Header modifying: how would GRM do it?

I'm building a 4 cylinder 190E 8V and I want to replace the ugly 50 lbs cast iron manifold as seen on this picture taken the day I got the car:

It is a cast 4-2-1 design. It could probably flow pretty good already.

Mine had a 4" crack on the underside. That was probably the mystery exhaust leak I could never find. I removed the manifold and it went straight to the metal recycler.

On Ebay I bought a damaged header from a 2.3-16 car. Also a 4-2-1 design. On one of the outlets, there is a broken piece:

Also, there is a crack around that brace thing near the outlets (all 3 pics are the same crack):

That 16V engine has the same block as the 8V, so same port spacing. Except the head has larger oval exhaust ports instead of round ports like the 8V head.

And finally, it seems to be bolt-in with the stock downpipe. It'll make a nice longtube design. If it doesn't make more power, at least it'll look good and will make the engine scream:

The joint at the downpipe is made with those flexible donut things. The donut is integrated on the header side.

The header's primaries are 1 3/4", the downpipe is dual 1 7/8" into a single 1 7/8". The headers and downpipe tubes are non-magnetic, I assume stainless steel. The downpipe flanges are steel and the head flange is magnetic and rusted, but not rusted like I expect a 30 year old piece of steel to be.

Now here's how I plan to modify it to fit the 8V. I've never done anything like this so let me know your opinion please.

I had a shop laser-cut a flange for the 8V head out of 3/8 mild steel. The diameter of the ports is 1.5":

I think I'll cut a pie-shaped piece in the bottom of all 4 primary tubes. The length will be like 1"-1.5". For the width, I'll calculate how much circumference I need to have a pipe that is 1.5" ID and cut the extra on the oval tube. Then I'll crush it back into a circle in a vise. Is there a good tool to get the tube to be in a perfect round shape? Something like a steel cone or round bar that I can hammer the tube onto? After the tubes are round, I'll weld the pie shut and weld to the flange. The flange seems to be already not straight... Can this be milled after welding? I guess it'll move a bit during welding, even if done in small passes to minimize heat.

For the broken piece, I think I'll get some 1 3/4" pipe and fab a new section and brace. I had a shop cut me 4x 2 bolt oval flanges that I can use instead of the donuts. There is a place on the transmission to hook up an exhaust brace. Will I need flex joints? 

As for the downpipe, I plan to run 2.25" exhaust and a spun cat. I'll cut at the merge to weld the 2.25" tubing. I thought about moving the collector upstream and putting the cat in that straight section but then I thought maybe Mercedes thought about this and tuned the collector locations and I should put the cat at the stock location (ie; in the middle of the car)... Also, can I put the O2 bung for the standalone EFI in the dual downpipe section or I absolutely need it after the merge? 

My welder is a small flux-core/gas MIG 110V. I don't have gas but I can get some. I have 0.030" mild steel flux core wire. I welded random stuff before and also redid the floors in the car. At the end I was doing okay and could weld without burning through the thin steel. But honestly I'm no welder... I might just tack all the header pieces and take it to a welding shop for final welding.

Rocambolesque said:

For the broken piece, I think I'll get some 1 3/4" pipe and fab a new section and brace. I had a shop cut me 4x 2 bolt oval flanges that I can use instead of the donuts. There is a place on the transmission to hook up an exhaust brace. Will I need flex joints? 

1) V bands are way easier in this case.  They seal great, and the orientation is not critical, unlike a flange where bolt holes must align.  I used ~$10 vbands off ebay for all the exhaust connections on the last exhaust I did, and I will certainly do so again next time.

2) yes, you'll need flex joints

I prefer band clamps over v bands. You only get one axis of misalignment with v bands, and the clamps are basically single-use because the stainless fasteners love to gall. Band clamps let you deal with gaps as well as rotation and you can use them over and over. 

The clamps that are on there give pretty good alignment options as well. You can be off in angle or in rotation.

I’d put in a flex joint and avoid too many braces. That’s where you’re most likely to get cracks, where the exhaust is constrained so it can’t grow. O2 sensor can go before the collector, but you won’t be reading all four cylinders if something acts funky.

Keith, do you have a link to the band clamp style you use?  All the ones I've seen rely on friction to keep the pipes together as opposed to the vbands which are mechanically locked in that direction.

Yeah, the fasteners on a v-band can be annoying, esp. on the ebay-grade stuff.  Gotta be careful with them and sometimes use lubricant.

