The Great (and stupid) DIY 3" Exhaust Tread
 

Let me first say that I'm fairly certain that I'm going to regret doing this in the long run, but the journey will be entertaining, if nothing else.

My objective: To replace my completely adequate, but somewhat leaky, 2.5", FM-based exhaust system with a (probably horrible, and completely crappy) home-built 3" system.

Purpose: Ostensibly, to achieve 3" exhaust Nirvana. Actually, to have something to do while the cold(er) winter months saps the life from my ever-aging body.

The Plan: Create a masterpiece of design, engineering and good old American know-how (but actually to cease asphyxiating myself when I have to drive with the windows up to avoid freezing to death because I've borked the heater vents the last time I had the dash out).

Right now, all I have is a glimmer in my eye, a shitbox HF welder and a few bits and pieces. Now, before you start with the "DUDE! Get a REAL welder!" comments; a) I don't have spare cash flowing out my ass, and b) I want to make this a difficult as possible on myself because "I'm that guy".

What "bits" do I have now? Just a China-made T25 outlet to 3" V-band adapter (see previous comment about the HF welder for reference), and a 90-degree 3" downpipe from Himni Racing. That's it. That's as far as I've gotten.

Next step, put the car up on a lift and figure out what shapes and lengths of tubing that I'll need, then order twice as much from Columbia River as I think I'll need because there will be MUCH wastage.

For now, it's just a pipe dream...yeah, pun intended.

Buy a used MIG off of facespace and save yourself the headache........as well as a useful measuring device

You will thank us later

I will take this under advisement. The search begins...

Yeah flux core sucks. I know you said you didn't want to hear it but you need a better machine. The orange or green units from HF aren't that terrible.

I use a Hobart 140 mig machine. It works well enough.

I know, I know! Punish me for my sins!! I REALLY want a mig machine (I really DO), but for now I'm stuck with my POS flux core HF boat anchor. I've gone so far as to do the AC to DC "conversion" on it - you know, big ass caps, a rectifier and re-wired it for DCEN. I know it's a POS, but right now I'm cash-limited so treat this as a "science experiment". I'm going to buy some sacrificial stock to practice on. I've been doing some practicing on thin, flat mild steel already - I'm not good (yet) but I'm improving using inferior tools.

If I hit the lotto - not a good bet, 'cause I don't do that - then I'll get something better to work with. Or, if something pops up for a good enough price, then I'll splurge. Until then...

"To infinity, and beyond!"
https://cimg2.ibsrv.net/gimg/www.mia...ebabd34152.jpg

I cobbled together my first 3" exhaust, first time doing something similar too. Only slightly banged off shit when the hose clamps fell off the hangers. Might still be a list of bends I ordered from summit in my thread somewhere.

https://www.lowes.com/pd/KD-Tools-Ex...Cut/1000972618

One of those things is super clutch, but you gotta be using steel and not stainless. Can also find them online a lot cheaper.

Godspeed my man. I (very stupidly) built a custom 3" exhaust for my NA this summer. I like to punish myself, so I did it all with Flux core wire and a cheap MIG/Stick machine from Amazon. Used mild steel all the way, and ceramic coated it in the end. Not the prettiest thing, but it gets the job done. I might still have the parts list lying around somewhere...

I'm not sure what type of muffler you're running, but the classic Dynomax Race Bullet 'Resonator' paired with a straight-through Magnaflow 5x11x22 Muffler suited me well.

Just remember to add flanges so you can take down a section or two at a time. Installing one complete huge exhaust all at once sucks. Ask me how I know..

Just get on OfferUp and Craigslist and keep your eyes peeled for a Lincoln or Hobart 140 mig or a HF Titanium or Vulcan mig. Welding with gas is so much better than smoky flux core that turns out like hammered dog shit most the time anyways. And you'll be able to build it out of stainless like God intended. The welder will pay for itself eventually. Also dump your HF one on OfferUp and get a little money back.

The gods have spoken. I repent of my flux core ways. CL, here I come.

I definitely was going to use mild steel. No way was I going to try this with SS, knowing that there would be many mistakes made. And it will definitely be ceramic coated once done.

My existing muffler will go to the scrap heap. It's 2.5" in/out, so no bueno. Thanks for the resonator suggestion, that was going to be a question for the community. And a nice Magnaflow is in the plan.

Also, this will be done in sections with a midpipe so I can swap in my existing FM cat when it comes time for the annual GA emissions check.

I picked up some practice stock today so "welding class" will resume this weekend. If I get something that looks better than dry boogers on a desert wasteland, I'll post pictures.

Otherwise, the search for a decent rig commences.

