Why no aluminum???
 

If everyone is so concerned about weight savings why doesn't anyone try making an aluminum catback single exit exhaust? i can only imagine it weighing a couple pounds. I plan on having one custom made to bolt up to the AP headers here in the next month. I think a mass produced version would sell great.

because metallurgy

Here you go: http://www.lmgtfy.com/?q=why+isn%27t...d+for+exhausts

Aluminum is not that easy to weld.

aluminum is very easy to weld and I've used it for exhaust before. someone should probably tell STM or Full-race they're wrong for using aluminum exhausts on their 9 sec+ race cars then....

Closed before this goes downhill, but did you read anything from the link I posted?

Using Aluminum on a race car doesn't mean anything. It will get brittle very quickly and it will break, that doesn't matter on a race car, it matters on a street car.

Decided to re-open, but will shut it down again if it goes too far.

In before thread is closed and re-opened again.

If you are concerned with weight savings in your exhaust system, you will want to wait for titanium systems to become available.

My GD Magnaflow Ti catback could be held up by one pinky finger.

epson,

Yes, AL exhausts are used in racing cars, that are under different situations than your everday DD.

Long story short--stainless steel strikes the best compromise between wear and weight. Aluminum is light, but would be worn out (for various reasons) after a relatively short time.

Ask Geoff from full race how well that exhaust worked out for him when his s14 w/R32 conversion caught fire because it blew a hole in the exhaust due to the ductability of Al at those temps


to the OP:

For future reference when asking why not, consider answers, consider source, before making yourself seem...over extended in your knowledge base

I wasn't going to mention the fire :P

My bad :(

There are a couple of Ti catbacks out from the JDM companies but they are super expensive. Freed Engineering is/was working on a single exit catback which weighs in the neighborhood of 10lbs.

I wish Magnaflow would make a single exist Ti catback for this car (I had a GD one as well) but I think they got out of making them a long time ago. I still have mine I was going to see if there was some way I could shorten it to make it work on the BRZ. :D

I would think titanium would be God-awful-expensive:eyebulge:

It is the same reason as alum. con rods. Race cars okay, street no unless you like to change your rods with every oil change.

I don't think that's a relevant analogy. The exhaust pipe is under very little stress. I don't doubt that an aluminum exhaust would probably start developing stress fractures or something after some time (possibly sooner than most owners would be happy with), certainly before any stainless or titanium exhaust, but since there's little structural integrity required from an exhaust pipe I think it's not fair to compare it to a critical engine component.

I have no idea what exhaust temperatures are typically, but I imagine that after the catalyst there should be enough cooling for the exhaust pipe to let it withstand the heat.

Yep. It was no joke being the first one on the block with the Magnaflow Ti System. 1,325 Shipped. Ouchie! Shortly there after they slashed the pricing to 7xx and then the system was discontinued due to lack of interest.
But the JDM will come correct with some different Ti systems for the BRZ. It's just a matter of time.

There are a couple out there now, one from Amuse and another from SARD. The Amuse is like $3200 for the axleback, I think it's close to $4k for the entire catback.

Stress reversals from vibration will play a significant part in fatigue. Add to the fact that aluminum's strength drops at those higher temps.

I think that axle-back could be ok for a couple years, but life will get shorter the closer to the motor. But there could also be more stress and fatigue issues with a hybrid stainless/aluminum systems from different expansion rates and galvanic corrosion.

Meh...

I think from my avatar and sig people know what my choice is...

I was waiting for you Dimman!

Someone said stainless steel is the best weight/wear compromise. Not true; titanium is. Unfortunately, stainless still is the best weight/wear/price compromise while titanium still remains expensive (not to mention requiring a far more experienced and knowledgeable welder).

You are forgetting about vibration :P

An aluminum exhaust simply won't last any reasonable length of time on a street car.

Perhaps a mix of SS and AL with SS closer to the cat, but that probably has it's own issues.

Let's face it, there's plenty of reasons why manufacturers don't do it, it's not because no one has thought of it before ;)

If you want light, Ti is the answer. It's not cheap though obviously.

I should have just finished reading the thread before replying eh? :P

Anyways, yes, SS is the best for the money. Ti is the best otherwise for a long lasting solution. :thumbup:

Aluminum is more expensive than mild steel, and is less durable at temperature, that's why they don't use it in street cars.

But what's a reasonable length of time? A street car is run mostly at part load where exhaust temperatures are low. Vibration and other stresses can be dealt with in the mounting system, at least enough to extend the life of the exhaust. Where I'd be rather concerned is a racetrack, where the exhaust temperatures are consistently high enough to melt aluminum. In the future as we see forms of exhaust heat regeneration that reject exhaust at temperatures that look more like 200-300C instead of 800C, then I think aluminum would be competitive against Ti/steel in all respects.

Between expansion/contraction and vibration I bet you'd be lucky to see 2 years of normal use. Also the risk of it running hot and developing a leak from melting. I've seen more than a few exhaust related fires (two this year racing) from exhausts cracking (accident, poor welds and simple fatique) and directing hot exhaust at fuel tanks.. boils for awhile first though :P

I would never run one. SS is fine, heavier, but low to the ground so shrug.

