Why isn't aluminum a choice for headers... is it just because of material cost? As far as I can tell, just the tubes would cost about $200 to make a set.

I don't see why exhaust heat would be a problem, it couldn't be too much hotter than aluminum heads run in the chamber. Even then, they could be ceramic coated inside and out. It's not as if the aluminum is gonna melt.

It seems like the weight savings could be worth the extra cost to some.

The heat lost from the headers actually won't make a significant difference over steel, believe it or not.

What about corrosion? Should be better than carbon steel, even at the high temperatures.

Fatique, thermal expansion, manufacturability?.... what's the biggest excuse?

 

If it's weight you are concerned about, the steel ceramic coated longs tubes are light anyway compared to stock. I have mine from EM and could not believe how light they were.

 

Pontiac tried aluminum exhaust manifolds on the 1963 super duty 421.

Apparently they would start to melt after about 14 seconds at wot.

 

Alum should be good to 1200 degrees. Without cooling, I could see a problem there. So maybe a jet hot coating would make all the difference?

And if your coating started deteriorating, then the headers would melt?

 

I remember hearing about that.

 

That could be a problem considering the catalina is a 15 second car. Just add nos.

 

Many companies make aluminum headers.

 

Got any links ?

.

 

I won't say you're wrong but I've never heard of any in 35 years of fooling with cars. Are you sure you're not thinking of aluminized steel?

 

Not sure if they are for vettes. But BBK & SLP are just two.

here is a link I just pulled up with a google search
http://www.lmperformance.com/1635/2.html

 

Aluminum-coated maybe.
Not aluminum headers.

They would collapse and blow out from the heat/pressure.
I'm with the Kid on this one.


BTW, the link shows steel headers that are aluminum-ceramic (also know as alumina; a very high temp ceramic) coated.

Larry
code5coupe

 

Yes, they are aluminum-ceramic coated. It says right in the text that they are constructed of 409 stainless, the same as my DRMs are.

 

Is stainless steel the new nick name for aluminum? Google again.

RACE ON!!!

 

Aluminum=409 stainless steel tubing

Aluminum headers would melt!

 

Although combustion temperatures actually exceed the melting point of aluminum in some engines, the heads survive because fuel acts like an insulator (a little of it coats the inside of the chamber) and the heads are cooled constantly. Not sure on the detailes, but a combination of the two is what keeps the heads together - at least that's what my poor recollection tells me.

 

I would think that alu. headers would have a short life span. Fatigue would set in very early in there life. I have seen tubing being stressed, same O.D. but the walls on the Alu., tube where thicker. The Alu tube failed at about 20% of the life of the steel tube.

 

The critical thing here is not melting.. but....

Aluminum looses its modulus of elasticity quickly past 600F. which means it quits acting like a spring and starts plasticly deforming.

Steel remains tough stiffer longer.

Another problem with aluminum has a thermal conductivity of around 3 to 10 times that of most steel and stainless steel alloys. This is horrible for a powerplants stand point. Not to mention it would probally be pretty hard to deal with 3-10 times more heat under the hood.

Finally, the melting point for pure aluminum is aroung 1200F (if I remeber right) which is easily acheveable at the exhaust port. Cylinder heads, pistons and blocks stay cool with oil and coolant. The faces of these parts may get hot but they shed it right out quickly.

1 last thing, aluminum alloys have I want to say 4x the coeff of thermal expansion as steel alloys


flat out melting is not your first worry as far as failure goes. Ever hear the wack job conspiracy theroy guys claim that the trade center was brought down by bombs and not the fires.. They claim "scientifically" that jet fuel isn't hot enough to melt steel.. It doesnt' have to melt.. If it starts plastically deforming under loads your going to have a failure. If they brought that scientific arguement to any engineer.. mechanical, civil, material they would be laughted at.

 

Alvin, there's another thread about heat tranfer on stainless that brought this up. In short, alum being more thermally conductive doesn't have a big impact because the convection from the air to the tube walls is a much bigger factor. And, even if the exhaust temperatures are 1500*, the inside wall of the aluminum will still be much less.

 

Convection isn't going to save your butt. check out the Cp of air vs. water

The reason why your heads/block/whatever stays cool is because it has a coolant pulling heat from them. If you depended on air alone it woudl take 4 times as much air as water in MASS. Considering the density of water is around 1000 times that of air your talking about moving quite a volume of air to remove the same heat.


thermal conductivity isn't the worst problem.. The problem is the parts would loose their toughness (resistance to plastic deformation) starting at temps much lower than you'll see here.


So in short.. to use a aluminum alloy for a exhaust manifold or header you would have to use some sort of coolant, build them to where the COT isn't going to break anything when the parts get hot, and build them thick enough to where the parts aren't going to deform when the modulus of elasticity goes threw the floor.

 

How do you figure? That is not the way these things work. This is a boundary layer problem. The temp at the wall of the tubing will be the temp of the boundary condition. IF its not the same your not talking about the same boundary.