Iron or aluminium for exhaust ?

[neich]

Hi all,

I am considering to build a new exhaust pipe for my AAZ because the one I already have (custom) is pretty narrow. I was wondering if it is worth to build it with aluminium instead of iron. I now that iron is much heavier, but I don't care about that (VW T3 syncro). I would say that iron is much easier to weld, but not sure. What about durability ?

Thanks
Nacho

[BlackTieTD]

i'd go with stainless steel if you're not concerned with weight, but want durability. aluminum is much harder to weld than steel.

[fspGTD]

Are you talking solid aluminum tubing or aluminized coated steel tubing?  Solid aluminum tubing will be much more fragile than steel tubing especially when used in an exhaust system.  It is not normally recommended for exhaust system components as its heat treatment can be lost at high temperatures, at which point its strengh drops a lot.  The main feature of solid aluminum tubing is low weight and it is relatively cheap when compared to other lightweight materials such as titanium tubing.

Aluminized coated tubing is mild steel tubing with a thin coating of corrosion-resistant alumininum material, similar to galvanized coated steel tubing.  They are as strong as uncoated steel tubing, but will resist corrosion better due to the coating (except where it gets burnt off near the welds.)

Stainless steel exhaust tubing resists corrosion better than coated mild steel tubing especially at the welds.  Popular grades are 409 stainless and more corrosion resistant 304.  The main drawback of stainless compared to coated mild steel is significantly higher cost.

[TDIMeister]

Combustion acids (and road salt, if your area uses them), combined with elevated temperature, would eat aluminum in no time.

[steve]

If there was an advantage to AL, somebody somewhere would use it in their super duper light weight sporty car.  I don't recall that AL has ever been used for that.  Plus, AL might have a tendancy to melt under the right conditions... Or at least suffer a great strength deficiency at exhaust pipe temperatures in addition to loosing it's heat treat.

[MacGyver]

If this is indeed about aluminum pipe and not just aluminized steel, also consider the expansion coefficient and the temp range your exhaust sees.
Aluminum expands significantly more than steel, necessitating stragetically placed flex or expansion joints.

[fspGTD]

I am using a couple aluminum sections in my GTD autocrosser's exhaust.  (As has JoeGTD, and 2383GTD.)  Although my GTD is not very often street driven (it is a dedicated autocrosser), I have not had any problems with the aluminum exhaust pieces yet.

Converting them to aluminum saved MUCHO weight over the stainless pieces they replaced!  Aluminum is AMAZINGLY light stuff... 1/3 the weight of stainless or mild steel for the same volume.  And it is actually reasonable price compared to say, stainless.

Although professional performance exhaust dealers do not recommend aluminum for exhaust systems, conventional recommendations are made for gas motors, that run higher Exhaust gas temperatures than efficiently intercooled turbodiesels...  so how are we to know it can't take the heat if noone has tried it?

Despite my success with the aluminum exhaust pieces, I would not recommend them on a daily driver car and would use caution when using it for highly-stressed areas.
 
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[neich]

Thank you all for the replies. Very informative !

Nacho

[Piper106]

My two cents.

Somewhere it was posted that exhaust gas temperature (EGT) on the VW turbo diesels runs about 750 degrees F crusing at a steady 70 MPH on level ground.   Normally aluminum is not used long term above 450 degs F because of the loss of strength.  At 750 degrees F aluminum is about as strong and stiff as cooked spaghetti.  

In short bursts, like an autocrosser or drag strip, like fspGTD says aluminum is fine, since the exhaust tubing doesn't have time to get to the EGT.    

Further complicating things is that all diesel exhaust pipes get a pretty decent layer of soot inside, and that soot acts as a insulation layer or thermal barrier.  Plus the exhaust is loosing heat from the OD of the tubing to the air under the car, so the metal temperature is lower than the exhaust gas temperature.  The part I don't know is how far below the EGT the actual metal temperature is.  Wonder if anyone has any Tempsticks and time to get some temperatures on the OD of the exhaust system?

Bottom line based on what I think I know, for a daily driver, I would not go aluminum.

Now as far as stainless, since it doesn't corrode nearly as fast as plain steel or aluminized steel, you could go with thinner gauge material than a steel exhaust system and save a few pounds.

Piper106