaging decision. I would generally refrain from putting an o2 on the inside of an elbow, but I have no actual data to suggest what you did is bad or wrong. I try to use the o2 on a straight section if possible but will settle for the outside radius of an elbow and then the inside radius if packaging is really tight.

Cheers,
Matt

Just as an update, I have basically everything built all the way to the rear diff. Not sure where to go from there but right now I am just running a straight pipe 3 inch. Everything seems to be holding up fine... for now.. but I fear the heat treat on my mild steel components will eventually give way and my parts will fail. Hopefully by then I will create a nice thickwall tubular manifold and recreate the downpipe. Right now the turbo I put on is a T3/T4 and spools WAY TOO SLOW anyway, so i'm looking at other options.

Schlammed, the O2 sensor boss is mounted at the 12 o'clock position (sensor is vertical and on top of tube) about 20 inches from the turbo outlet. Readings seem fine as far as I can tell. The sensor cannot collect condensation or any other liquids in its current configuration.

Which t3 turbo did you get? There are only 2 that works good on the miata, 1 is OOS currently, and the other you have to piece together if you want v-bands and such.

I bought this one, https://www.ebay.com/itm/CXRacing-Un...ZSzmw9&vxp=mtr

I read underdogs build here https://www.miataturbo.net/build-thr...w-48386/page2/ and just went with the first turbo I saw. As I said it's way too big. I make about 12 PSI at 5000 RPM. The turbo screams on the highway though, haha.
I've been thinking of moving to this turbo here https://www.ebay.com/itm/T04E-T3-T4-...-/182672207985 Is that a good decision?

I have a tacotaco T25 manifold as well, so maybe I should just go with that and run a chinese T28, like this. https://www.ebay.com/itm/GT28-GT25-G...lXSF~Y&vxp=mtr