If your POS HF welder is like mine, you’re a tank of gas, a $20 Amazon regulator and a roll of decent wire away from running MIG. I converted mine and get very okay results. Plus you’ll need that stuff anyway even if you get a new welder so you really have nothing to lose. If you have money to burn, spend it on a good helmet instead- the HF ones are hot trash and you’ll suck even with a good machine if you can’t see what you’re doing. There are some tricks to get around the lack of voltage adjustment; if you want it cooler, move the ground further away. I’m in ATL and we’ve met at the track, if you want to come check out my setup just let me know.

There's an even cheaper one that is flux only. I'm guessing he has that one.

Seconded on the helmet but if he doesn't have enough money to get a MIG machine then I don't know if he can get a decent helmet either. Maybe just go with a fixed shade one. I've got a Viking 3350 and it's pretty good. The older model that doesn't have 4C tech.

Gonna be that asshole who goes againsy the current. You can 100% do this job with a DCEN converted $80 HF flux core. We did my friend's downpipe with his still running AC. It's ugly, but it works. You are going to have to chip a lit of slag, grind out a lot of welds, and fill a lot of holes though.

If you are blowing through, you can switch to doing a series of tacks instead of trying to run a bead. Also, a piece of copper pipe smashed flat is going to be your friend, you can use it for backing while filling holes, the weld won't stick to the copper.

That said, I went and got myself a $100 Pocket Turbo mig (it's the same as the 120v one snapon sold in the 80s and 90s) off facebook and then a tank of gas. It's way, way better.

I'd be down to take a look at your setup - what plans do you have this weekend? PM me.

Yeah, I've got the bargain-basement one. Bought it about 5 years ago and it sat in the box until last month. And you guessed it, I got the HF helmet as well. My only redeeming quality in all of this is that I didn't know what I was doing at the time, and I just got what I saw, without consultation. Not an excuse, and explanation - mea culpa.

I'm all for being "contrarian"! I know all about the slag, and the BB's, and the smoke, and the grinding, and the blow-throughs - it doesn't take too much time to figure that out real fast. Like I said, I'll be practicing until I can come up on a better rig.

I've been known to do things the hard way, and the wrong way, using inferior equipment. Been doing that for a long time now.

Just some initial bits I've acquired; T25 to 3 in vband adapter (from Himni racing) and a generic EBay 3-in 90* downpipe. These will be the starting point.
https://cimg7.ibsrv.net/gimg/www.mia...e63279eab2.jpg
https://cimg8.ibsrv.net/gimg/www.mia...7b2a2683e1.jpg
https://cimg9.ibsrv.net/gimg/www.mia...82f98f0073.jpg

You're gonna have to cut the shelf with that.

Yep...It's been trimmed already, and the area underneath has been "massaged" with a 5-lb sledge. I'm gonna do a test-fit tomorrow to see if more clearance is needed.

Did a quick visual check of the adapter/DP combo (holding it against the Churbo outlet upside down just to see where it landed). It looks like this adapter is about 1/4" shorter than the 2.5" adapter that's on the car now, and the DP is shallower than what's already there. All told, I'm pretty confident that this will clear the shelf with room to spare.

Crappy picture follows;
https://cimg1.ibsrv.net/gimg/www.mia...a167c7ac3d.jpg

Quick update - I've been sidelined for a few days with a leaking oil fitting on the turbo. The Churbo fitting came with a weird thread pitch (7/16 x 24 - straight), so I'm tapping it out to a more "normal" 1/4-NPT and using a 1/4-NPT to -4 fitting. My challenge has been getting a tap that'll go deep enough into the "blind" hole so that the taper is wide enough at the top to engage enough threads on the new fitting.

Ah, the joys of wonky parts on Chinese turbos! One day, when I grow up, I'll get a 6758 and all my troubles will be gone.

Are you keeping tabs on expenses? Curious to see what it totals out to...

This is going to be on hold for a while. I spun a rod bearing this past Thursday, so I'm doing a rebuild right now. I just got down to the crank today and the #1 rod bearing was toast.

Strangely, everything else looks fine. There were big pieces of bearing material on the pump screen, but the oil didn't look silvery at all. I'm getting the block and head hot tanked and I'm on the lookout for a used crankshaft.

For anyone interested...

