My downpipe (because no one looks at my other thread) - Page 2 - Miata Turbo Forum - Boost cars, acquire cats.

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DIY Turbo Discussion greddy on a 1.8? homebrew kit?

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Old 05-12-2008, 01:03 PM   #21
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Quote:
Originally Posted by m2cupcar View Post

I'd be a real bitch to weld the pipe at that shallow an angle. The TIG torch just doesn't fit in some spots. you can do it but you'd end up welding half way up the pipe.

I don't think you'd see much difference between the two though. Leave it as is and motor on.
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Old 05-12-2008, 02:34 PM   #22
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Looks good,

The only question I would have would be how the thermal expansion would be handled? Prolonged highway cruising heats up the main exhaust and lets the wastegate line heat up only from transferred heat. Considering the stainless steel expands more than carbon steel, there might be some cracking in the joints where these two tubes connect.
Rob suggestion of longer wastegate pipe would allow to put another small expasion bellow into the wastegate line before the two connect back together.

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Old 05-12-2008, 02:49 PM   #23
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Don't follow that drawing like a cad file Tim But I get what you're saying.
Here's a merge angle representative of what I was aiming at (courtesy of splitime)
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Old 05-12-2008, 04:18 PM   #24
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Looks great Chad, props for doing it yourself, I only wish I could do the same.

Unless you are pushing 300+ I don't think that juncture will cause any issues, you should be good as is.
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Old 05-12-2008, 04:27 PM   #25
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What TIG welder do you have? I'm looking at buying one in the near future.
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Old 05-12-2008, 04:39 PM   #26
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Quote:
Originally Posted by m2cupcar View Post
Don't follow that drawing like a cad file Tim But I get what you're saying.
Try it on your next downpipe if you want; the current one is already done and will work fine. Looks great to me
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Old 05-12-2008, 04:51 PM   #27
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next time. do this:




in all honesty I think his design makes good use of the venturi...the speed of the exhaust should slow as it hits the transition cone, thus creating a vacuum. Dumping the wastegate outlet here should be a good spot to mix gases.
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Old 05-12-2008, 05:07 PM   #28
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it should work now:


I wasn't knocking chad's design - I'm all about doing it the easy way. My internally gated t3/t4 bolted to a BP turbo mani bolted to an FE3 bolted to my Miata is proof of that.

Speaking of Chad... he never shows back up in his threads.
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Old 05-12-2008, 05:17 PM   #29
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I have never welded anything, but I wasn't sure if the weld should go all the way around the flange in the picture that you provided in the first post. And what welder do you have?
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Old 05-12-2008, 05:23 PM   #30
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its welded completely on the inside.

Quote:
Originally Posted by Corky Bell
This part of the exhaust system is subject to temper*atures ranging up to 1500F a factor that dictates much of the configuration of the components. This is perhaps the most highly stressed section of the ex*haust system. Therefore, strength is of prime importance. Strength starts with the thickness of the turbine outlet flange. This flange can arguably be as thick as 1/2 inch and still call for additional ribs or braces.

As flanges do not stay flat during welding, the mating surface to the turbine must be surfaced prior to installation. Welding is generally harmful to the base metal, A weakened condition thus exists at the flange/tube weld. An easy way around this weak point is to weld the tube inside the flange and only intermittently on the outside, making the weak segments discontinuous.
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Old 05-12-2008, 05:27 PM   #31
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look on the inside
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Old 05-12-2008, 08:30 PM   #32
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Sorry guys i won't be able to dick off on the forum all day anymore. I started my new job in Industrial Automation today. It's pretty cool and I'm happy thus far. Lots of new things to learn.

I'm not going to modify the downpipe by any means just to gain 2.3765% of flow by tieing the wastegate in later. Not just because it would be damn near impossible with the 3" tubing and space available (another 4-5" down would be possible i bet) but tubing is expensive guys. I wish i had known to buy another 45* stainless bend so I could have done it all in stainless but I don't really mind.
I figure for the sake of simplicity and me being a complete newb at tubing fabrication i took the easy way out by keeping the tube short.

It was a big enough of a PITA that i'm pretty sure the next time I build a downpipe I will be dumping the wastegate under the car just so I don't have to "fit" the wg tube to the downpipe which took FOR EVER. Definitely the most difficult part of it all.

Splitime's downpipe looks great, wonder who did his welding.

I really do need more time on the tig welder as my coordination with the torch and my ability to "feed" filler rod through my hand is something that needs work.

When welding tubing to flange, i found that i needed to have the pedal maxed out to keep a weld puddle flowing. I had the machine set up so that it would max out at 125 amps with full pedal.
I'm using a Airco Heliwelder with watercooled torch. It's old as dirt but its free and available. I used 2% thoriated 3/32" tungsten for all of it.

For filler I used 3 different types of rod.
-I used ER308 1/16" for all the stainless tubing joints and hated the way it welds, it just flows poorly it seems.
- ER309 1/16" filler for all my stainless to mild steel flange welds. It welds way better than that 308 garbage. It flows better and looked better as a finished product.
-Random crap in the lab, some kind of 3/32" or 1/8" stainless rod that has a swirled band along its length with no stamp designation telling you what it is. It welds pretty nice and it was free so I used it when i ran out. It was used for all the O2 bungs and everything after the final SS tubing to SS tubing joint.
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Old 05-12-2008, 09:36 PM   #33
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dood I was just pumping up you're thread, I'd trade dps any day awesome job for the first time I've never tig'd anything :(
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Old 05-12-2008, 09:38 PM   #34
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TIG is a blast, i just wish i could afford a nice AC/DC tig welder of my own. And a garage to use it in :(
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Old 05-12-2008, 09:59 PM   #35
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Looking good Chad.
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Old 05-12-2008, 11:24 PM   #36
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I think a problem might exist with the very short coupling of the wg vent tube. I don't see an expansion joint.

Consider that the turbine exit tube stays hot all the time, whereas the wg vent runs along cold most of the time then wells up with heat when the gate opens. The on again - off again nature of the thermal expansion will wreek havoc on the intersections.

I've not measured it, but the gut tells me there will be an expansion of about .060 in the vent tube, and its going to go somewhere. Thermal expansion IS the irrestible force.

Without a slip joint, I would expect a crack in about 200 miles.
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Old 05-12-2008, 11:37 PM   #37
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I've been thinking about that actually corky, mostly due the the difficulty of install the way it is now. I feel like I want/need two slip joints. One on the WG tube and one before the flex section because its a royal pita to slip the downpipe past the engine with a 3" flex section and flange. The flex section is 3" ID so its quite "fat".

But considering my current position regarding access to a welder due to starting the new job i'm going to leave it as is. There are two people Y8s and Splitime running wastegate tubes in similar fashion successfully so I might get lucky. If it fails on me I'll re-engineer it to be better.

Edit: I have exactly enough 1.5" SS tubing left to add a slip joint to the WG tube. For the sake of testing things out on my own though I'll wait for it to fail before changing anything.
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Old 05-12-2008, 11:49 PM   #38
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you want me to check for cracks I bet.

edit to add the picture so corky can see what you're saying:


(sorry, no shots of just the merge)
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Old 05-12-2008, 11:54 PM   #39
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Man, yours is short and stubby. Considering you made good numbers at low boost i'm not going to worry about it.
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Old 05-13-2008, 03:33 AM   #40
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Lookin good!
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