The 2025 Pebble Beach Concours d’Elegance Best of Show award was given to this 1924 Hispano-Suiza H6C Nieuport-Astra Torpedo on Sunday. The car certainly looks unusual compared to any remotely modern vehicle, but its particularly bizarre exhaust caught my eye immediately. I did a little research and figured out what the deal is with it.
Quick context dump: The car was originally commissioned and owned by André Dubonnet, who sounded like quite a character—he was a distinguished aviator, Olympic athlete, racing driver, and designed car suspensions. His grandfather invented the apéritif brand bearing his namesake, and he helped create several unique vehicles. This particular machine raced in the Targa Florio and the Coppa Florio before becoming a collectible. It sold in 2022 for $9,245,000, and since then, owners Penny and Lee Anderson commissioned it to be restored to its original spec at what I’m sure was monocle-shattering expense.
“Known as the ‘Tulipwood’ Torpedo, the car is in fact constructed of strips of mahogany, each individually carved to shape and joined to the inner ribs by 8,500 rivets. It reportedly weighed just 160 pounds when first built,” the Pebble Beach show shared in a press release. That release also included a quote from Mr. Anderson about what attracted him to the car: “I like wood! I’m a wood boat antique collector. I started collecting them 40 years ago, before it became vogue-ish to do that. So I’ve always loved varnished wood, and when I saw this I said, my gosh! This is right in my wheelhouse. This is what I really like.”
Must be nice! Anyway, onto the exhaust. What the heck is going on with that weird shape?
As you can see, the exhaust dumps out of the left side of the car before going through a double-triangle-plus-hamburgers-looking maze that terminates ahead of the rear wheel. Peeking closely at pictures from the show, we can see “Steigboy” branding, referring to Steigboy Apparatebau, founded by Friedrich August Boysen in 1921. His Leipzig-based outfit made vacuum exhaust systems that were mainly used on motorcycles and some aircraft, but obviously also had at least one automotive application as well.
What we’re looking at here is an early vacuum muffler, also sometimes referred to as an ejector muffler. Those big, round hamburger-looking sections allow for a large expansion of volume right after the header. That sudden volume change is supposed to knock down the sharp pressure pulses before they exit. In other words, it’s a reactive silencer tuned by volume and geometry rather than the fibrous packing you have in modern exhausts.
According to German literature about the Vacuum-Auspuff from when this stuff was new, a beveled baffle inside sits opposite the outlet, and the outlet pipe has helical vanes. That’s supposed to swirl and accelerate the gas flow out of the neck, inducing a slight vacuum in the main chamber. The idea was to scavenge cylinders between pulses and cut back-pressure while still muffling.
The Steigboy’s round sections are for creating an optimal volume-to-surface ratio and symmetric wave reflections for pulse cancellation. The short-side neck needs to be at right angles to the inlet to shed and redirect pulses and to house the spiral vanes. And the overall “spheres with spout” silhouette is basically just the smallest packaging that could be achieved in the 1920s.
At least, that was the theory based on 1920s methods of measuring such things. Since the design didn’t persist for very long, its real-world viability was apparently limited.
In 1922, Boysen’s exhausts were on the winning bikes in that year’s AVUS motorcycle race, and of course, the company leveraged that fact in period ads. Looks like the ’20s were the brand’s high point; Steigboy did dabble in some other aspects of transportation, even creating a three-wheeled cargo truck.
Since then, the company has evolved into The Boysen Group, which is not only still making exhaust systems but also battery cases, hydrogen tank systems, control elements, electronic components, and other industrial items.
It’s pretty cool to see such an exhaust on a car at all, and it’s quite fitting to see it on Dubonnet’s Nieuport-Astra-bodied Hispano-Suiza. Boysen himself had aviation ties, and his silencers were marketed for cars, bikes, and aircraft—so the Steigboy technology and design match the vibe and era of a gentleman aviator’s racing car very well.
Got any other insights about odd exhaust designs? Drop the author a line at andrew.collins@thedrive.com
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