The block is clean, there are minor scuffs on the #1 wall, but those pass the "fingernail test" so they'll get honed out. The only bearings that were damaged were the rod bearings on #1 & #4 . The cap end on #1 was completely gone and the cap end on #4 was beat into foil. The #1 piston is toast, when the bearing "left the building" there was enough inertial slop for it to lightly kiss the squish area on the head (there are witness marks on the crown of the piston to show the contact). I was considering "refurbishing" it, but I decided to be cautious and order a new one (should be here tomorrow). The only other thing that I'm considering is to swap out the head for one that's been collecting dust in my garage. I might just do that - again, for safety's sake - and do some experimenting on the old head; radical porting, new guides, larger valves, etc...

One other adjustment that I'm making during this rebuild is to use King bearings instead of ACL's. Twice now I've had to rebuild because of a spun bearing and both times I've used ACL Race Bearings. The first time I blamed the failure on poor assembly technique - I didn't check the bearing clearance then. The second time (now) I think it was because I had low oil supply and roasted the bearings. I can't guarantee either cause, but I can say with certainty that I used ACL bearings both times. Maybe it's "Bad Ju-Ju", but I'm changing to King bearings just in case.

Anyway, tonight is "Valve Lapping Night" on the new head and tomorrow is "Piston Balancing Night"!! Re-assembly begins on Saturday.

FWIW I made an 3" exhaust w/ cutout with my buddy with nothing but amazon pre bent piping and a shitcago welder from HF. Fitment is meh and welds are meh but its totally sealed and works for how cheap it really ended up being. I already had the welder for a couple years to use for random odds and ends just like this. It's definitely hot garbage but it will make metal stick together.

The worst part about a cheap welder is the duty cycle, i could only get maybe halfway around one decent weld(2 minutes?) before it would start to get super choppy.

What kings did you use, race or street? All race bearings are more tolerant to poor tunes, but less tolerant to. Imperfect bearing surfaces. I haven’t had a set of race bearings survive on a crank that wasn’t ground for .25 over sized bearings. AKA You can almost never slap them on a used surface.

I've been on the same crankshaft since 2003 with std size ACL bearings. I did a rebuild 3 years ago and the bearings looked perfect. New bearings went on and I've got 15k hard miles on it now and it's perfect. The crank is literally the same one I've been using since 2003 when I blew up a b6 motor and went BP. ACL bearings are very very good.

When I get to that point, I fear mine will be similar...

The ACL's that I used in the past were "Race". The Kings are "Standard" and the journals mic out within tolerance and are / seem to be "round". I was going to mirror-polish them, but didn't know if that would inhibit the ability to retain an adequate oil boundary layer - I'm open to advice there.

I'm not saying that ACL's are "bad", just that I've had bad luck with ACL Race bearings. Change is good, right?

No for sure. I was just sharing my experience. By all means King is a quality bearing as well

While I've got a little bit of attention and totally unrelated to what's been said before...does anyone use a steel gasket between the exhaust manifold and their turbo. And if so, from where? I have a Kraken manifold and a churbo from EMUSA. Initially, I used the el cheapo steel gasket that came with the turbo - predictably, it was junk and soon failed. I did a tiny bit of research and came to the general conclusion that using a gasket was a waste of time, so I've been running without one for about a year. I know that the sealing surface on the turbo is not 100% so I'm planning on having it "machined" (i.e. flat, level steel plate, progressively finer sandpaper on the flange until I achieve reasonable "flatness").

If I do that, will that be good enough without a gasket?

I run a gasket from ATP turbo and its been good for a long long time, but back in the day I ran no gasket there and was also fine. If you make it flat, once the car heats up the metals expand and there is no leak there. So yea youll be just fine.

I also used the cheap one that came with the turbo and had it decay away. After that I lapped the mating surfaces flat with wet/dry paper and used Permatex copper spray gasket; it seems to have held up very well.

Similarly had a cheap one fail. Huge pain... Haven't seen any reports or personally had issues from mating the surfaces so that's the route I'd go to cut down failure points

I used churbo gasket on taco manifold and had the same problems. So I took it out and coated both flanges in some axle grease and never had an issue.

On my current setup I set it down bare and left it that way, no gasket. It's been fine so far. I don't want more unnecessary shit in that joint causing potential slop than I absolutely have to have.

I have never used a gasket between manifold and turbo with a dozen different manifold and turbo combinations.

Engine is mostly back together again - hope to have it in the chassis this weekend. In celebration (and in line with the original intent of this thread) I've ordered some bends from Columbia River. When they get here, I'll pick up where I left off.
https://cimg0.ibsrv.net/gimg/www.mia...6e4ab15585.jpg

Columbia river and Ace race are where i get all my exhaust materials

Exhaust tubing should be arriving next Tuesday. Does Columbia ALWAYS take this long?

