Pursuit of the "perfect" Boxer exhaust

I have a Nouvalari Sport exhaust on my '83 BBi. It's okay, but I know the car can sound better. My theory is that I prefer the sound of a quad exhaust (12-into-4); which has a late 60s / early 70s Le Mans sound (eg, 512 S/M and the '69 917) – and of course, the BBLM. This is essentially the opposite of what most modern exhausts do (lots of collectors, cross-overs, etc.).

As far as I know, the only 12-into-4 Boxer exhaust is the Borla. Swap out the mufflers for something more aggressive (eg cherry bombs), and you're in business. Unfortunately, Borla discontinued their Boxer exhaust ~15 years ago, and it seems no amount of bribery will convince them to make me a one-off.

So my plan is to create an exhaust from scratch, similar to the Borla, but more modular (easier to swap out mufflers). I removed the Nouvalari, and (after the obligatory open headers run), I started 3D scanning.

I haven't decided if I will 3D print the entire thing out of stainless steel, or only the connecting parts (eg, brackets via SendCutSend, 3D printed inlets and tips, etc). I am designing it to go either way. The cost is not that different if using good quality steel, mandrel bends, and high quality tig welding (not just some sloppy "Exhausts-R-Us" shop).

My first test is essentially straight pipes. This is mostly an experiment to make sure things line up, experiment with the 3D printed stainless steel, etc.. The sound will likely be the same as the open headers.

Once the "straight pipe" experiment is finished, I'll make a decision on 3D printing the whole thing, or having it fab'd. If I opt for 3D printing, I may tweak the design to have more organic bends.

I'm still working on the designs, but figured I'd share work-in-progress. Both are using 2" diameter tubes, 1.5mm thick stainless steel, with a consistent 50mm bend radius, and 2.5" tips. The muffler section is 18" long. It's mocked up using Vibrant glasspack styled resonators, but the idea is that a lot of different mufflers / resonators could be tested with minimal welding (if any).

"Straight Pipes"
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Borla-Inspired Quad Exhaust
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Thoughts? Am I missing anything? Anyone have a Borla exhaust they want to part with and spare me the exercise?

 

I could try that. When I spoke to them, they didn't seem at all interested in bringing it back or making a small run. Not quite "no way!", but pretty close. Maybe I could try higher up the chain.

That would be very interesting. 3D printed titanium is currently 10x the price of stainless steel though (Inconel is 30x!). There just aren't enough shops overseas printing large pieces in these metals yet. It's rapidly changing though. I bet in a year or so the price would be half. A super lightweight exhaust would be awesome, but I think I'd rather spend that money on other projects.

 

After sleuthing through all the exhaust threads discussing Bell, Borla, Nouvalari, Larini, Tubi, etc., it was your videos that triggered the "that's it!" moment; which led me down the path of trying to figure out what it was about your setup that I liked – which I think is the 12-into-4 configuration. So yeah, your exhaust is essentially the inspiration for this design.

It'd be great if I could find an existing Borla system to modify, but it seems I'll need to create one from scratch if I want to go the quad exhaust route.

 

After doing a little more research, it looks like some of the pre-muffler parts are outside the bounds of most Ti printing machines; which makes it much more expensive. So a few strategic cuts that are convenient to weld can make a big difference in cost. If you do that, Ti is about 4x the price of SS (vs 10x without breaking the larger parts up).

The nice thing about this design is that I can have it fabricating or printed. I can also have it printed in SS, and then swap out components for Ti – or do a hybrid (SS pre-mufflers, Ti post-mufflers).

Speaking of the design, I made some small tweaks. I used standard bend radiuses (2", 2.5" and 3"). The piping is all 2" ID; which I think is slightly larger than what is used on the original Borla design. I also made it so the mufflers can be swapped out (I think the original design has the muffler outlets welded on).

Attached is the current design.

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While working on the CAD for the full design, I ordered four 2" ID cherry-bombs, and 3D printed side exit exhaust pipes out of ASA. They can only handle a few minutes of exhaust temps; which was just long enough for me to confirm this is the sound I'm going for. This experiment was equal amounts of ghetto and awesome, but it did the job. I much prefer this sound over the Nouvalari I had installed – particularly at idle. My plan is to make the full version in plastic (ASA) first, make sure everything fits, and then have it printed in metal (SS or Ti – I haven't decided).

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Mostly for the weight savings. It does sound a little different too, but that wouldn't be main reason (for me, at least).

 

A little update.

I've been tweaking the design and 3D printing the exhaust parts in plastic for mockups. I've shortened the pipes as much as possible, while using as large of diameter tube as possible, and as large of a bend radius as possible – without having the parts touching or in danger of radiating too much heat near body parts, factoring in O2 sensors (my car is EFI), etc.. The 3D scan is invaluable in this process.

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I've got it mostly there. The manifold attachment area is necked down to 1.75" OD to make it easier to install hardware. After 50mm, it goes up to 2" OD throughout. I then have it expanding to a 2.5" OD megaphone on the exits with a 3mm rolled edge. I'm currently testing the exits / tips to see if I like the look, but it's inspired by some of the white BBLM exhausts I've seen with rolled edges.

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I also some of the parts being made that I know aren't changing. Eg, the "brackets" that everything hangs off of are being cut via SendCutSend in stainless, then powder coated black. I designed the muffler flanges to incorporate 2" length of straight tubing, so I can easily swap mufflers using straight tubing from Vibrant (no welding necessary). The "flanges" are being printed in stainless via CraftCloud.

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My plan is to A/B a few options:

- Straight pipes (control group, for science!)
- Cherry bomb glasspacks (the real thing).
- Porter "shorties". Similar to cherry bombs, but have a lower growl due to a coil instead of perforations and steel packing vs fiberglass.

I was able to shorten all of the tubing to make them 3D printable in SS or Ti without breaking into smaller parts, so once I am sure those parts are fully designed, I'll have them printed in stainless steel. Then it's just bolting it together. Not sure how I plan on treating the parts. Eg, cerakote, powder coat, "white lightning" ceramic coat, etc.. I've never 3D printed SS before, so I think I'll see how it looks with glass blasted before making a decision.

If all goes well, I should have a custom 3D printed exhaust in a few weeks.

 

I've only done metal printing a few times, and only aluminum (I printed custom intake manifolds for my EFI/ITB conversion). This is my first time printing SS.

My guess is that the cost is similar to a custom made exhaust. Eg, good quality stainless, mandrel bends / pie cuts, purged tig welds, etc..

Right now, the price for the printed parts is coming in at $3,000 USD via CraftCloud. The SendCutSend brackets (both the shock mount and hanging brackets) came in at $100 (with black powder coating). Factor in another $400 for the cheapo cherry bombs, lap joint clamps, and gaskets. So $3,500 – which is less than I paid for the Nouvalari exhaust it's replacing.

Assuming this all goes well and works, I still think I would prefer to have purchased an off-the-shelf exhaust from Borla, but that's not an option. Also, I don't know of any exhausts for the Boxer that are this modular. With this setup, I can swap out mufflers and experiment with different sounds. I can swap out the post-muffler sections for Titanium if I want to reduce weight, etc..