Winning Aero For Your FR-S: Available Now!

Of all the questions I get about our Scion Tuner Challenge-winning FR-S build, by far the most common is “Where can I get that rear diffuser?”

(HINT: Now you can buy it here!)


After receiving that email at least a dozen times, I figured it was time to give the people what they want and put the diffusers into production. I never thought so many enthusiasts would be willing to cut into their cars to install my part, but hey, I’m not going to discourage it.


Since this was a one-off part, I verified some measurements while the car was back in my shop for some final touches before making an appearance on Jay Leno’s Garage. I then updated my Solidworks model, split the diffuser into a shipping-friendly three piece design and produced the first batch. They sold out instantly, and now people all over the world have Charvonia Design diffusers on their FR-Ss, BRZs and 86s!


With the diffusers now flying off the shelf, the next most common question became “How do I install it?” After guiding several folks via email, I decided I had better get my hands on a local FR-S and document the fitting process.


The first step is to unwrap all the parts and peel away the protective coating from the brushed 5052 aluminum panels. There’s no finishing required, unless you desire a different color. A couple guys have threatened to powdercoat their diffusers black, but I haven’t seen it yet.


The hardware comes kitted in a heat-sealed plastic sleeve by yours truly, so cut the poly bag open and organize the stainless steel fasteners


I mostly used 1/4-20 stainless button head cap screws for assembly. These will accept a 4mm or 5/32″ Allen wrench. Place the flat washer below head, and make sure the sharp edge faces down, otherwise I might have a fit when I see your car.


On the other side of the diffuser, you’ll secure the screws with a nylon locking 1/4-20 nut using a 7/16″ wrench or socket.


Now comes the fun (and totally not scary) part of cutting out the plastic portion of the rear bumper. I like to use a pnuematic body saw because it’s easily controlled and the plastic won’t melt back together as with a cut-off wheel or Dremel.


As you can see, I taped off the painted portion to prevent any mishaps. Then I slowly and patiently ran the saw along the crease between the textured black plastic and the painted part of the bumper. This took a solid 10 minutes of pretty intense concentration.


Okay, now you can take a breath and remove the plastic part. Unclip the reverse light wires and chase them up into the trunk to disconnect the plug. If you want to keep your reverse lights, check out this kit that relocates them into the tail lights.


This is the plug you want to disconnect to free the reverse light harness. It’s the white plug shown here, accessed by removing the plastic panel you immediately see when opening the trunk.


With the plastic portion now out of the way, you’re greeted by the new stripped-down, mechanical looking rear end of your FR-S.


You won’t be needing these brackets anymore, so remove them. Notice the 6mm threaded stud on the left side of this image. That’s where you will attach the L-brackets.


This Scion had an Invidia N1 cat-back exhaust, which just barely squeezes in above the diffuser. I compressed the front exhaust hangers using hose clamps and moved the rear hangers to the middle hole to fine tune the height of the mufflers. Other exhausts that are known to fit are the Nameless Track Pipe and Berk Muffler Delete.


The Invidia mufflers were also too close to this sheetmetal, so I marked it, removed the mufflers and carefully tapped the metal up at a 90-degree angle to buy some valuable space.


Here’s the Invidia cat-back in its new raised location, which will now fit with the Charvonia Design rear diffuser.


With the help of a friend, or maybe just some jack stands, hold the diffuser up against the bottom of the car. Using a silver Sharpie, mark the three holes on each side. Two will land on these black plastic boxes, and one will end up on the painted bottom edge of the bumper.


The next step is to remove the black plastic boxes from either side of the bumper. You can use a flathead screwdriver, but my favorite tool for this is a set of push pin pliers. There are a couple retaining pins up top too, so use a flashlight to identify them. I’ve found it’s easier to reinstall the upper push pins backwards when you put it all back together.


Drill 5/16″ holes on your silver Sharpie marks, then trim back the plastic until the provided 1/4-20 speed nuts fit in the correct position.


Now you can bolt up your Charvonia Design diffuser! Spin the 1/4-20 x 3/4″ long button head cap screws into the speed nuts, then use a fender washer and nylon locking nut for the screw that goes through the painted portion of the bumper. You’ll also need to install the two provided L-brackets (not shown) in the center of the diffuser, and use the 6mm nylon locking nuts to secure them to the body.


On the underside of the FR-S rear diffuser you’ll find the Charvonia Design logo has been engraved.


And on each fin, the diffuser proudly wears the SH Speedhunters logo too!


Since this is a street-driven car, we opted not to remove the factory crash beam for safety and liability reasons. If it’s a track car, go ahead and shed the weight, at your own risk of course.


The factory crash beam could also be painted to change the appearance. Black will make it less visible, or you can clean up the color-matched factory paint and let it blend in with the body.


Thanks for following along to see how installation of the rear diffuser is accomplished. Hopefully this takes some of the mystery out of the process and gives you the confidence to tackle this project at home. As always, I’m down to help in any way, so shoot me an email with your questions.