Anyway, reassembly has been delayed because I managed to crack the old Kia thermostat cover while tightening it down on the spacer for the coolant re-route on the back of the head. I've NEVER had that occur before, EVER so now I'm paranoid about it happening again since there are no Kia's in any of the local pick-n-pulls, the dealers have told me that the part isn't available anywhere in Georgia, and the online order (which arrives today) took a week. Does anyone have the recommended torque setting for that cover so I don't repeat this fiasco.

Yes. Columbia is running on a skeleton crew. I think they almost closed up shop sometime in the past year or two. They are also moving into a new building and not producing new bends until sometime in the new year.

I used Mandrel Bending Solutions out of MD fir majority of my pipe needs now.

PS - 14-19 ft lbs

Okay folks. Things are going to start getting stupid 'round here (as promised). The engine is rebuilt, and resting comfortably back in the chassis. It hasn't been started yet, but that's soon to come. But the most important thin (as it related to THIS thread) is that the first "stuff" has come in to begin cobbling together an exhaust system. But before getting to the action, here's a brief explanation of what I'm going to be doing...

(Preparing flame-suit for action)

What I'm starting with is 80% of a 2.5 inch stainless steel system from a Flyin Miata VooDoo kit purchased many moons ago. It's been a stalwart performer over all of these years, but it currently suffers from a few problems; first, it's 2.5 inches. Second, it uses several slip joints that have been prone to leaking, Third, the FM muffler bit the dust a few years ago so was replaced by a cobbled-together set of pipes comprised of a sawed-off section from a stock muffler from the axle-back flange, poorly welded to a 1.5" to 2.5" coupler, welded to an Autozone muffler. Yes, it's a hot mess, but it has served me well.

What I intend to do is to use my shiny new 5-bolt to 3" vband exhaust flange and 3" DP into a custom made (by me) 3" pipe, going into a flex pipe, then to a 3" to 2.5" reducer. This will feed back into the 2.5" FM exhaust, to the axle-back flange. From there, I'll replace the abortion between the axle-back flange and the AZ muffler with another custom-made (by me again) section to round out the system.

Now, you may be asking (perhaps, screaming) the question; "Why keep that 2.5 in, FM relic in your system?" Well, there are 3 reasons; cost, cost and a lack of welding skills/proper equipment.
- Cost #1; That FM section has two key pieces that I can't afford to toss into the shitcan at the current time - the cat and the resonator. Since I still have 5 more years of GA emissions tests to suffer through, I still need a convertor in the mix and I just can't swing a new one of those right now (having blown a wad on the rebuild).
- Cost #2; As much as I want to get a badass Magnaflow muffler, again I can't cut that cost right now. So I will keep the AZ muffler but ditch that cobbled-together, restrictive crap that is in front of it.
- Inexperience/bad equipment; I still have my POS HF welder and it just can't cut it welding SS, so the stuff I'm making will all be aluminized mild steel. Where there are junctions between that and SS, there will either be a vband, or a flange. Only one slip joint will remain and I'll have to live with that for the short-term.

Once I get through this project, and I feel a bit more assured of my "skills" (and replenish my wallet), I'll ditch that "FM section" and build a full 3" system front to back with a new cat, resonator and muffler. So, in the meantime...DON'T HATE!

PS - Here's a shitty, hand-drawn picture that I'm using as my build-guide.
https://cimg5.ibsrv.net/gimg/www.mia...ef56f9de33.jpg

(removing flame-suit)

I did some test fitting this evening and the results are encouraging (after a bit of a scare).It looks like I'll be able to make my plan work. The "scare" was that, at first glance, the very pretty, low-profile 90-degree vband fitting - intended to take me from the 5-bolt flange on the turbo, to the exhaust pipe, was going to be too long. The 90* from the FM kit is about 2-3 inches shorter than the new 3" 90*. It was looking like I would not be able to "turn the corner" and get a 3" pipe to fit in the gap between the trans and the subframe. I was thinking that I might have to take a chunk out of the new 90* in order to make it work, and that would mean taking that to a shop to have it done since I don't trust that HF rig to properly weld stainless steel.

Anyway, just for grins I tried the 90* - 3" pipe in the space just to see how bad it was, and IT FIT! There might be some clearance problems, but nothing some "massaging" with a hammer wouldn't fix. So, I think I can make it work. Here's a picture that might help visualize what I'm talking about. For some perspective, you're looking down at the ass-end of the downpipe into the abyss between the trans (at the top right) and what remains of the shelf above the steering shaft, The -AN fittings are for the water lines that feed the heater core;

https://cimg5.ibsrv.net/gimg/www.mia...a6d5fc1445.png
So, the next step is to get the car on the lift and tack this together and see where I land. Maybe tomorrow?

