The Last Of The BS: FMU’s Exhaust-Blown Diffuser

The Last Of The BS: FMU’s Exhaust-Blown Diffuser

If you’ve noticed, we never explained what all that tailpipe-related business was, nor did we release a set of ‘last shots’ of the Black Series. Well, let’s change that.


When showing off photographs or videos of the rear end of the Black Series, up until today, we kept the full assembly off the internet, choosing to let SEMA be the big reveal. Now that SEMA is in full swing, we finally figured we would clue you all in to the final piece of the puzzle — the exhaust-blown diffuser (EBD) — and our reasons for implementing it as we did. The idea for utilizing an exhaust-blown diffuser came from Formula 1, as have other ideas we’ve spawned. Given the sheer amount of money being thrown into F1 year after year, even with all the constant guideline changes and engine requirements, F1 constructors still manage to theorize cutting-edge technology and implement it within guidelines thicker than the Oxford English Dictionary.

While theorizing ways to make the Black Series sit apart from all the other cars at SEMA, we came upon the idea of using an EBD. Being the ones in charge of making the exhaust, we knew that SEMA was a place where the ordinary just won’t cut it. And regular ol’ exhaust tips weren’t really seeming like an interesting idea, or one that would turn heads. So we set about hiding the exhaust tips within our own custom aluminum creation. After plenty of time at the drawing board, and even more time on the CNC mill, we’d figured out a way to incorporate the basic functionality of the EBD while still maintaining a form that was easy on the eyes (and easy enough to create in our time with the BS). A perfect mix of form and functionality.


The science behind the EBD isn’t too tough to grasp. You can see on each side of the carbon-fiber, there’s an aluminum diffuser that we’ve created. In the middle of each of those diffusers is what we call a “shovel.” Essentially, the shovel’s job is to direct the flow of the exhaust as it exits into the diffuser. Without the shovel, the air would be turbulent and wouldn’t produce any extra downforce. The exhaust gas is almost always moving faster than the air around it, especially at lower speeds; also, if you remember science class, air is viscous or “sticky.” The faster-moving exhaust gas exits the tailpipe onto the shovel, which directs it into a laminar flow (the opposite of turbulent flow) as it exits the diffuser. That laminar flow of exhaust gas will “grab” the viscous air around it, speeding it up as well. This increased velocity contributes to the pressure differences in the diffuser that create downforce.

Now, a proper Formula 1 exhaust-blown diffuser relies on a boatload more theory than this, but it was this basic idea that spurred us to create the EBD you see on the Black Series. We’ve flow tested it in SolidWorks, and having the diffuser blown by the exhaust almost doubles its effectiveness at lower speeds. Additional increases can be extracted from the exhaust by taking advantage of its thermal load. Using the exhaust to heat the air traveling under the car causes thermal expansion which further reduces pressure, increasing the effectiveness of the diffuser system. For that to be possible, you’d need a much more complete undertray system and other changes to the system as a whole. For those of you interested in this sort of aerodynamic work, good luck finding it online; as it’s really a trade secret between various F1 manufacturers at this point, there’s not very much official information on how EBDs work. Forum posts on this topic are abundant, but you can never really tell who actually knows what they’re talking about on the forums.


So there you have it. Not only did we eschew the long-standing notion that exhaust tips had to be just that — old-fashioned plain metal tips — but we also managed to bring in some extra functionality with it. Essentially, an EBD just uses what the car is already producing (exhaust gas) to have some sort of beneficial attribute above what it normally does. These last three pictures will show the parts of the car that you don’t normally see in a regular picture. First, we have the engine bay. The M156 should be well north of 600 bhp with the Weistec Stage 1+ supercharger kit, and it’s helping to stay cool with an Aquamist HFS3 water-methanol injection kit, supplied by Howerton Engineering.


Next, we have our Silence To Violence switch. The toggle on the right controls the electric cutouts in the exhaust system; with the cutouts open, the mufflers are bypassed and the unmuffled exhaust is let straight into the EBD. Both settings empty into the EBD, so loud or not, you’ll still be producing downforce. The switch on the left controls the WOT Box, which we’ve installed for one reason and one reason only — violence. It’s not a normal 2-step like you get with a WOT Box; instead, we’ve wired it to dump fuel into the exhaust, which in an overrun situation will cause a fireball like none you’ve ever experienced. Your humble narrator first heard the WOT Box fireball inside the office as the Black Series was being road-tested. It’s not your typical fireball; this sounds more like a rifle shot happening five feet from your face. It’s that singular POP you hear in the Black Series feature video. Why would we create a switch that wastes fuel for the sake of mind-blowing fireballs? Because we can. CAFE standards quake in our presence.


And finally, a shot of our EBD system, which we incorporated into a rear undertray, connected to the BS through that bar-and-arm system we showed you a week or so ago. Since you haven’t seen it before in its completed state, here’s the back half of our exhaust system:


And with that, we’ll sign off for the day. We’ve already given you more text than usual, and we don’t want to overload you guys, especially since it’s so close to the weekend! Look for more BS-and-SEMA-related blog goodness tomorrow!

1 Comment
  • Ryan
    Posted at 15:21h, 05 November


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