If you’re ready to get your own Charvonia Design x Speedhunters Scion FR-S Rear Diffuser, you can purchase it HERE.

Keith Charvonia
Instagram: SpeedhuntersKeith



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That's one well made solidworks model Keith! Will you also be selling them with the MAXIMUMATTACK logo or is that a one off for the tuner challenge?


Maart3n Good eyes! :) The MAXIMUM ATTACK logo was a one-off for the Scion Tuner Challenge, that's what we named the car. The new diffusers have the Charvonia Design and SH logos as pictured later in the story.


That looks amazing! Here's a suggestion. Since you are very evidently capable of doing so, a matching stainless steel cover for the crash bar would really clean up the back. You can offer it as an option for the street guys. Just a thought.


Looks great, is there a reason you can't cut the stock diffuser to match the profile of your new part? I also think it could look rad anodized black:


FuhksgivenZheroh This has crossed my mind. I made aluminum crash beams for the Tuner Challenge FR-S but obviously they're not street legal. A cover would give a similar look without sacrificing any safety.


I just wish I had an FR-S/GT86/BRZ


Is this just a body kit or has a car fitted with the kit actually been in a wind tunnel ?
The body kit looks ok but the rear diffuser looks a bit tacky to me.


for a diffuser to actually work it must extend out past the rear of the car. That is useless apart from hard parking.


Nice work Keith. It ought to do a decent job of shrinking the turbulence pocket. Looks clean too.  I'd like to see how much of the stock bumper cover can be retained.


@yoloswagemperor  I don't always consult an Aerodynamicist for my aerodynamic needs but, when I do, they like to use y.o.l.o. and swag in the same statement.


Nice work, Keith. Now it's time for some tuff-testing or messy flow-viz. I'd love to see you post some of that!


So you're basically saying it's another dress up part ? Not a fan of any of these kits but at least it's on the conservative side.
At least it doesn't make the car look like a toy hot wheels car like the Porsche kit featured a month or two back.
Each to their own I suppose.


Your work looks nice; clean and thorough.
Were any CFD simulations done on this part? Without evidence to show the defuser actually works, it's tough to justify putting this on a track car.


Will this fit a 510 wagon, Keith?  ;)


@yoloswagemperor Diffusers are more complex than that, and they don't necessarily need to extend far past the car to work. A diffuser works as part of a system, ideally comprised of a splitter and flat bottom. It is known as a pressure recovery device. Basically with a splitter and flat bottom as the car moves the air traveling under the car increases in velocity, which in turn turn reduces the pressure of under the car drastically. However, the air pressure at the rear of the car, near the bumper is at a very high pressure (and very low velocity) and works against the low pressure, high velocity airflow under the car. A diffuser is used to gradually slow down the airflow under the car as it reaches the rear bumper of the car (or beyond). By doing this, it brings the very low pressure airflow under the car back to the higher pressure levels near the rear of the bumper.

A diffuser only has one very low pressure zone: at its entrance which is sometimes called the throat. This section is very sensitive and is one of two places in a car underbody where you get the largest amount of low pressure drop. An aerodynamicist has to design a diffuser considering how much expansion volume regulations allow, how the aero balance (straightline, braking, roll) will be affected, etc, etc, etc.

Not sure why I wrote this long post, but I guess I see a lot of incorrect assumptions about automotive diffusers posted online, and while I don't know everything about them, I've done a considerable amount of research and design on them. I guess if there is any take home message from this post its this: diffusers (and aero in general) are very complex and can't be described solely with statements like "it must extend past the rear bumper to work" , "it must have an AOA of under 10 degrees", "CFD is the only way to validate that a diffuser design can work." Like all things engineering, the end design is optimized for the rule set, design constraints, cost constraints, and timeline that you have. Just because something works in F1, Lemans, WTAC, BTCC, etc doesn't mean its necessarily the best solution for your car. 
...But back to the original comment...

The design above, like you imply, probably wasn't designed for track duty. However, considering aesthetics, the piece looks really nice and I bet 90% of the people buying this are for show cars anyways.


The Charvonia Design logo is really nice Keith


@No Salsa Seems that you love to complain on here.. you posted two negative comments on the same article..take the hate somewhere else. Go build something better and then you can talk.


@yoloswagemperor There are many factors involved in getting a diffuser to work effectively, extending past the rear of the car is a minor one.


90nissanS13@my350z BURN! this made me laugh alot!


Real nice look to it. What did you do with the reverse and fog light though?


KeithCharvonia FuhksgivenZheroh Keith, what makes a crash beam cover more/less legal than a whole diffusor instead of the standard bumper?


@cheechthechi Good explination, but to me, all you've done is validate No Salsa's position that this is a show piece only. Maybe he's being a bit of a dick about it, but he's not wrong. Without an aerodynamicist working with the part, there's really no way to know if this piece helps or hinders performance.
Asking someone to purchase your product (and chop up their rear bumper) without showing evidence that the part actually works, seems a bit odd to me. I mean, we don't even know how much it weighs.


