posted on November 20, 2011 21:00
Project Miatabusa part 12: Why You Can't Make Aluminum Exhausts... And Why I Did it Anyway.
by Dave Coleman
From exhaust port to tailpipe, project Miatabusa's exhaust system fabrication has stretched well over a year and has followed a circular evolution from brilliant hack job, to exotic race tech, and back to brilliant hack job. From custom-fabricated race header, to a lightweight aluminum tail section, to scavenged RX-8 silencers and hangers made from an S13 power steering cooler, the exhaust system is as awesomely eclectic as project Miatabusa itself.
I'll get to the header in a second, but let's talk about the aluminum back section first. In theory, there's no reason we couldn't use any old off-the-shelf aftermarket cat-back exhaust for the Miatabusa. Exhaust flow is pretty closely correlated to horsepower, and our power output will be well within norms for Miata exhausts. There's nothing at all off the shelf about the exhaust we just built, though.
The aluminum exhaust adventure began when I was browsing through Vibrant Performance's very tempting collection of fabrication components, and I stumbled onto their 4-pound aluminum race mufflers. Whaaa? I had always written off aluminum as an impossibly soft, temperature sensitive and fatigue-prone material for exhaust use. But if someone is making aluminum mufflers, that must mean someone else is building exhaust systems with them.
The temptation to use aluminum should be obvious. Steel exhaust systems are really heavy, and an aluminum exhaust has the potential to save dozens of pounds. Burns Stainless has a very thorough, well-reasoned explanation for why they don't do aluminum exhausts. The short version is this: Aluminum's strength falls off dramatically at elevated temperatures. At room temperature, 6061-T6 aluminum actually has a slightly higher yield strength than 304 stainless, but by around 500F, it's about half the strength of stainless.
|For Burns Stainless' full dissertation on why they don't build alminum exhausts. That blue line really sums it up though. That's the yield strength of aluminum plunging as temperatures approach those we'll see in an exhaust system. The purple line on the bottom is the yield strength of annealned 6061-0 that you'll find around all your welds. Yikes!
The heat treat of aluminum is also critical. 6061 is the alloy (basically the only alloy you're likely to find in tubing suitable for exhaust fabrication), and the T6 is the heat treat. Annealed 6061 (called 6061-0) is roughly one quarter the strength of 6061-T6 at room temperature, but at 500 degrees, they're both equally weak. Annealed 6061 is relatively stable vs. temperature, but heat brings the T6 aluminum down to the strength of the annealed material. Another tricky bit is that welding 6061-T6 basically anneals it, so a fabricated exhaust will be as weak as 6061-0 around the welds.
On the other hand, the aluminum tube Vibrant sells (1.5mm wall thickness) weighs roughly half as much as even thin, 18-gauge stainless. That's the kind of temptation I can't pass up.
At first blush, an exhaust shouldn't really need to be that strong. All it does is keep the smelly gas on the inside and the slightly less smelly gas on the outside. How strong does a metal tube have to be for that? Turns out there are a lot of sources of stress on the exhaust that could make hot aluminum's strength issues problematic.
You can't see them, so they're easy to forget, but any fabricated component is full of internal stresses from different parts of the material disagreeing over what shape they should be. A mandrel bent tube, for example, has been stretched and pulled into a bent shape, and welded part will have stresses left over from the expansion and contraction that happens as material around a weld heats and cools. Heat treating the final product is the only way to get rid of these internal stresses, but it still won't change the fact that there are huge temperature swings across the material during operation, with hot sections expanding more than cool sections and fighting each other over how big they should be.
Easier to visualize (and manage) are the stresses the exhaust sees from driving the car. Go around a bumpy corner at 1g and you're effectively hanging the exhaust sideways on its mounts and bouncing it up and down. This can concentrate tremendous stress where the hangers weld to the exhaust, and where the rubber-hung back-section connects to the front section that's inevitably rigidly mounted to the cylinder head. If the rubber hangers are soft enough, and there's no flex joint, it can be like you're holding the front end of the exhaust and swinging it around, concentrating incredible stress on the front flange of the exhaust.
