Project Miatabusa part 12 – Why You Can’t Make Aluminum Exhausts, And Why I Did It Anyway

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In spite of the thin outer shell, the RX-8 silencer is still quite heavy. To survive rotary heat, the inner tube is unusually thick. Again, thinking about the stresses upstream on that thin-wall 18-gauge stainless downpipe, I wanted to support this weight with a hard mount to the drivetrain. Luckily, I've never thrown away a bracket in my life. Somewhere in this bucket of crap is something that will work! 

Indeed, this thick, 90-degree bracket used to be some S13 exhaust manifold brace, I think. Now it bolts to the back of the Miata transmission. 

This chunk of cooling fan spacer bracket is welded to the RX-8 silencer and bolts to the S13 brace. Downpipe installation is now surprisingly simple. From below, simply lift it up, align the slip-fit at the upper V-band clamp, thread this little nut into the S13 bracket, and everything holds itself up while you fine-tune the alignment.  

As you can tell form the intentional misalignment between the cat and silencer, I'm a big fan of using angled cuts to make things go where they need to go. If I had made this adjustment by welding in a section of mandrel bend, this adjustment would have added a few inches and an extra weld to the downpipe. Making small tweaks a few degrees at a time is very effective and doesn't really hurt flow, it just hurts your pride. Of course, I can't do it up in the engine bay next to that damn sexy header, but under here, nobody will see. Just you guys. Shhhh.  

OK, time to start planning for that aluminum back section. First, Vibrant's aluminum mufflers only go down to 3″ ID, so I have to step the exhaust up to 3″. This seems excessive for 170 hp now, but once it's turbocharged, the exhaust will make perfect sense. Don't start with that “you need backpressure” horseshit. It isn't true.

Second, the aluminum part of the exhaust needs to be designed with minimal stress concentrations. Any place stresses are concentrated, there's a risk of cracking. This means exhaust hangers, and any place the rubber-mounted back half of the system meets up with the hard-mounted front half. 

That's where this 3″ Vibrant flex joint comes in.   

Vibrant sells joints both with and without this interlocking inner liner. The liner provides a much smoother surface for airflow, but also stiffens the joint somewhat. If you're going to have a lot of engine movement, say from transverse engine mounting or just soft mounts, you should shy away from the liner. Our mounts are stiff, the rubber exhaust hangers are pretty beefy, and the lightweight exhaust won't be exerting that much force on the rubber mounts, so we don't expect all that much movement. This joint should be enough to handle the movement we'll have. Hopefully…