exhaust sizing for a turbo system

[brian89gp]

This is for the DOHC. Thinking of building some headers and a crossover pipe out of schedule 40 (yes i know, it'll be heavy but I want to header wrap it). Thing is that the only pipe i can find is either 2" or 2.5", while the stock crossover is 2.25". I'm not sure on the piping size and can't seem to find anything relevent to a turbo system, just NA.

[dykz34]

I would go with 1.75" primaries and a 2.5" crossover. And we all know that backpressure after the turbo is bad, so you should do a 3" downpipe and catback.

[Jacque8080]

I don't know about the primaries or crossover, but I definatley agree on a 3" downpipe and after the turbo. Are you going for the single or twin turbo setup?

(ha ha ha, bi-turbo/twin dual cam - that'll make non-gm people think for a bit)

MOe

[brian89gp]

With 1.75" primaries and a 2.5" crossover I'm afraid I'll loose exhaust velocity, but I don't know if 2" crossover is too small.

[Anonymous]

Before the turbo a FI and a NA car should use pretty much the same setup as far as size goes. However it is less important that lengths are kept equal. IF you have the way and space, go all out equal length, but if not, it isn't too big of a deal. I would use 1 5/8" primaries(thats what I'm using), and a 2.25" crossover. For sure use 3" after the turbo.

[RacerX11]

Keep in mind the schedule 40 uses ID for measurements, so the 2" Schedule 40 would be 2" ID (actually 2.067"). Normal exhaust pipe measures by OD, so 16ga (0.060") 2-1/4" pipe would only be 2.13" ID. This makes the 2" sched40 pretty close to the stock pipe.

I would use the 2" for the crossover. Stepping up to the 2-1/2" pipe would hurt your velocity. The 2" should flow plenty, since each half of the crossover only needs to flow for 3 scylinders.

Your next step down in size is 1-1/2", so I would use this for the primaries. The 1-1/2" schedule 40 measures 1.61" ID, while 1-5/8" 16ga pipe is 1.505"

Equal length isn't as important as minimizing the average distance from the exhaust valve to the turbo. The closer the turbo to the valve, the more energy content in the exhaust, so more heat goes into spinning the turbo.

Another option is the Schedule 10 304L stainless weld-els. It is thinner than the mild steel, but the stainless is more durable when heated. The schedule 10 pipe is about the same OD as the schedule 40, but with a larger ID (don't ask me how they came up with this measurement system). The wall thickness is about 0.109" for the sizes you would need, so it will be a lot more durable than any pipe header. The 304L alloy is much easier to weld than standard 304 stainless.

Let me know if you have any questions on building the weld-el manifolds. I have been playing around with the stuff the past year building turbo manifolds and O2-sensor housings for some turbo 4-cylinders.

Marty

[brian89gp]

Let me know if you have any questions on building the weld-el manifolds. I have been playing around with the stuff the past year building turbo manifolds and O2-sensor housings for some turbo 4-cylinders.
Marty

Whats the wall thickness of normal exhaust pipe?

I sure do. I was considering the Sch 10 pipe but I didn't have a regular exhaust pipe to measure wall thickness and compare. Which should I use, mild or stainless? I hear horror stories of SS cracking.

For the design I am thinking of the design of the top header in this pic:

http://www.turbomustangs.com/forums/attachment.php?s=7104cc1f9c15d1e9c27fe5d3e81fe4a3&postid=224569

Would there be any benifit from an equal length shorty header?

Either way, the heavy pipe is mainly just so I can header wrap it. I've coated some of my stuff with the Techline coating and made heat shields, but its nothing compared to when I just had it header wrapped.

[brian89gp]

Oh, and is it alright to weld 304L with an oxy/acetaline torch? And what do I do about a flex in the crossover, if any at all?

[RacerX11]

Most regular exhaust pipe is either 16ga (0.060", most common) or 18ga (0.049"). So even with the schedule10, you are twice the wall thickness already.

If you go with the 304L stainless (L means low carbon, allowing easier welding), you can get by with the 0.109" wall thickness. In fact, I think the stainless is the only alloy available in the schedule10. Stainless is notorious to crack, but if designed properly, you can get by. The biggest thing to remember is that the stainless expands a lot more than mild steel when hot, so that is usually what cracks it. On the head flange, it is common to cut reliefs between the individual exhaust ports, effectively making three individual flanges. This allows the flanges to expand and contract, without shearing off the exhaust manifold studs/bolts. If you design the header so that it can expan/contract freely without binding, you are more likely to be crack-free.

I'm not sure about oxy/acetaline welding. My only experience is with MIG and TIG, either of which works fine with the weld els. I have seen header collectors that were gas welded, and looked good, but I haven't done it myself. The stuff is cheap to get, so you could always buy some pieces and practice to see how it works.

I'm not sure about the flex in the crossover. The problem will be getting one that will hold up to the heat from the turbo. I know JC Whitney sells some pretty cheap, but I am unsure of their durability. I haven't shopped around yet for turbo-quality flex joints. You could always build it without, and if it cracks, add one later.

For the header design in the pic, which kinda resembles the shorty Fiero headers, you really wouldn't gain anything by going equal length. You would keep the long runner the same, and effectively add length to the other two, which makes the average length longer.

Its a tough call on stainlss vs. mild. I would lean towards mild, just because it is thicker and easier to weld. The thinner stainless could probably survive with the header wrap, but I don't have enough experience with it to recommend it yet.
The mild will end up being heavier by a couple pounds, but it will be pretty much bulletproof.

Marty

[brian89gp]

Bullet proof is good, as long as it doesn't weigh to much to lift in and out of the car :P. I wouldn't even bother but the elaborate heat sheilding and expensive coatings just to keep stuff from melting is rediculously expensive. With the header wrap I could touch the pipe immediately after a nice romp in the car and not burn myself, too bad that in 6 summer months it had rusted everything pretty seriously.

[Lyle's GTP]

Oxy-welding stainless, I don't believe is possible, at the veryleast not recommended. I would TIG weld it, before MIG. TIG would be easier to get a secure, uniform weld, with the best possible penetration. MIG would be real easy to get holes.

Lyle

[RacerX11]

I have used MIG quite a bit with this stuff, and it works pretty good. As long as you know how to weld, you won't have a problem wih hols. Plus, with the thick material, it is very easy to use more heat and get good penetration. I agree, TIG would be the best, but this material is very easy to weld with MIG as well.

Marty

[Anonymous]

I MIGed mine and it worked well. I did burn a few holes becuz of my lack of experience, about 1 hour, but after playing around I figured out how to fill the holes.

Keep the primary length as short as possible.

Now you are already turbocharged correct? If not remember on the DOHC you can just flip the front mani over and it will fit the back. But they do not match very well, and a header type would be more beneficial. Crazy idea here, but have you considered twin turbo? I don't know about the autos, but with a 5-speed there is plenty of room to plant the turbo right behind the block.

[brian89gp]

I MIGed mine and it worked well. I did burn a few holes becuz of my lack of experience, about 1 hour, but after playing around I figured out how to fill the holes.

http://www.brian89gp.com/misc/weld2.jpg

I like oxy/acetaline

Now you are already turbocharged correct? If not remember on the DOHC you can just flip the front mani over and it will fit the back. But they do not match very well, and a header type would be more beneficial.

Matches quite well. The manifold ports are oversized enough that there isn't any overlap, at least on my motor.

Crazy idea here, but have you considered twin turbo? I don't know about the autos, but with a 5-speed there is plenty of room to plant the turbo right behind the block.

Turbo might fit, but what about all the piping?

[dykz34]

Somebody said it best.... "Go big or go home".