I haven't had one fail on the exhaust I built, but its use has been limited thus far.  It has come on and off a few times and they have been very handy in that regard.  However, the one downside to the lack of orientation alignment is sometimes it feels like you need a 3rd or 4th hand to install the exhaust.

I just use those friction ones (like this). They keep the pipes nicely aligned. They may not stand up to an axial pull, but when does that happen on an exhaust?

I actually cut the V bands off my Miata exhaust because they were such a pain to deal with on a car that gets torn down fairly regularly, and they actually were prone to damage because they were a low point in the exhaust. Going to the bands allowed me to build a better skid plate with more clearance and made it easier to pull the exhaust apart with no loss of any sealing or function.  My v-band clamps weren't "ebay grade", we got them from our high-end exhaust manufacturer. Galling is just the natural condition of tiny little stainless threads, even if you use anti-sieze. V bands are very sexy but I don't like to live with them unless I need the mechanical connection, such as on a turbocharger mount.

Hey thanks for the replies! The band clamps sure look easy, but I looked around quickly at different online part stores and I can only find 2" and up. I would need 1.75"... I'll search more, maybe I'll find something.

I sawzalled the pipes off the flanges this weekend. Then I started shaping one of the primaries back into a round:

I decided to see if I could shape it without having to cut and weld to reduce the diameter. I think I'm doing good. My vise broke, so for now I'm only using a hammer and the old flange as an anvil. Also, a large c-clamp can help. But really I gotta find another vise. I'm at the point now that the tube will fit over the 1.5" ID of the round ports:

The material has a lot of springback, so to make it yield you have to exaggerate the shape a lot. I'm taking my time as I don't want to crack it.

I know the bigger the primaries, the higher it's supposed to shift the powerband... But now in a mostly street application on a relatively stock and low power engine, do you think this would matter? If anything I can chamfer the outer edge of the flange to make the transition smoother. The ID of the tubes will be 1.7" and the ID of the ports is 1.5".

Here is one in 1.75" although it isn't as long as the 2"+ ones. Maybe that would be a good thing?

https://www.amazon.com/Torca-AccuSeal-Exhaust-Band-Clamp/dp/B004G8UTCQ/ref=sr_1_2?s=automotive&ie=UTF8&qid=1540992510&sr=1-2&keywords=TORCA+BAND+CLAMP+1-3%2F4

I think i would try to find something cone shaped to put over the end of the tubes, or even use a muffler clamp tightened around the tube to round it out. (Rotating it so it doesn't flatten, of course)

If you aren't able to get it round enough, I would suggest welding a short round tube to the flange and then use it as a transition, possibly sleeving the original tube over it, if possible. 

I got the tubes back to almost round. I used a vise and a hammer at first but in the end the muffler clamp trick worked best. 

Then I prepped the end of the tubes for welding and tacked the pieces to the new flange:

But I can't seem to get the settings right on my machine. I have a little 110V flux core/gas welder that I am using in flux-core mode with 0.030" wire. I know, not the best setup... I can only seem to get beads like this:

I am trying to weld the tubes which are about 1/16" thick to a flange that is about 1/4" thick. I can either have penetration on the tubes and barely melt the flange, or I can melt the flange and blow a hole in the tubes. I welded car floors before, I know sheetmetal is more difficult...

The machine I have lets you set the feed from 1-10 and 4 different voltage settings:

The weld pictured above was done with the machine set at those parameters. I tried putting the voltage switch to 2 and backing off the feed a little bit but it blew holes instead. What am I doing wrong? There is probably a technique where you start the puddle on the flange and move to the tube after or something?

Heat that flange with a torch so you don't lose a ton of energy to it and that will make welding to the dissimilar thicknesses a lot easier.

     You want to point the weld wire more at the thick plate, and basically weld to the plate, with the edge of the weld picking up the pipe.   Tack it in a few spots first to get it positioned, then you can focus on the actual weld.   Is the pipe stainless?   You can get stainless flux core weld wire....

Might want to get a steel plate to bolt on the exhaust manifold plate so it won't warp.drill all the same holes ( all if them) tighten down all the bolts so the exhaust manifold plate won't warp

 Then instead of doing full welds on the tubes do1/4 welds  th en5 move on to next  then after those are done .repete the same tubes untill each one is completely welded, this will ensure no warping of tubes,  done th is a few times for friends custom work 

In reply to Rocambolesque :

If you try to weld the whole tube,you will burn thru, just do small sections at a time, sort of like a couple spot weld at a time, this way you won't overheat the thinwall  tubing ,and burn thru it.

In reply to Rocambolesque :

You can also to a lot of tack welds, then just go back over them with a better weld bead