Cost isn't the reason, otherwise Ti exhausts would never be created ;) The fact that it gets brittle and very soft when it gets hot is the reason. That and liability of course.

Ti is expensive but durable as you say. Aluminum as you say, can be a disaster waiting to happen, which would never be acceptable for a commercial product. Liability at higher cost isn't worth it.

Crazy people like me though, get the idea of swapping out sections of tube for aluminum :D Thanks for the warning about the fuel tanks.

Gettin' slow old man!

Whoa, whoa... you guys realize there are millions of cars with aluminum exhaust components including turbo engines?

This is a theory that makes sense at first glance, but when you look at real vehicles on the road today it turns out that you've got it in reverse. There are basically zero axle-back exhausts on street cars, but millions of vehicles with aluminum exhaust manifolds. Most of them are cast into the head.

I have four examples for you, two non-turbo and two turbo.

First is the Chrysler Pentastar V6 engine you find in all sorts of longitudinal configurations, like on the V6 Challenger.

http://www.ft86club.com/forums/attac...1&d=1343702843

next is the Honda R18 engine used in a gazillion Civics

http://www.ft86club.com/forums/attac...1&d=1343702656

third is the new EA888 1.8T VW engine for European markets that I have discussed in the past. This engine also features port + direct injection like the FA20:

http://www.ft86club.com/forums/attac...1&d=1343702843

note that this manifold is aluminum and water-cooled. It's also cast into the head. The last example is the Honda K23A1 engine found in the outgoing RDX models. It's aluminum and water cooled, but it is not cast into the cylinder head:

http://www.ft86club.com/forums/attac...1&d=1343702843

http://www.ft86club.com/forums/attac...1&d=1343702843


Again, this seems to make sense on the surface except for the fact that exhaust temperatures have gotten very high over recent years. Engines run much leaner during transients to keep CO emissions down, as measured on the US06 test cycle. And you don't see gazillions of aluminum exhaust manifolds cracking.

The most common manifold design for modern emissions standards is probably the dual-wall stainless steel type, but aluminum exhaust manifolds are becoming increasingly more common. As for the rest of the vehicle, well I don't design catalysts but I'm sure cost is a big issue and I imagine there are a lot of problems making aluminum shells for catalysts.

Just because small companies with low production volumes haven't made it work doesn't mean there are insurmountable technical challenges.

^ So basically if the exhaust is very thick-walled, rigid and water cooled, aluminum will be perfect?

:laughabove:

I was about to ask what manifolds cast into heads have to do with thin ass piping.

It is interesting that manufacturers are moving in that direction, but I suspect it has a bit to do with reducing the amount of assembly time and parts. Could also be used to control EGTs better so they don't overcook the cats, too.

this is starting to remind me of the arguments when plastic intake manifolds where coming into production,

The Civic and Chrysler V6 engines are not using water cooled manifolds.

The water cooling is for turbo engines so that they don't have to enrich the mixture nearly as much to keep cat temps down. Any way you look at it, it's still an aluminum exhaust manifold whether it's bolted or casted to the head.

Well since they are already making a casting, anything you can combine in to it saves tooling and production cost as well.

I know you know your stuff and that manifold/extended head is interesting. But I was just presenting things a little differently, since I have this image of some noobs welding up a bunch of aluminum intercooler piping as an exhaust and wondering why it fails 6 months later.

Comparing manifolds to cat-back/axle-back exhaust systems is like comparing apples to oranges. They are constructed different, as Dimman said.

And whoever said it was correct--304 stainless (which is the standard in the industry) is the best price/weight/durability currently for exhaust systems.

:thumbup:

But c'mon guys. We can combine 4 turbo cylinder heads and two non-turbo cylinder heads to make an extremely light weight exhaust? Whiskey Tango Foxtrot? What the hell is going on inside this thread?
If Aluminum was the way to go, wouldn't all of the aftermarket Mfrs already be doing it? They don't make it because it makes no sense.

Yes, you can use Aluminum all over a car in high heat applications, just not the actual piping of an exhaust.

This would all make sense if the bottom line wasn't "that is too loud for my local track."

Aluminum axleback is about a 2lb savings over a 304 single exit. Factory midpipe is 18ga (thin) stainless. We might take a crack at an aluminum midpipe. But thus far we haven't found good enough pricing on Ti 2.5" tubing to make it worthwhile.

And from what I have read in the metallurgy literature, bonding is a better solution than welding in most cases. Not sure how the high strength and fatigue resilient epoxies will withstand oxidizers and temperature like found in exhaust gas. Same can be said of aluminum in general. Kinda tough to anodize a 7' long pipe. But if it was a known seasonal consumable, that'd be a different story.

J

How about not fixed at all? Silicone seals (good up to 500C or something) and some kind of slip fit thing? And then for mounting maybe add a structural support extrusion of some sort, held onto the pipe with silicone as well.