Instead of inconel, you could use platinum or gold, both have great corrosion resistance......

 

Send me the material & I'll make one!

 

I am actually in the works for building a stainless turbo manifold. Inconel would be outrageous in price (more than just a little more). The problem isn't so much with cracking but corrosion with mild steel. They will all crack about the same. At high temps, which turbo cars easily reach when raced, mild steel will badly oxidize and flake off. With repeated use, it will actually errode clear away and make holes, or get weak and crack. Can't quite seem to get the picture link to work, but look in my gallery for a pic of what it will look like.

 

If the weight of the turbo and rest of the exhaust system is braced properly then cracking wouldnt be as much of an issue. But the extreme heat a turbo manifold generates with all that extra weight hanging from causes too much stress on the manifold which is what will cause it to crack. Stainless is a more durable metal then mild as well as having a higher temperature resistance. And inconel has an even higher temperature resistance. But yeah I guess the "little" part was kind of an understatement. The best thing you can do for a turbo manifold is weld in some bracing to help support the weight of the turbo. Good TIG welding with good penetration and back gassing will also help to increase the chances that the welds dont crack either. MIG will do the trick but it's ease of use cause a lot of people to incomplete welds cause of the fear of burning through. You can be a lot more accurate with TIG and the odds of burning through with TIG are close to zero. proper choice of welding rod and preperationg of the metal and rod is also a step that can lead to cracked welds if not precaution isn't taken. But mild steel in a turbo application WILL end up cracking before a more heat resistant metal. It's not just about corrosion. Mild steel just cant handle the heat & stress the way stainless (and not all stainless either) and inconel can.

 

Mild steel actually has a longer fatigue life than stainless. Mild steel also has less expansion with heat than stainless too. It is only a matter of corrosion resistance that makes stainless the better choice.

 

actually there is a lot more to it then just corrosion resistance.
I'll list off a few:

1) Stainless steel has a lower coefficient of thermal conductivity. In other words, it will transfer the exhaust heat down the pipe instead of into the engine compartment the way a mild steel will. Mild steel will conduct over 200x the amount of heat that stainless will.
2)That heat retention will cause a mild steel header to collapse on itself.
3)weight. A SS header can be made using much thinner wall tubing then mild steel tubing.

4)321 and 347 stainless have awesome fatigue resistance. I don't know which type of stainless you're referring to (there are a lot), but not all are meant for this type of application. Any other stainless is going to be cheaper but it's not going to be as high a quality for this particular app. My guess is most people use 304 cause it's the cheapest. This whats usually used in exhaust systems, but it's not qualified to be used as an turbo header material.

Now let me break this down for you:

Tensile strength of mild steel: 55,000psi
Tensile strength of 321 stainless: 90,000psi
Density of mild steel: 0.283 lb/in(sq)
Density of 321 stainless: 0.290 lb/in(sq)
Coefficient of thermal expansion: 7.228
Coefficient of thermal expansion: 9.6
Coefficient of thermal conductivity of mild steel: 26.98 BTU/ft(sq)
Coefficient of thermal conductivity of 321 stainless: 9.30 BTU/ft (sq)

Those numbers tell me this: Stainless is stronger and so is less likely to suffer fatigue from the weight of the turbo system. The stainless barely suffers from a higher thermal expansion rate but it has almost a 300% lower coefficient of conductivity. So the amount of heat neccessary to expand the stainless to failure would have destroyed the mild steel long before that. Not to mention the mild is far more stressed right off the bat just from having all that weight on it. Stainless has almost twice the tensile strenght so stress from the turbo weight is not as much of an issue.

Corrosion is the main concern in exhaust systems and in NA headers where the heat is not nearly as high. If you tried to use an NA type header with a turbo system then it would demolish itself.

 

good stuff to know for my Strength of Materials class next semester.

 

If Concerned About Weight, Why Not A Bracket That Holds The Turbo To The Block, Simmilar To The Cavilier Turbos. Without That Those Crack Everytime And There Even Stainless.

 

Yeah a good quality manifold thats actually had some thought put into it will have some sort of bracket to support the weight of the turbo. An easy way to do this is to join the turbo flange to the head flange with some steal rod. A race manifold can be re welded after every race and doesn't have to deal with day to day driving. It doesn't have to be durable, it only has to last the race. A flex pipe after the downpipe will also aid in preventing cracks. Theres no point in this debate, it's not like anyone was arguing with the fact that stainless is a better choice. On an inline 6 though you have 6 runners all running to the same turbo flange. The weight gets distributed to all 6 runners so weight isn't as much of an issue with a stainless manifold on this car. A mild steal manifold will still be prone to failure however.

 

Like I stated above, all tubular manifolds will crack. Every manifold I have ever seen or had made for me cracks, It may take a couple years, but eventually it will crack. My 4g63 tubular manifold almost went 3 years and multiple engines, but in the end it cracked. And cracked bad. And that had 4 runners to hold up the turbo. It is just the way it goes. Try to think of manifolds as replacable parts, even though they can cost alot, they are only temporary and are a wearable part, just like brake pads

 

Thats why the manufacturers use cast manifolds or log type on occassion. It's just more durable.