I have no experience with either for making your own headers, but I was just curious on what was/is the conscensus for either material. 304 is obviously cheaper, but 321 is supposed to be better for the large temp swings a turbo header may encounter. Comments?

 

IMHO, ceramic coated mild steel is the way to go. It's easy to work with and is very durable.

Stainless is very strong, but brittle. I've seen alot of pictures of cracked stainless turbo manifolds. But, if I had to use stainless, I would use 321. I would use mild steel over 304 even if it was the same cost. I'm sure people have used it without issues, but that is my personal preference.

 

I would think that mild steel would be fine as long as you used brackets to support the turbos. The headers are only going to get red hot for a few seconds on a run.

How well would a coated stainless header hold up to repeated heat/cold cycles?

 

The reason a lot of shops and manufacturers like mild steel is because it's easier to bend than stainless. That said, I'd rather have stainless any day. On a header, assuming you have the appropriate grade, they'll last darn near forever. 304 stainless is excellent material. It's highly corrosion resistant, and can be polished to a mirror finish (unlike most 400 series ferritic stainless). Its drawbacks when compared to mild steel are that it is somewhat less formable, and its coefficient of thermal expansion is higher. The 304 pipes will expand more under high heat. 321 is similar to 304, but is alloyed differently and can withstand higher heat. I don't have the specs with me right now, so I won't get too specific, but 321 is a good choice for a turbo application. I think that for turbo headers and downpipes, 304 would probably work, but you'd be at the limit for its heat tolerance in some situations. 321 would likely be fine. For almost any other application, 304 is a good choice.

For pricing, you have expensive, and really expensive.

 

Nah, they use it because it is cheaper and it works. IMO, 304 pipe is a terrible choice for 'pre-turbo' exhaust... The stuff expands too much, and you're asking for cracks and warping. Like someone mentioned above, if mild steel and 304 were the same price, I'd take the mild steel over 304. 321 or 400 series are both good choices but a tad pricey. The 400 series turns brown, but it works well and will last a long time. 321 will get you the bling and it'll last, but the stuff isn't cheap! Save the 304 for the down pipe to the tail pipe.

 

Here is what ProTurboKits has to say about why they use mild steel for all their street car kits.

Stainless Steel expands and contracts 5 times more than Mild Steel, think about it, a daily driven or often used street car will be started ran to 180* and cooled back off to ambient temperature 4-10 times a day will have serious expansion and contraction, it will just be a matter of time before the stainless cracks.

 

Again, I don't have the exact specs in front of me, but I believe the 304 expansion isn't 5 times. More like 2 or 2.5. If you're careful with the design, I think 304 could be made to work (flex coupling anyone?). 409 stainless is only slightly worse than mild steel in expansion, not enough to worry about. It just can't take the high heat.

PTKs blanket statement about stainless steel expansion is a little erroneous. Thermal expansion of stainless runs all over the map. Some grades are quite acceptable, and others less so.

EddieP, you're right, it's cheaper and it works. Those are certainly reasons, but not the only ones.

Oh yeah, another reason the aftermarket doesn't like stainless: Fewer repeat customers. If the replacement product lasts too long, they don't need to come back.

 

Again, not a smart move to use 304, but I'm sure you can make it work. Just throwing a flex coupling in there isn't going solve all the problems, though. 400 series can definitely take the heat ... in fact, it was designed for use in ***very*** high temp applications. OEMs use 400 series almost exclusively in turbo exhaust systems (everything from diesel semi trucks to the factor headers on Grand Nationals), not to mention catalytic converter casings (can you say hot!).

 

lmao........in case you didnt notice.........people that buy a turbo kit definetly would not buy again from a company that they had problems with the product. you dont get repeat business with a crappy product.

and proturbokits has excellent feedback on the quality of their kit, from what ive seen the mustang boards (the majority of their kits are for stangs). throughout the mustang community, companies that provide turbo kits are made or broken by their reputation with the consumers. PTK certainly wouldnt design anything to wear out. exactly the opposite, they choose mild steel because it will not give their customers any problems.

 

I did a big seach on this before I made my kit. From what I gathered after a while mild steel you will be able to push the header in with your finger because it will become weak. Mild is not made for the high temps that turbos produce. This is just info that I got from guys on message boards such as turbobuick.com.
I made my headers out of 304 stainless. My flange is not connected. If you have seen SLP's header flanges that is the one I use, because of the quick expansion/contraction. These headers have~8,000 miles on them, taken the car on the power tour, and use it as a daily. In other words no problems.

