DIY 2.5" ---> 3" Downpipe w/Cat + 3" catback build
 

Just thought I would post some of the progress I have made on fabricating a full 2.5 to 3 inch exhaust for my turbo build. You all inspired me to go out and build my own so I bought an Alphatig 200X and went to town.
Only one image for now, but I will update as I go. This picture is of the 2.5 inch inlet side vband of the 2 stage cat I bought from amazon
Feel free to tell me how ugly the welds are.

Go look at the first page of my MKTurbo for sale thread. Copy my exhaust exactly. Win at life.

Wear good welding gear when under the car and trying to tack things together. TIG welding upside down to tack is absolutely horrible, you need 3 or 4 hands at any point in time. Use additional jack stands and jacks to help hold things in place while you are figuring out fitment. Buy the huge magnaflow muffler, the part number is in my FAQ. Build a 4th exhaust hanger for it to even it out. Get all your pipes from Mandrel Bending Solutions in Maryland, or Columbia River Mandrel Bends in Colorado. Bother have great prices and fast shipping.

Things I learned about welding V bands.... they warp. Really really bad if you aren't careful. What is beautiful now, will look like a tulip in 5 minutes. So, how to avail this?

1. Tons of short beads runs. I think I count about 6-10 beads at a time.
2. Alternate 180° each bead set. Cool as needed to keep it not-hot. I went so far as to wiring a wet sponge in place. It helps cool things and let's you know when things are getting hot.
3. Avoid having to run back over to smooth things out with the torch. This usually causes more harm than good on a V band.
4. Check with a straight edge when you are done. This is when you get to know if it's junk or not.
5. Buy a bunch of extra everything when you are learning to weld.
6. DO NOT TRY TO WELD WITH DIRTY TUNGSTEN. Especially when you are learning. The last thing you need while learning, is an unsteady and u predictable arc column. Sharpen like 5 tungsten on both ends, change them the second you spit contamination on one.

Filler- your weld looks like it needs more. Overall, it looks like it got REALLY hot.. use the pedal, thays why you have one. Back off when you don't need the heat. stand on it to melt in the rod, ten back off.

This is all just my own practice, others might do it different. I am extremely particular about how my fab work looks and performs, and this is what works for me.

I like the v-band assemblies from MBS out of all the ones I have found because the collar part of the flange is longer then most other brands. Whenever I am welding a v-band flange on I always have it clamped to another flange that is welded to a another pipe. This way the heat has more places to move, and it does a much better job at keeping the flange straight.

Cool beans. As long as it doesn't leak. At least you are learning. And if your welds are ugly, but don't leak, and still hold, and you've got mad grinder skills, who am I to argue?

excellent idea. I'm going to steal that one from you, and go make one now. Thinking about it now, I think I might turn one from aluminum or brass. Just like my heat sinks for welding spherical bearing cups. Damn I feel like an idiot for not thinking of this before! You has the smart.

https://cimg8.ibsrv.net/gimg/www.mia...c36eae683d.jpg
I have the alpha tig 200x as well and this is what my vbands look like. The vband is 3" stainless and the cheapest one I could find on eBay that came with a stainless connector. The exhaust is aluminized. After tacking I got away with 4-5 beads. I didn't notice any warping. I just placed it on the end of the pipe and lined it up straight and went to town. Where does everyone else source their vbands?

Would welding stainless exhaust to stainless vband make a huge difference to warpage vs aluminized exhaust to stainless vband?

Not enough to matter on an exhaust for a miata. All my exhausts are aluminized while all the v-band flanges I use are stainless.

LOL that was quick. I have been graced by the presence of god emperor shuiend himself.

And yeah, I think the weld was too hot, but it definitely penetrated, if a little undercut. Another thing that sucked was the fact that there was this huge thick blanket of white residue and oxidation that developed on the both the outside and inside of the weld, but mostly on the inside. I looked it up to see if it was because I was welding galvanized metal, but the guys on welding forums never post pictures for some reason, so I don't have a visual comparison. It was completely white, and had the mechanical properties similiar a very very thin sheet of PTFE or something. It also broke apart and floated in the air easily. I'm not dead yet and this was 2 days ago. Tell me what you think. I cleaned the metal very well before welding including using acetone, so it had to be a coating on the metal or something.

As for warpage, I tack about 8 tacks alternating 180 degrees from each other. I then wait about 10 mins and do the final passes. The vband is a little warped, but not enought to create a sealing problem I don't think. They are the mating kind with a machined lip and groove to center it, so that should help.

EDIT: Damn I just looked at Mandrel Bending Solutions' prices.... thanks for ruining my day. I could have saved $100 by now if I had gone through them....
I also plan to use this fancy high heat polyester tape to hold joints together for fitment and then tack it all together off of the car. Again we'll see if that works.

I'm also curious as to how you all mount your exhaust hangars. I've seen people weld reinforcement plates onto mufflers and tubing where the hangar is supposed to meet.

In my opinion this is a perfect place to be using silicon-bronze filler. The white is probably from te aluminum coating if you didn't sand it off.

Would silicon bronze hold up to the heat?

Also Joseph Conley, I source my vbands from Tough Turbo Parts on eBay. They are here in GA so I get them super fast. They are nice quality, but I think they are aluminized or zinc coated or something. I ordered mine in mild steel so I guess they do this to prevent rusting before they ship.

http://www.ebay.com/usr/tuffturbopar...p2047675.l2559

EDIT: Yeah they say zinc coated right in the description. I'm going to die aren't I...

Good luck using tape to hold things in place. I have not had much luck with it when it comes to exhaust parts.

I weld exhaust hangers straight to the pipe. I try to put a decent side bead on either side of it. As far as I am aware I have had a single one break out of 50+ exhausts. That guy things it was far more because of his crappy roads and potholes and such more then my welding.

Well the way I am going to attack it is to build the downpipe/cat section, and then hang hang and mount the muffler where I want it. This way most of the weight of the exhaust system is already held to the chassis, so the tape shouldn't have too much work to do hopefully.

I also just researched how to get rid of a zinc coating. Brb getting pool cleaner (HCl)

A quick pass around the weld are with nome sand paper should be just fine. I use the cheap stuff on a roll from harbor freight. 150 grit. 1 inch wide.

too late

EDIT: Where should I dispose of this?

Sorry for the delay, I decided to build the intercooler piping first before tackling the exhaust. Anyway, here is the downpipe. I'm running a TacoTaco T3 on a 1.8 and a Cxracing garrett T3 copy. Needless to say there was no space for the downpipe and I really didnt feel comfortable going all faeflora on my firewall shelf. This downpipe is the result.https://cimg9.ibsrv.net/gimg/www.mia...09b1f83733.jpg
https://cimg5.ibsrv.net/gimg/www.mia...592e91ab0e.jpg
https://cimg0.ibsrv.net/gimg/www.mia...6960c80555.jpg
https://cimg3.ibsrv.net/gimg/www.mia...06c0703259.jpg
https://cimg7.ibsrv.net/gimg/www.mia...9c2e1bb4c8.jpg
https://cimg1.ibsrv.net/gimg/www.mia...e849372bad.jpg
https://cimg2.ibsrv.net/gimg/www.mia...5a79ea2e92.jpg

yuk dude.

http://weknowmemes.com/wp-content/up...right-meow.jpg

Savage :rofl:


I wish I had any ability to weld. I'm at the mercy of shops charging whatever they want

Brain are you cringing at the welds or the abortion pie cut bend right after the outlet?

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