The difference Between 304 and 321 - Subaru WRX Forum
 
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#1 Old 02-17-2003, 11:35 AM
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The difference Between 304 and 321

While searching for upipes I noticed basically 3 different materials being used. Cast-iron, 304 and 321 stainless steel. The difference between stainless steel and cast-iron is obvious but what about the difference between 304 and 321? Well, here is what I found:

321 and 347 are known as stabilized grades of stainless. These are alloyed with either titanium (321) or columbium (347), both of which have a much stronger affinity for carbon than does chromium at elevated temperatures. This eliminates carbide precipitation leaving the chromium where it belongs for corrosion protection. Both 321 and 347 are top choices for exhaust headers, especially turbocharger systems and rotary engines. Since 321 is much more available than 347, that leaves 321 as the first choice, with no sacrifice in needed qualities.

304 is the most inexpensive and available stainless in the 300 series. It is suitable for normally-aspirated header applications, and has been successfully used by many racing teams. It does not have the high temperature fatigue resistance that 321 does, but is considerably less costly and much more available. Most 304 tubing these days has the dual designation of 304/304L.

Practically speaking, there are overlapping applications of 304 and 321 stainless in header construction, but knowing you've got the insurance of the aircraft-grade 321 for the job is definitely worth consideration of the extra cost... if your application requires it.

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#2 Old 02-18-2003, 06:57 PM
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Hey Matt,

Just wanted to add on to your findings for FYI

Both of these steels (321, 347) are basically 304 stainless. They differ by a VERY small addition of either Titanium (321) or Niobium (347) The real difference is their carbon content. The higher the carbon content the greater the yield strength.

The real problem with most headers/upipes is a difference in the coefficient of thermal expansion (CTE) As you block gets hot it expands, as it cools it contracts. What you want is a material that expands and contracts at the same rate as your cast iron block. This allows the seals (gaskets/flanges) to undergo less stress. Most leaks (besides improper installation) are caused by this unmatched CTE. That is why stock exhaust manifold is cast iron, which really meany it has a 2% or more carbon content.

321=(17-19Cr, 9-12Ni + Titanium)
347=(17-19Cr, 9-13Ni + Niobium)

As for the dual designation theory, that is incorrect!!! L stands for low carbon.
304 L grade Low Carbon, typically 0.03% Max
304 grade Medium Carbon, typically 0.08% Max

Hope this helps out. I am interested in how your turbo back design is coming along. Be sure to take pics for us all.
Brian
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#3 Old 02-18-2003, 10:49 PM
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Quote:
What you want is a material that expands and contracts at the same rate as your cast iron block.
So by this theory a cast-iron uppipe would actually be best. Right? I can only think of 1 1/2 uppipes that are cast-iron, TurboXS and 1/2 of the APS uppipe.

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#4 Old 02-19-2003, 12:46 AM
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Hey Matt,

Whoops!! The EJ20 is an aluminum block, I forgot to put that in there!! Sorry for the confusion. I've been a rotor head (RX-7) for the last ten years so I just kinda went on autopilot. Cast aluminum alloys CTE vary on composition by a lot, so it would be hard to comment specifically on our block vs. cast iron without knowing what they are using. I'll look around and see what is going on.

The idea of cast iron is that it can handle thermal fatigue very well. This is due to the high carbon content in the alloy.

Also, if you are still building this yourself, you may want to look into cermic coating your down pipe/headers. Flow characteristics of air are dependent on density. Hot air is less dense, so it is easier to create larger areas of laminar flow. As the air cools on exiting the engine, it becomes denser, making the flow turbulent (not good for exhaust). If you can keep the heat in your pipes by coating them with an insualtor (ceramic coating) then you hopefully create a better flowing exhaust. I now it sounds counter intuitive at first. It depends on how much time and money you want to spend on this project! But I'm digging it so far.

Brian
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#5 Old 02-19-2003, 01:02 AM
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syncopatio,

I am creating my own turboback solution (see pict below). I have ordered the muffler (3" BorlaXS) and the Hi-flow cat (3" MagnaFlow) and should be getting them any day now. The reasons I'm creating my own is because of budgets reasons, I'm a cheapass, and aftermarket exhaust is over priced. Not sure what to do with the uppipe yet. http://www.gruppe-s.com has them for $149 (2" ID pipe) or I was thinking of buying an extra OEM uppipe and modifying it (gutting it or cutting it out and replacing it with straight pipe). Remember, I'm on a budget so be nice.

Anyway here is my turboback if you have not seen it yet:

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#6 Old 02-19-2003, 08:38 PM
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Hey Matt,

Like I said, I dig it. I read your reasoning on another thread and it seems like your on the right track. I looked up the numbers on what the expansion will look like, and it appears that our cast blocks expand about twice as much as normal cast iron. There are special gaskets for applications such as this, made from molybdenum disulfide and PTFE (teflon). But this all might be far enough down the exaust chain that it might make very little difference. I would be careful around the header and uppipe areas though.

Be sure to get a nice gauge piping. This is going to make a difference in the tone or your exhuast. I don't know what is being offered too you, but go with a heavier gauge (at least 1/8"). Don't forget we want to see pics!!

Brian
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#7 Old 02-19-2003, 10:29 PM
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Brian,

I'm sure the $150 quote I got from the muffler shop was not for stainless steel. And I'm not really sure what it is. I think it's just standard stuff (aluminizied steel?). All I know is that it is in my price range. I'm not to worried about it because the climate here in oklahoma does not rust out exhaust pipes. And in the winter they don't lay down salt on the streets, they lay down sand. When I get a car wash I always get the under-bath to clean the belly of the car. My dad has aluminized steel exhaust on his truck for the last 12 years, he got rear-ended last month which bent up the pipe so he had it redone. But even after the 12 years under the truck it still showed no signs of deterioration, it could have gone another 10 years.

I'm assuming that the thicker the gauge pipe the less vibration and the lower the tone of exhaust. Is this correct?

I am creating a page on my website dedicated just to the MattSpec Turboback. I will take pictures of everything and add them on that page.

Matt

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#8 Old 02-20-2003, 06:02 PM
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i researched the differences between these alittle when i was deciding on uppipe and downpipe here are some of pages i found useful.
304 technical info
304 321 347 grade info
steel grades info

"it aint cool to be cheap, it aint cheap to be cool"
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#9 Old 03-04-2003, 12:58 PM
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I also found a price difference between the two:
both are 16 gage and
- 1 3/4 O.D.
- 1.17 lbs

321 straight pipe is $19.65/foot
304 straight pipe is $10.18/foot

reference:
http://www.burnsstainless.com

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