The biggest one, of course, is the huge stress of bottoming the exhaust out on a driveway or trailer. The bottom of this car is already covered with trailer loading scars, and that's on good, old-fashioned mild steel frame rails.
Still, none of this was enough to keep me from trying an aluminum exhaust. Knowing where all these stresses come from means I can design the system to minimize them. I've effectively designed the alumium exhaust to be gently cradled under the car in a way that should keep it from being bottomed out, and designed the hangers and front flange to minimize stress concentrations. I've also limited the aluminum use to the cat-back section where temperatures should be lower. The hotter front half (which is also subject to more cyclic stresses from engine vibration) is all stainless.
I'll get to the aluminum details in a bit, but first, let's back up to the exhaust ports and follow this whole system from front to back:
In the concept stage, we had hoped there would be enough clearance under the engine to retain the stock header and just turn the exhaust 90 degrees once it entered the collector. When we first bolted the Hayabusa engine to the Miata transmission, this simple idea looked doomed. The header was occupying space that would be needed by the subframe.
As you can see in this photo, the Hayabusa header has unconventional pulse primary pairing. With a 1-3-4-2 firing order, you generally want to pair the primaries of cylinders that aren't firing right after each other. This means 1 and 4 should be paired, and 2 and 3 should be paired. Suzuki paired adjacent primaries instead.
This strange pairing is probably the reason the intake trumpets are two different lengths and the fuel and timing maps are cylinder dependent. This pairing means the timing of the exhaust pulses hitting the center two cylinders is different from the outer two, requiring different intake tuning and different maps. Suzuki probably did this to spread out the powerband so the engine's massive power output was easier to put down to one little tire. It also seems to package better inside the aero-sensitive fairing of a bike designed for top speed.
Sunday, November 20, 2011 9:41 PM
Mother of god!
Sunday, November 20, 2011 9:57 PM
Fuck me this is gorgeous !
You should swap the muffler with a Yoshimura :)
Monday, November 21, 2011 3:42 AM
Coleman, nice discussion of all the engineering-based decisions that must be made tp create even an exhaust system . . .
Monday, November 21, 2011 4:01 AM
On that header, I've had FF headers about as close to the fan (and not melt anything) if it makes you feel any better. In addition to that, EGTs tend to be lower at idle. If I had to guess, this car will, at some point, get a heat extractor hood so it likely won't be an issue. You could always shell out some more green paper and get it ceramic coated too.
Monday, November 21, 2011 5:06 AM
So with a lot more words you're saying that Miatabusa customers should just buy a catback exhaust and go on a diet instead?
Seriously that's pretty nice looking. I need to get a welder and start practicing.
Monday, November 21, 2011 6:58 AM
Nice! I cannot wait to see this thing doing doughnuts in the parking lot.
Dave, I did have a quick question about the bell housing adapter for this project.
Do you think that the adapter will will work with the Mazda FWD manual transmissions out of the Protege?
Monday, November 21, 2011 7:02 AM
I've seen honda drivers running aluminum exhausts and I just figured they were going to be doomed to have a really short lifespan but thanks to this article I might take them seriously. I also had a thought a while back...why does no one make a chromoly exhaust? I mean it's not the most expensive material around, it has some corrosion resistance and it's fairly light and strong.
Monday, November 21, 2011 7:16 AM
Only take them seriously if 1: they've taken all the precautions I have, and 2: this turns out to be a good idea...
Monday, November 21, 2011 7:21 AM
Interesting question. I honestly don't know. The way the engine is offset to the passenger's side will, in a front-drive orientation, offset the engine to the back. Great for weight distribution, bad for putting the pass-side axle and engine block in the same place. Too close to call if they'd actually occupy the same space, but it would be close.
Tim has 323 GTX bellhousing pattern in his computer, we just never thought to check. Hmmm....