Okay, I did a thing today;
Vband onto 3" downpipe;
https://cimg5.ibsrv.net/gimg/www.mia...07f438ed82.jpg

Flex pipe onto 3"exhaust tube (w/ store-bought 3" to 2.5" reducer)
https://cimg6.ibsrv.net/gimg/www.mia...53fe5dde7e.jpg

The whole monstrosity in it's entirety;
https://cimg7.ibsrv.net/gimg/www.mia...3f13cdf270.jpg

This will v-band onto the FM midpipe.

Tomorrow's task...re-do the axle-back abortion! My welds are ugly AF. I don't know if they are gas-tight (yet). On the plus side, I didn't blow through the tubing, not once!

PS - this will be a tight fit, it may contact the front subframe and rattle like hell. But I'll cross that bridge when I get there.

Looks good for the first time, I can't do that, so props to you! Just doing, not talking, I like that

Progress!

I did something else today;

The Axle-back "abortion" is gone! It is now a proper 2.5" into the muffler. PS - please don't laugh at my Autozone mini-muffler. I'll get a REAL muffler when all of this gets swapped out for a full 3-inch system.
https://cimg9.ibsrv.net/gimg/www.mia...d210a51d79.jpg

Per my hand-drawn schematic earlier, this is the transition from the 3" downpipe, into a 3" - 2.5" reducer, then into the FM system. The ONLY reason for doing this is the FM system has a cat and a resonator. Cost is prohibiting me (now) from replacing all of that. Also, there are bungs for the WBO2 sensor, and the two stock NBO2 sensors in the FM system. I didn't want to tax my (marginal) welding skills trying to put a bung into the 3" piping - I'll save that for later. Is it ugly? Yes. Are there too many v-bands? Yes. Will it work? We'll see.

https://cimg8.ibsrv.net/gimg/www.mia...05b5382f21.jpg

Tomorrow's "adventure" will be to fab some hangars (right now, it's all held in place with tie-wraps). My car is 50% tie-wraps...

PS - Yes, the lower two bolts mating the trans to the engine are missing. I'll get that done soon.

PPS - All of the mild steel pieces will get a coat of VHT to delay any rusting. I just wanted to mock it all up.

I'm baaaack!

I've been running with the above "hybrid exhaust" for the past few months without issues. Now it's time to finish the job...which is to have a complete 3-inch exhaust all the way to the exhaust tip. To that end, I've assembled;
- Some 3" straight tubing
- Some assorted 3" mandrel bends (a 45* and a "U-J" bend to fabricate the axle-back)
- A generic resonator
- An O2 sensor bung
- Assorted v-band clamps and flanges
- A teeny-tiny Flowmaster (pseudo) muffler

Why not the MT-approved Magnaflow? Because I want "loud and deep", but not "fart-can obnoxious". I'm designing this so that I can swap in different mufflers until I get the sound that I want. I might end up with something that drones and rattles, but that'll match the rest of FrankenMiata to a tee.

Cutting and welding commences on Saturday.

Out with the old...
https://cimg0.ibsrv.net/gimg/www.mia...a0cd6a3431.jpg
and in with the new...
https://cimg6.ibsrv.net/gimg/www.mia...f2f3d53e2e.jpg
https://cimg7.ibsrv.net/gimg/www.mia...7eabf01391.jpg

3" stainless from stem to stern. There are a few touch-up's I need to take care of, but that's it for now.

So I’m in a similar predicament to you. I currently have everything to build a 2.5” exhaust, am I going to regret not just going to 3”?

My sage advice is, "It depends".

If you're running an El Cheapo Chinese turbo like I am, 2.5" is probably good enough. In that instance, the turbo itself is most likely more flow-limited than the rest of the exhaust system. If, however, you are running a "Big Boy Turbo" then a 3" exhaust might be the better option.

Of course non of this is based on scientific theory, or rigorous testing - it's just my opinion, so take it for what it's worth. I went wit a full 3" system because I'll be ditching the churbo for an EFR in the very near future.

I have what is probably one of the least likely to spool well setups and a 2" exhaust is still doing well and a total blast. 1.6, Kraken TD04 manifold, 2.5" kraken downpipe to a 2" test pipe that just changes at the flange, then a 2" ebay cat back that came on the car. Makes 175kpa at 3300rpm and spools up in first gear while trying to pull into trafic (that's "fun" if you aren't ready for it).

Build the 2.5 and send it.

kind of what I figured. I have an entire 2.5” diy exhaust kit I bought for another project that never got finished but have yet to own anything to use it on until now.