Now that we’re starting to get serious about turning a fast lap in Project RX-8, the time has come to start exploring ways to reduce aerodynamic lift and increase aerodynamic downforce. Rather than trying to sort this out through trial and error, we turned to Simon McBeath, a well-known aerodynamics expert who literally wrote the book (called Competition Car Aerodynamics: A Practical Handbook, available on Amazon.com) on making downforce on just about any body shape imaginable, from Formula cars to production-based road cars.
For those of you looking to sort out the aerodynamics on your track car, McBeath offers an aerodynamics advisory service called SM AeRo Techniques, where he surveys the external and internal details of your whole competition car and then provides a detailed report containing recommendations on ways to improve its aerodynamics. We sourced our wing from his website, WingShop.co.uk, and he’ll sell you one that makes serious downforce, too.
The airfoil we opted for is the 183 single-element, high-efficiency wing, manufactured by DJ Engineering Services in the UK to McBeath’s specifications. McBeath developed the 183’s profile via both CFD and wind-tunnel testing at MIRA’s full-scale aerodynamic wind tunnel. The 183 wing is his original main element design, and it has been used to great effect on a wide variety of machines ranging from lower-power single-seaters all the way up to GT race cars where the rules only permit a single-element wing with a 300mm cord (the distance from the leading edge of the wing to the trailing edge).
As you can see from the wind-tunnel data at 100 mph in free-stream (unobstructed airflow), the 183 wing profile is a highly efficient design, generating 887N (or 200 pounds) of downforce at the 8-degree angle of attack McBeath recommended we start at and just 71N (or 16 pounds) of drag. So with this wing in place, our RX-8 gets the benefit of meaningful downforce in the corners, which should go a long way toward taming its rather active rear end while producing surprisingly little drag down the straights (that 16 pounds of drag at 100 mph is the equivalent of losing just 4 hp).
To install our DJ/SM 183 wing, we had Aaron Weir at Weirtech fabricate some custom uprights to mate to APR carbon-fiber GTC-300 baseplates. These baseplates can be bought separately from APR and make for a convenient way to trunk-mount a custom wing like this one. In an ideal world, we’d chassis-mount the wing like we did with Project S2000’s dual-element DJ/SM 203 wing (also a McBeath design), but since the RX-8 is still a street car, cutting holes in the trunk lid or rear quarter-panels really wasn’t an option.
The uprights were designed to position the wing based on McBeath’s recommendations, which specify it should be at least a wingspan (300 mm) above the trunk’s deck and have about an 80mm overlap of its leading edge with the trunk’s rear lip. With our Weirtech uprights, the wing is 310 mm high, so bang on where it needs to be, but it’s a little farther forward with about 160 mm of overlap. We’re going to do some testing with the wing in this less potent position, but in all likelihood, we’ll have Weir design and fabricate some new uprights that move the wing farther aft so we’re closer to the ideal 80mm overlap. Plus we’ll design them to be lighter and more aerodynamic (thinner and with a rounded leading edge).
To balance out the overall aerodynamics package, we’ll be fabricating a custom front splitter to mate to the Mazdaspeed front bumper, but you’ll have to wait until next issue for details. In the meantime, we’ve completed the Mazdaspeed body kit by installing side skirts and a rear underspoiler. The side skirts help overall aerodynamics by reducing the amount of high-pressure airflow from the sides that spill underneath and disrupt all that nice low-pressure airflow that helps keep the car planted. The rear underspoiler, on the other hand, is probably a bit of a parachute given its shape and lack of venting (we may add some venting if we feel like defacing a rather expensive piece of out-of-production bodywork), but it does give the rear of the car a lower profile that matches the lines created by the side skirts and front bumper.
Installing the sides and rear underspoiler isn’t for the faint of heart. The rear installation doesn’t take long, but you have to cut some of the OE rear bumper to make room for the backside contours of the Mazdaspeed piece. This means taking a hacksaw to the urethane rear bumper cover and trimming a few inches off the black section that covers the muffler. Follow the instructions and you’ll be fine (or do what we did and accidentally cut off a bit that was meant to remain—nothing a few well-hidden zip ties couldn’t fix, though).
The side skirts come with very detailed instructions, and for good reason: You have to drill eight holes in each side of the car. Drilling holes in the factory sheetmetal isn’t something we normally enjoy doing, but thanks to the templates provided with the side skirts, finding the exact spot to drill is easy, and once the holes are drilled, the side skirts snap/bolt into place with OE fitment. And that’s really the beauty of this kit: It fits perfectly and enhances the car’s body lines without screaming, “Hey, Mr. Police, come look under my hood for illegal modification!”
We’re very pleased with how Project RX-8’s shaping up on the outside, and it’s only going to look meaner once we install that front splitter. And maybe some dive planes. And a rear diffuser. And a vented hood. That’s right, kids, we’ve got plans!
Our Simon McBeath-designed wing is built by DJ Engineering in the UK, the outer wing skins being laid up and cured as one-piece in high-pressure aluminum molds and then combined with exceptionally stiff and lightweight carbon box spars (also made using high-pressure tooling).
Custom Weirtech uprights and APR baseplates complete the setup, along with a wrecker-sourced trunk lid so we can use the original one for wingless street mode.
The 183 wing has undergone extensive CFD and full-scale wind-tunnel testing, as well as having been proven in the field on a wide range of race cars.
Installing the Mazdaspeed rear underspoiler isn’t rocket science, but you do need to read the instructions carefully before taking a hacksaw to the OE rear bumper cover. Don’t ask us how we know.