Monday, November 21, 2011 7:22 AM
Check your filler rod alloy before looking into heat treating that aluminum section. There are some alloys that are not heat treatable and will net you no gains by heat treating.
Monday, November 21, 2011 8:14 AM
This seems a lot cheaper than the paper-thin Inconel that some of the NASCAR teams use. Not the same operating conditions though, and you don't need a tube roller and seam welder to turn Inconel sheet into Inconel tube.
Monday, November 21, 2011 8:43 AM
I love the way Coleman writes. Yes, save the puppies please.
Monday, November 21, 2011 10:41 AM
Ah, to live in a warm climate. Those stainless clamps and aluminum pipe wouldn't last a single Canadian winter due to galvanic corrosion.
Nice progress so far Dave!
Monday, November 21, 2011 11:27 AM
Dave - I just love how much of your writing gives me a big grin :) I would've laughed at anyone trying to make a back pressure comment with how much bigger that collector looks than anything that would've come on the bike. That thing is set up so well for a turbo! PS So looking forward to Audio and Hooning videos!
JDMized - Been bugging him the last two installments to go Yoshimura, too late now. Unless the experiment fails.
Monday, November 21, 2011 12:18 PM
If/when a turbo comes into the picture. Will you run twin or single scroll?
Monday, November 21, 2011 1:52 PM
I'm curious as to a motorcycle style slip-fitting will be leak-free enough for a turbo. Fascinating!
Monday, November 21, 2011 4:19 PM
I really enjoyed the article, lots of cool recycling and great writing! I see dgerryts already brought up galvanic corrosion. I would most be worried about the joint that has stainless/mild steel/aluminum with the water, heat, and exhaust gases. For a car that's not daily driven it probably won't be an issue. Did you weigh the whole catback?
Monday, November 21, 2011 4:34 PM
Hey Dave, next time you do something awesome, hint hint, nudge nudge ...
Dry sump, 215hp stock, light as a bike engine, closed loop cooling system.
Monday, November 21, 2011 6:13 PM
I'm pretty sure the reason T6's strength drops to equal that of 0 is that it's getting annealed by the temperature, thus becoming 0.
Monday, November 21, 2011 9:51 PM
Still not sure if a twin-scroll will matter. Two reasons: 1: twin-scroll's benefits are really all at low rpm, and it isn't clear what the functional RPM band wil be (ie: where will it cruise). 2: Having the twin-scroll turbo at the end of a long-tube header might negate any need for the twin scroll. As I understand it, most of the benefit of a twin-scroll is that it keeps exhaust pulses from mucking with each other, same as a header does. Not really sure you need both...
Still debating that one.
Tuesday, November 22, 2011 5:02 AM
Can´t wait to see just how this all turns out in the end. Anyway, I hope there are a few more installments in the way, before the project comes to an end, as they are all quite funny (and instructive) to read.
Tuesday, November 22, 2011 10:21 AM
Did you check the GTX fit in CAD yet?
When this project was still getting under way I was wondering if it would work. I own three GTXs and I think one of them should go to 11,000rpms!
Tuesday, November 22, 2011 10:27 AM
Just curious if you looked into 2024 tubing (yes a bit more pricey) or any other aluminums that do 'better' with heat. Just curious :)
Tuesday, November 22, 2011 10:29 AM
Own 3 GTX? I would assume you are on the yahoo list, right?
Tuesday, November 22, 2011 10:57 AM
I wouldn't be too worried about exhaust heat killing your fans. My SE-R's fans would've turned to slag LONG ago if that were the case.
Tuesday, November 22, 2011 12:50 PM
Haven't checked yet. I keep waiting for Tim to read this himself and do it, but I might have to finally call him...
I never considered other alloys because the Vibrant mufflers are already 6061, and one-stop-shopping all the tubing and bends from them is irresistibly easy.
Tuesday, November 22, 2011 6:46 PM
Turns out the bellhousing patterns are close enough that the adaptor will bolt up. Still very unsure about the axle clearance, but we just happen to have a 323 transmission and a GTX shell sitting around Tim's shop, so we'll try a little test fit on adaptor #2. Stay tuned...
