aluminium exhaust, a good idea?

[kewp]

Hi, I've just been wondering about making my own exhaust out of aluminium mandrel bends, the same as what you would use for intercooler piping...
Is this a bad idea? i can't see why it wouldn't work, except that aluminium is more prone to cracking than steel.
Any thoughts?

[Talasas]

The temperatures that you get from an exhaust are much higher than intake charge. Certain sections near the middle of the exhaust may be rather lower than the obvious red glow one may get from the headers, but even this heat will not do any favours for ally. There is one exception that I have heard there of, which is a specific grade/alloy of alumium used on aircraft exhaust systems but I only heard this recently and have no idea what it's called and/or what it's capable of.

[legend]

aluminium may be eaten away by the unleaded exhaust gases and prone to cracking

[wombat]

Just stick with stainless steel.
Thats what all our dyne cell exhausts are made of, no problems, and light. Plus being stainless you can polish it up if your that way inclined.

[Draven]

Plus being stainless you can polish it up if your that way inclined.

euphemism for homosexual

[Screamn_Sleeka]

All good advice so far.

Aluminum has approximately TWICE the thermal expansion of steal, so its going to be expanding a lot, then reducing when cooling. This will lead to problems and cracking.

Aluminum is a lot softer than steal (Al = 3 and mild steal = 5 on mohs scale [where diamond is 10], and thats logarithmic remember) so the light scratches and dings on the underside of most exhausts would be deep gouges and will cut straight though the pipe.

Aluminum is more elastic so after heating and driving around it will bend is stress areas and then remain that way after cooling. This could lead to a "sagged" exhaust.

Also, most aluminum alloys (note you wouldn't actually use 100% pure aluminum) melt at around 600oC. The temp at the headers can reach this, as well as the cat (probably not on a stock engine though). Even without reaching the melting point, the metal would be very soft, which exaggerates the effects of the above problems further.

You might think polished alloys look good, but if you heat/cool/heat/cool them a few times they get the aluminum oxide layer on them really fast and look like shit really fast too.

[Draven]

Ya... it's why people tend to use Ti for weight reduction in their exhausts rather than Al

[oldcorollas]

stainless steel is a bit heavier than steel. (8 vs 7.8 ish)
Ti is lighter, but not that much (5.4?)
AL is lighter (2.7) but needs to be thicker to make up for lost strength


really, you should be able to use Al for post cat exhaust and muffler, as temps will not be so high, BUT you would need to have it well supported. to be safe, having a length of normal steel or stainless after the cat for a few feet might help.

i am not 100% certain on the corrosion from the inside with Al, but it might passivate.

main issue is cost....


found diagram with vague idea of strength vs time
basically, if it is an age hardened alloy (anything with T after it) then it will die at higher temps. the precipitates will be too big and will make it brittle/weaker etc
so you need more pure alloy which has low strength to begin with... but if it is well supported, it doesn7t really need strength....

[MR22ZZ]

Al oxidises very easilly. I can't think pumping a whole heap of CO and other associated exhaust gasses passed it would be a good thing.

[oldcorollas]

cylinder heads are made from aluminium.....

aluminium passivates realyl easily.

titanium axidises easier than aluminuium (iirc) but also has stronger passivation.

[MR22ZZ]

fair point... didn't think of that.

[Draven]

you could also passivate the Al prior to installation...

and the good thing about aluminium's oxide layer is that it's fairly non-porous, so in itself the oxide layer acts as protection (unlike iron/steel, which has a very porous oxide)


Disclaimer: OC is free to rip the above to shred if it's wrong

[MR22ZZ]

yeah that's my (poor chemist, non metallurgist) understanding of it as well.

While Al oxidises easilly it is only a surface oxide layer, where as steel the oxidation diffuses and grows and speads into the metal itself, so the oxide on the Aluminium actually forms somewhat of a natural barrier for protection, but a very brittle layer which can be removed relatively easilly.

[oldcorollas]

yup, Al (at room temp) forms dense continuous protective layer, whereas iron forms non-continuous non-protective layer. this is due to stuff like oxide volume etc.. anyway...

Al is good at room temp, but if it comes into contact with salts and whatnot, it can oxidise badly. i can7t remember how it goes at higher temperature, BUT superalloys and turbine parts are protected by aluminium oxide...
just as stainless steel is protected by chromium oxide

ie, aluminium, stainless and titanium all resist corrosion by forming oxide layers. (compared to gold or platinum which don't really oxidise much)

normal steel exhaust pipes have aluminium coating to protect them anyway.....

i am not sure abotu using alloy for an exhaust system (past the really hot bits...) but might be worth a try if you have some alloy lying around
i wonder if having it in contact with the steel part will cause accelerated corrosion of either side?

[Lambolica]

Would galvanic Corrosion become an issue (is there any acid in exhaust?)

Cheers
Simon