build thread attenpt 2...diy manifold and exhaust
 

Posted a turbo build thread for my '90 miata back in July. Things have changed since then (I am now living in a different state) and so has my build. So here goes:

sr20 turbo w/ internal wastegage
begi s3 downpipe
ebay intercooler setup
fm blow off valve
ms1 engine management wired for boost control
left over boost solenoid from hondata
rx7 red top injectors cleaned and flowtested by witchunter performance
upgraded competition clutch
cx racing upgraded radiator
maf delete and afm addition per the miataturbo guides

The biggest part of this build will be the custom fabrication of my manifold and exhaust both of which I will be doing myself with a Lincoln 110v mig welder

I have been heavily referencing https://www.miataturbo.net/fabulous-...anifold-66261/ for my manifold. This appears to be solid work but I do not have access to tig. I will be using mild steel weld els and flanges to compensate for this. I have been able to get welds down to 16ga steel with good success so this shouldn't be an issue so long as I am careful. I will be using oem 300zx inconel studs to hold the manifold and turbo flanges together.

I am split between a top mount and bottom mount manifold for my setup...a bottom mount should work better with the dp i plan on running and wont affect ac or ps since both of those have been removed from the car. Any advice on whether a top or bottom mount setup would produce a better manifold?

The exhaust will be 2.5" tubing to match the downpipe outlet made out of aluminized steel purchased from Mandrel Bending Solutions. I am living in Alabama which has few emissions laws so I will be deleting the cat and replacing it with a resonater. All parts have been purchased and I am awaiting delivery. Any advice on exhaust routing, other things to watch out for? I am new to all this. Really appreciate all the help.

You can get stainless wire and a different gas for your 110 mig, if you want to build it with stainless. I think I used 307 wire and tri-mix gas, for my exhaust. You won't get the cool gold/blue/teal welds, but you do get nice shiny ones that are fairly corrosion resistant.

Wait a minute. You're going to build this setup around the downpipe?

That won't end well. The begi downpipe was made for the begi manifold...

Also. MAF delete, and AFM add? What is this about?

you know what's easier? buying a BEGi S3 manifold...

If you diy a manifold to fit the downpipe it will probably have to end up looking something like the one the picture.
(Less shiny thing next to the golf ball / 3.5" downpipe.)

https://www.miataturbo.net/attachmen...ine=1394569706

Yeah, don't build the mani to fit a downpipe. Another way to look at it: if you have the skills to build a manifold, you can certainly build a downpipe. Especially if you are going with sub 3" piping, it will be super easy to fit up. And if you don't already have the rx7 injectors, rx8 injectors can be had almost as cheap and are much better injectors.

I definitely meant to write iat...not sure why I wrote afm.

Ryan-I got the stainless wire. Would it be possible to weld stainless with the stardard 75/25 argon co2 mix or is a trigas mix essential here? As long as the welds hold under boost pressure appearance isnt all that important to me anyways.

Baron- I do have the rx7 redtops and they have been cleaned and flowtested by witchhunter performance.

Everyone else- I seriously considering just fabbing the downpipe myself now. What gauge tubing would you guys use? 16ga exhaust tubing? The exhaust gases would be cooler by the time they hit the dp so I'm thinking that would be good. Anyone with experience who can chime in here?

Thing is I got this downpipe for a really good price from a member on here a few months back so I've really been trying to work it into the setup. Yall are right though I'm gonna sell it if its gonna be that difficult to make it work. I could get the manifold through begi which would be easier but I'm definitely commited to the whole welding thing as of now. Would be a good learning experience anyways. Thanks again guys.

IMHO. You should just make a mild steel manifold. Use some nice thick tubing. I wouldn't "experiment" with trying to wire feed weld with stainless.
Also, the exhaust sees no pressure. It will rattle and crack over time.

Either sell the begi downpipe and make your own 14 or 16 gauge should be fine, i've only ever used 16 gauge. Or you can chop up the downpipe you already have and make it work. But i'm pretty positive the begi downpipe is stainless. I wouldn't experiment with that. It is my job and i still wouldn't.

I gotcha. Everything is mild steel right now except for the resonator which i could only find in stainless...really annoying. It was cheap though so if I do find anything mild steel ill definitely make the switch. I'll stick with the 14ga just to be safe. That begi downpipe is definitely mild steel. I remember the seller saying something about how it wasn't stainless which is why it was being sold for less money. I'm gonna sell it anyways so it shouldnt matter. Thanks.

Just for the spread of good information, this isn't quite correct. The turbo mani most certainly does contain pressure. Without pressure in the mani, the turbo could not create pressure of its own. The exhaust will also see a very small amount of pressure, albeit small.

Experiment? Certainly don't use 75/25.

My mistake, you're right and i knew that.
But that's not what will eventually crack it.

What do you guys think about welding in support brackets for the manifold? Will that help prolong cracking?

Gussets. Couldn't hurt. Downpipe brace could also help. Crane brace. There are options.

You should be able to use the mild wire for everything including the stainless resonator. The welds will rust with the mild steel, but the weld will be fine strength wise. Use sch.40 weld els for the mani, 16 gauge tubing for exhaust. You could go with sch. 10 and 18 gauge, but using the stuff that is a bit thicker will be more forgiving to your welding skills, and the weight won't be significant in your case.

And yes, build your own downpipe. You will learn a lot building your first downpipe. It will be a bit of cutting, test fitting, aligning pieces with sharpie marks, tack welding, test fitting, and then final welding.

What you really need to do is choose your turbo location and go to town on your first manifold. Your first manifold will not be pretty unless you have previous fab experience, but that isn't the point. If you build one mani, you will build another one within a year that will be 10 times better. Then you may build a third one that is 10 times better than the second.

Convincing yourself to build it yourself is the hard part, now go to town.

And I wouldn't deal with gussets or bracing for your first one. Build it simple. If it cracks, analyze if it is worth fixing (with the help of the forum) or build another manifold.

Ok. I ended up going sch 40 on the weld els. Just got all my stuff in yesterday so I should be posting up some pics of the manifold tonight if i get it done. Thanks again.

Well the manifold was a no go this weekend as I ran out of gas...I really need to get a bigger tank. Once I grab some tomorrow I'll post some pics of what I get done.

Started messing with Megasquirt instead after tracking down and fixing a few vacuum leaks. Installed the rx7 460cc red tops and began tuning the ve tables. Any of you guys have issues getting the coolant sensor to read? It starts out at 108F but doesn't change the whole time the car is running. The resistor installed on the board is a 2.49kohm so it should be reading correctly. I calibrated the sensor beforehand using an rx7 coolant sensor in easytherm. I attached my datalog as well.

with the datalog

Well here is what I got so far...the manifold is done and I've started work on the exhaust. Apparently my old exhaust was held in place by rust and hose clamps? Was too lazy to take off the bolts so I used a sawzall ftw. The manifold looks horrible but hopefully the welds will hold for a few months or so. The exhaust fabrication is going much better its looking like I should be done with that by the end of today.