Tuesday, November 22, 2011 7:05 PM
Also, if you`re concerned about heat, you can kill several birds with one stone by ceramic coating the inside of the exhaust. You keep heat outside the headers to a minimum; you (theoretically) keep the exhaust flow high because of the retained heat in the gas flow; and you protect the aluminum a bit more.
Wednesday, November 23, 2011 4:53 AM
COOL thanks for checking on that!
Soo, if it fits will a 323-busa or prota-busa be next?
Give Mike Kojima's Sentras a little competition?
The idea popped into my head driving to work one morning last week. I was thinking AW11 Mr2 + Mazda FWD transmission + custom axles = Awesome!
But I'm just dreaming.
Wednesday, November 23, 2011 6:30 AM
If it fits, YOUR 323 is next!
Wednesday, November 23, 2011 6:36 AM
I have a car addiction, I own eight cars currently. My true love is the GTX. You would assume correct, I am on the yahoo list, but not often and mostly just to see what people are selling.
Thanks! I'm so excited to see if this could work.
Wednesday, November 23, 2011 4:56 PM
Great bit of work and writing. I can't wait to see how well it stands up to use.
Thursday, November 24, 2011 7:38 AM
Amazing work Dave on the exhaust. Some of what you had to do with aligning and cutting the sections I had to do the same.
But it's wonderful to know that what I did to mark and align were the same as you did. Kind of makes me feel good about it.
I do hope that there will be an audio of all this when that time comes. I'd really like to hear what this is going to sound like in it's complete state.
Again, good work Dave.
Friday, November 25, 2011 1:19 PM
"hole-in-the-wall aircraft surplus shop" your probably talking about G&J off of Sultana. Was it next to a church, and did it have a wall full of Race Cars? That place is becoming the stuff of folklore.
Thursday, December 15, 2011 5:39 PM
T6 represents temper or "age" of the material, 0 is only 0 until it reaches room temp, at which point it is basically T4 it will age at room temperature eventually to T6 (years) or with applied heat the time shortens if the exhaust gets the pipes hot enough to worry about they will also be self hardening as they get heat cycled...the biggest concern is keeping it cool check the temps when you get off track and if you find they are higher than you'd like you can always add more surface area via fins
Saturday, February 25, 2012 11:21 AM
I actually check this thread twice a day. I'm thinking that it mighty be better to have fewer projects than to have projects where an update is made every two or three months. Or another thought - a fait accomplis type of project where you do it and THEN let us know how you did it.
I was hugely interested in this project but find my attention waning and am actually aggravated at the pace. Better to have never known about it!
Wednesday, February 29, 2012 10:05 AM
I'm with TLR, I check this numerous times a week. Not burned out yet, but am anxiously waiting an update. If I was building this, I dont know that I would have the patience to let it sit this long-I'd want to be out flogging it.
Saturday, March 03, 2012 8:51 AM
Wow, I can't believe this last update was in November! There has been progress, both forward and backward, since then, but mostly I've been incredibly distracted traveling the world doing Mazda's dirty work (Hiroshima doing secret future stuff, and Toronto and Monterey launching the new CX-5) and buying a house. I'll try to put together an update before I get distracted by moving...
Monday, January 14, 2013 3:40 PM
Mike, updates please?? I just reread all this while laying under my Miata looking at my exhaust routing options with my V8. I'd like to know how the aluminum held up, thanks!!
Monday, January 14, 2013 3:58 PM
Sorry, I don't think Mike is EVER going to do an update on this project ;)
Monday, January 14, 2013 6:24 PM
I have NO idea why I typed Mike o_O. Wow. Dave!! Not Mike haha
Monday, January 14, 2013 6:56 PM
Believe it or not, there has been significant forward progress on the Miatabusa just this week. Nothing to make a story about yet, but specs on the new adaptor are falling into place in a very real way, with actual bearing specs and such.