STM Custom Aluminum Exhaust

STM recently prepared a custom aluminum exhaust for my RX-8.

The exhaust was built for SCCA Autocross "Stock" class competition. This means it can be replaced from the catalytic converter backwards, and must comply with a 95 db sound limit. It must exit the stock exhaust location.

We went from a dual to a single setup. They used thick wall aluminum with their welding expertise, retained the factory resonator and went with a single muffler. The header collector is 2.5" so the exhaust bottleneck is up there, and I wasn't going to see any power gains by going larger, so the piping is 2.5". Power is useless in autocross anyways.

As you can see, the weight savings are substantial.

The sound is wonderful and completely streetable/easy to live with. If you're familiar with RX-8s, most exhaust setups sound like a robot berkeleying a sewing machine. This is just a nice, rich somewhat exotic gurgle. Minor little pop-pops on throttle lift off.

STM was a pleasure to work with, and stand by their work.

I say this because several people have informed me that this setup is absolutely 100% going to fail, but so far its been great.

Heat cycling can do funny things to aluminum. Brittleness may be a real concern. Sure looks like nice fab work tho.

I knew someone with an aluminum rx8 exhaust, worked out great! but his was LOUD.

fidelity101 wrote: I knew someone with an aluminum rx8 exhaust, worked out great! but his was LOUD.

This is very quiet. Quieter than my wife's 24v VR6. I'd say it's on par with a stock Coyote 5.0 Mustang or a stock Abarth 500.

The key is keeping the stock cat and resonator, and only going 2.5".

Most people on the RX-8 delete that stuff and go 3". Makes the ears bleed.

In reply to Cretinx:

I think he had removed the resonator and used a high flow cat (STX class)

Keep us updated. I like the idea a lot, but I would say it's a brave move. The RX-8's push some really high temps in the exhaust (will melt glass-pack internals under prolonged racing). Let us know how it holds up. Looks awesome though.

I actually campaigned an RX-8 in C-stock for a couple years and was pretty successful in the Atlanta region with it. Unless they changed the rules this year, I don't think there's anything that says that the exhaust has to exit in the OEM location. As such, my entire exhaust after the cat consisted of one of this:
It cost me nothing (made it from old exhaust turns I had), weighs less than a pound, has essentially no flow restriction and the down-turn kept the car exactly 1 dB under the limit. It exited right behind the driver as required by the rules. However, I would not daily drive it - loud as berkeley! So, I would swap it with a full cat-back before and after every event, which you probably don't want to do unless you're gunning for a top spot in a really competitive class.

For reference, here are the rules that apply to stock class (from 2013). I'd be curious to know if you've come up with an additional rule that applies or if the rules changed for 2014. I don't autox the car anymore, but still curious.

Stock Class Rules:
13.10
E. Any part of the exhaust system beyond (downstream from) the header/ manifold or catalytic converter, if so equipped, may be substituted or removed provided the system meets the requirements of Sections 3.5 and 3.3.3.B.15.

Safety Rules:
3.3.3 B
15) Exhaust must exit behind driver or to the side of the car.

Rules were changed for this year.

In reply to Gimp:

Do you know what the new rule is on this? I didn't find anything newer on the SCCA Solo website than the 5/1/2013 updated rulebook and that's what I quoted above.

Again, not really important since I'm not campaigning the car anymore, but I'm still curious.

Or am I not understanding the comments correctly and it was recently changed so that the exhaust doesn't need to exit near the OEM location?

I do seem to remember that back in 2009 or so there was some rule about that.

Stock class exhaust has to exit a stock location. It's in the rulebook.

Do you know what rule and version of the rulebook? I know people sometimes remove the entire cat-back at nationals and I'm pretty sure it was not required when I was racing.

I scoured the book I have (which I think is the current version) and couldn't find any rule about that.

Ah, finally hunted it down. So, it's the new "Street" class that this applies to. "Stock" class (now called "Street-R" still runs under the old rules).

For anyone interested, the new rules are here:

http://scca.cdn.racersites.com/prod/assets/13-fastrack-nov-BoD.pdf

Man, I have no idea why anyone would want to give up R-compounds. They cost SOOOO much less and are SOOOO much more fun to drive on. But I digress, that's a whole other debate.

So, yeah, back on topic - nice exhaust!

In reply to JohnyHachi6:

SCCA AutoX classes makes my head hurt...

In reply to fidelity101:

Haha, yeah, no kidding. It is kinda fun to build a car to the limit of the rules though. You've just gotta hope they don't change any time soon.

What is the story on the muffler? I've never seen (or searched for) an aluminum muffler. Did they make it or is it some other aftermarket off-the-shelf part? Any idea what grade of aluminum they used? How much did the assembly cost you to get made? Are you daily driving with this, or do you swap the exhaust out for competition events?

As mentioned above, heat cycling will definitely have an effect on the aluminum (especially with a rotary running a catalytic converter). I think the idea of an aluminum exhaust for autocross is pretty neat and can be done fairly cheap if you're handy. However, I've never seen anybody report back about durability after an extended period. It would be really interesting if you reported back once a year or so on how the setup is holding up.

Thanks for sharing!

Bryce

JohnyHachi6 wrote: Man, I have no idea why anyone would want to give up R-compounds. They cost SOOOO much less

I think you are mixed up on that one

Hoosier A6 and Khumo V710 are about $1200, R1R's are $840 for my mustang..

OR

$436 for ZII dunlops vs $956 for Hoosier A6 for my Miata...

On the origional topic, having seen an RX8's cat have its core melted, I am surprised that aluminium would hold up.

In reply to Apexcarver:

Maybe if you buy new... The thing about the R-comps is you can get the fastest rubber (Hoosier A6's) used for ~$100 a set from the top road racing teams that run qualifier laps with them and throw them out. I've purchased 3 sets of tires like this. They were all in excellent shape, I get a whole season out of them, and it costs a fraction of what even the cheapo "street" tires cost.

In my experience, it is extremely difficult to find deals like that on used street tires and the deals you do find are hit-and-miss. During race season, the used A6's are available any time you want them for consistently low prices.

Was a good ride while it lasted.

In reply to JohnyHachi6:

I agree. Used street tires for some reason cost no less than half their original new sale price. Where as race rubber sells for less than a quarter of their original purchase price all the time.

Having done the same, never again. Weight loss vs 20ga ss is nil, and the ss is way easier to fab.