 

Armed with some information:
304 is an austenitic stainless. The main difference between it and say, ferritic 409 is that 304 has more nickel alloyed in the metal. Mild steel has virtually no chrome and no nickel. 409 has 11% chrome, .2% nickel. 304 has 18% chrome and 8% nickel. 321 is similar to 304, but it is stabilized with Titanium. 304 isn't as good at high temp corrosion as 321, even though it scales at a higer temperature. This is a very short, nutshell summary.

With any steel, the temperature at which it starts scaling (rapid oxidation) has a lot to do with which material you choose. Mild steel actually has the lowest temperature tolerance in this respect. It starts scaling at 1200 degrees. How do you get around it? Use thicker material so it lasts longer, hope it won't get too hot during short bursts. It's cheap and it works, but it wouldn't work well if the system were to often get hot and stay hot. It would deteriorate rapidly. Manufacturers get by with mild steel because few of us push our cars long enough for the tube skin temperature (not gas temperature!) to reach critical levels at which significant scaling occurs. Even when we do, it's not often enough to cause too much of a problem. It works for many. Is it good enough? You decide. Oh, incidentally, mild steel is easier to form, and thicker tubes make it less likely to crinkle on deep bends, so thick material is a good thing in mild steel exhaust parts.

409 starts scaling at about 1500 degrees F. 304 scales at 1600, 321 at about 1500.

No disrespect intended on any of the aftermarket headers, including PTK's kit. They have a nice setup. I'm just saying that the material isn't the best. It may be the only thing close to affordable though.

Now, let's look at the thermal expansion differences.
Mild steel - 8.3 microinches/in/degree F (up to 1200 degrees)
304 - 10.2 microinches/in/degree F (up to 1200 degrees)
409 - 7.6 microinches/in/degree F (LESS than mild steel)
309 - 10.0 microinches/in/degree F
321 - 10.7 microinches/in/degree F

304 is worse, but not that much. Not even close to double. Certainly not the problem it's been made out to be.

Formability: 304, 321, 309 are better than 409, 439, but the austenitics work harden rapidly.

304 is OK for headers. Kooks make theirs out of 304, as I believe, do Burns. You can't argue too much with their success. After doing a little research though, I'd be interested in 309 for headers. It doesn't scale until 1800 degrees, it is very formable, it polishes nicely, and is better all around for corrosion than 304. I'm sure it's expensive.

If I wanted stainless headers in a 4XX series, I'd probably use 439 stainless. It's only marginally more expensive than 409, but resists heat better. It's biggest downfall is that it isn't quite as formable as 409. Moving up in tube thickness will help that some for the tight bends.

As mentioned before, OEMs use 4XX series stainless on their parts. Many times, ahead of the cat they use something better than 409 like 439 or 18CrCb. Some still use 409 in the hot end though, depending on the application.

The original question was on 304 vs. 321 and we got off on everything in between. Hope the additional info helps.

 

nateh,

Thanks for the info. 321 and 304 have similar expansion rates, but that's really only half the story. Take a look at the hot strength - 321 is the clear winner.

I was unaware that burns made headers (I knew they supplied components)?? In any case, I'm sure they will be the 1st to tell you to go with 321 over 304 for applications like turbo headers. Every Kook's header for a turbo app I've come across is done in 321.

On a similar note, didn't Jordan Musser build his turbo headers out of a set of hacked up ss304 dual cat headers? I remember seeing a pretty nasty pic of the 3/8" flange @ the #1 primary failing!

 

I haven't seen the pictures, but I definitely agree that 304 is not the best choice for a turbo. I thought it might work in some instances, but would be at its limit. If what you say is true, maybe it's over the limit.

Was it the flange that failed, or the primary tubing? Was the flange stainless as well?

 

It was the flange that gave... the headers were 100% 304 construction, and I think he did the cross over pipe in 304 as well. Just checked and the pic isn't up any more, but it showed the flange bowing out ~1/8" from the head at the #1 primary tube. I think he didn't have a flex section or slip fit built into the cross over pipe at the time, and expansion caused the flange to give when the parts got really hot.

 

No expert here but...

I built my headers from 304 and the collectors and crossover were done in 321. 50+ dyno pulls, and 20+ passes and all is holding up very well. On the dyno, I see the crossover glowing before the headers. Wouldn't stick my tongue on either

This was certainly more noticable with a smaller A/R and wheel. Making the switch to a bigger A/R/wheel combo reduced my backpressure and temp by 160*.