Jun Imai Let's find out! :)


LukeEVOVIII Thanks! Designed by my friend Ruggy Ahumada.


Colouryum On this car nothing, however if you want to keep your reverse lights, check out this kit that relocates them into the tail lights.


Twitch_6 I had a guy working on CFD but he never came through. :( Once I have that data I can easily tweak the design to optimize it, which could then become V2.

The reality is that most guys are putting these on street cars (yes, for aesthetics). It was initially designed on a car I built in 90 days for the Scion Tuner Challenge, and we got enough requests to buy them that we started production.

I have them on a couple track cars and have requested any data that supports a positive or negative effect, lap times, trap speeds etc. We will see!


bluestreaksti Interesting, you're the second one to mention retaining part of the bumper. I would like to see this too.


Twitch_6 "Without an aerodynamicist working with the part, there's really no way to know if this piece helps or hinders performance." 

You're correct. I'm not an aerodynamicist, I'm a car builder and engineer in the automotive aftermarket. This diffuser (and all the other ones on the market) does not reach speeds on the street where the part helps or hinders. 

"Asking someone to purchase your product (and chop up their rear bumper) without showing evidence that the part actually works, seems a bit odd to me."

This view only considers aerodynamics on a track car. Believe it or not, most people running a diffuser put it on their street car, yes, for looks. This is the same as not buying a Rocket Bunny kit, front lip or rear spoiler because its not proven to "work."


jaretron Cool idea! I would like to see this in real life too.


flyingjolly KeithCharvonia FuhksgivenZheroh The crash beam cover would be in addition to the diffuser. It would cover the white part with holes seen on the car above.


are you willing to share the .sldprt ? I would love to play with it and print it out on the 3d printer? dropped scale of course :-)


JasonWidelitz Like it's structure is that hard to recreate by hand....the laziness is unbearable


KeithCharvonia flyingjolly FuhksgivenZheroh damn, i didn't read that right, i mixed the cover and a whole aluminium crash beam up.
I thought you meant a cover wouldn't be street legal and i didn't get why a cover would be more illegal than already putting on the diffuser. But a aluminium crash beam wouldn't be street legal vs. a diffuser i get that now.


DanielSem JasonWidelitz Actually figuring out the angles and the fitment is not something you could easily replicate without either having a physical model or a car to test fit and measure.  With that said I find both your comments a little surprising as it is quite "nervy" to ask for something for free that someone spent a lot of time and money on to develope, as well to assume that it would be easy to recreate is also a little insulting.  I am a product designer who uses Solidworks, if you have the skills to make a cardboard copy of this and then 3D model it, it usually means that you have probably invested some time and money obtaining those skills. I agree that this not the most complicated part, but even the simplest of parts take more time than you would think.


Nice work Keith, I'm sure it will be a great seller. Don't let the negative nellies out there put you off designing any more parts. The design and quality look great. You might build one up for Bryn's volvo now!?!

Keep up the great work.



jaretron DanielSem

       Jaretron, I guess it's a bit ostentatious to ask for the model. As I said before I really don't have any plans to replicate it to full scale. Hell I don't even drive an FRS I just saw solidworks screen shot and thought it would be fun to modify the file with a mount for a stand and spit out an .STL. Then scale it way down and send it to the printer. It would make some nice "Desk Aero". I would be more than happy to send Keith a copy of the print. But I guess there really is no way to prove my intentions so I could see it being disregarded. Either way if you're reading this and you want a replicated copy in ABS to sit on your desk hit me up. No strings attached just thought it would be cool.


JasonWidelitz jaretron DanielSem Hi Jason, while it sounds like you have the best of intentions, I would prefer not to have my files out of my hands. Hope you understand. :)


KeithCharvonia JasonWidelitz jaretron DanielSem

No, that makes plenty of sense. I can understand why you wouldn't want to release the file to the public. Never hurts to ask. If you change your mind and want a printed copy let me know. Either way Cheers!


Will this work with Perrin exhaust? I have a muffler in the middle area


€. 420 ex tax


Awesome work Keith!
For those that were talking about the performance of diffusers, I'd recommend checking out this website regarding bluff body aerodynamics and diffuser angle comparsons. I used these as a reference in my disseration! very interesting to read!


The only problem in my opinion with this very cool diffuser on a road car is the stock crash bar. This is a very interesting idea to cover it up. I was also looking around for ways to possibly custom fit a grill mesh to cover the gap / crash bar while adding more of a textured transition between this wild diffuser and the plain stock car. Many supercars use this type of transition such as the LFA shown below. This idea for the frs looks good in my mind but it would be nice to see some kind of virtual mock up.


the car was very good 
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Je voudrais acheter les même rétroviseur que cette magnifique scion frs!! Vous pouvez m'aider.


I would like to buy the same mirrors as your beautiful Scion frs !! You can help me, because I do not buy it or buy it. thank you very much