yoshimitsuspeed
Automotive
- Jan 5, 2011
- 191
If you listen to aftermarket car talk there is a ton of back and forth and many opinions on what materials are best for automotive exhaust systems. Once you get past the downpipe you can get away with just about anything. Some have even done aluminum with surprising success.
But at the manifold especially on a high performance motor there is a lot of heat and over time heat cycles and fatigue will make just about anything crack eventually. I have formulated opinions of my own but I don't know enough about materials engineering to feel I am any more qualified than most the other opinions out there and I would like to have a better understanding of things on that level and be able to develop better products as well as explain things better when asked.
I have a basic understanding of materials properties. Understanding tensile failure of a bolt and how that changes relative to heat and things like that are pretty simple stuff. But when it comes to exhaust systems if it's designed remotely properly yield and tensile strengths really shouldn't be a concern unless you get the metal hot enough to eliminate it almost entirely. Ductility could be important but across a wide range of temperatures. I don't imagine it's a big factor though it could play some role in fatigue which I figure has to be in the top three along with I believe thermal cycling and possibly carbon absorption. High performance motors can definitely get into the range of heat that should allow pretty quick and easy carbon absorption but it's my understanding that metal can absorb carbon at lower temps over time. I would like to know a little more about how that could play into the longevity of a manifold. I have always believed the best answer for any alloy is something like a ceramic coating on the inside that would prevent carbon absorption.
Now a lot of pipe resellers and a lot of fabricators will swear up and down that stainless is the best material for manifolds but I have always had a bit of a hard time believing this. Largely because of Stainless's very high CTE and also the fact that even the most high temp Stainless is usually specced for very close to what high performance exhausts can hit. 304 is the most commonly used but for high performance applications they will try to sell you on the benefits of 316 or 321 but are these benefits applicable to headers? Tech sheets brag about things like creep and rupture resistance but rupture isn't going to be a big concern. Would creep help lead to fatigue cracking? Would 321 be less prone to fatigue cracking than 316, 304, or mild steel?
If the manifold is hard anchored in more than one place it seems the CTE of stainless would definitely be a huge disadvantage and make it more likely to fatigue in areas that flexed more. Even in a system that allowed more movement I still imagine that moving and flexing must fatigue the material faster, especially in something like a manifold where the temps can change drastically across the surface and over time.
Aside from it's corrosion resistance is there anything that would make stainless better than mild steel in terms of longevity and crack and fatigue resistance?
What about mild steel with thermal barrier inside or in and out vs uncoated stainless? Would coated mild steel not be at least as good if not better?
One other thing I have heard that seemed odd to me were a couple claims that thinner was better. This goes against all my materials understanding and I would always want to go as thick as could be justified in terms of weight. But other than weight is there ever any advantage to going with thinner piping?
But at the manifold especially on a high performance motor there is a lot of heat and over time heat cycles and fatigue will make just about anything crack eventually. I have formulated opinions of my own but I don't know enough about materials engineering to feel I am any more qualified than most the other opinions out there and I would like to have a better understanding of things on that level and be able to develop better products as well as explain things better when asked.
I have a basic understanding of materials properties. Understanding tensile failure of a bolt and how that changes relative to heat and things like that are pretty simple stuff. But when it comes to exhaust systems if it's designed remotely properly yield and tensile strengths really shouldn't be a concern unless you get the metal hot enough to eliminate it almost entirely. Ductility could be important but across a wide range of temperatures. I don't imagine it's a big factor though it could play some role in fatigue which I figure has to be in the top three along with I believe thermal cycling and possibly carbon absorption. High performance motors can definitely get into the range of heat that should allow pretty quick and easy carbon absorption but it's my understanding that metal can absorb carbon at lower temps over time. I would like to know a little more about how that could play into the longevity of a manifold. I have always believed the best answer for any alloy is something like a ceramic coating on the inside that would prevent carbon absorption.
Now a lot of pipe resellers and a lot of fabricators will swear up and down that stainless is the best material for manifolds but I have always had a bit of a hard time believing this. Largely because of Stainless's very high CTE and also the fact that even the most high temp Stainless is usually specced for very close to what high performance exhausts can hit. 304 is the most commonly used but for high performance applications they will try to sell you on the benefits of 316 or 321 but are these benefits applicable to headers? Tech sheets brag about things like creep and rupture resistance but rupture isn't going to be a big concern. Would creep help lead to fatigue cracking? Would 321 be less prone to fatigue cracking than 316, 304, or mild steel?
If the manifold is hard anchored in more than one place it seems the CTE of stainless would definitely be a huge disadvantage and make it more likely to fatigue in areas that flexed more. Even in a system that allowed more movement I still imagine that moving and flexing must fatigue the material faster, especially in something like a manifold where the temps can change drastically across the surface and over time.
Aside from it's corrosion resistance is there anything that would make stainless better than mild steel in terms of longevity and crack and fatigue resistance?
What about mild steel with thermal barrier inside or in and out vs uncoated stainless? Would coated mild steel not be at least as good if not better?
One other thing I have heard that seemed odd to me were a couple claims that thinner was better. This goes against all my materials understanding and I would always want to go as thick as could be justified in terms of weight. But other than weight is there ever any advantage to going with thinner piping?
