What improvements i aerodynamics (EXCEPT wings) can be made to the '93 - '95 RX-7 body? I have a factory front air splitter/lip. Now I think an RE Amemiya rear underbody diffuser is nex. I don't want a wing for both esthetic reasons and I don't want the aero drag.

Are there other tricks? I've seen a replacement taillight cover that has a fairly pronounced turn up to create downfporce but this too will create some drag.
I've transferred my battery to the passenger side storage compartment for better weight distribution both in corner weight and to offset driver weight and to center the mass of the car a bit AND to permit a bigger Pettit intercooler in the stock location. Since I live in 'Vegaswhere the heat is torrid in summer I'd like a VIS carbon fiber/frp hood with vents just behind the intercooler. I understand this increases the front downforce as well as helping the intercooler to flow like crazy.

Since I've done away with my spare tire in the trunk and will soon loose the unnecessary 22 pounds of Bose "Wave" speakers there (any buyers out there?)
Also I feel I either need to add weight to the spare tire well or hope the Amemiya diffuser works ("sucks"?) very well and also adds a little weight of its own.
Any cool underbody tricks?
P.S. I will not add any body "cladding" other than the front lip and rear diffuser. I'm trying to retain the original beauty of the design and keep weight to a minimum.

 

What are you trying to build?

Daily Driver
Scca Racecar
Drag car
Autocross

 

Cool underbody tricks? No, not really except the Amemiya rear diffuser. Maybe the racing guys know about some, but the stock underbody with all the covers does quite well.

Aero tricks? Yes, adjustable rear wings and front canards can increase cornering downforce, but you said you don't want that. I have to ask why, if you're concerned about aerodynamics, since the stock FD body is designed for very low drag coefficient and less for downforce. Most mods then trade off drag for more downforce, you won't get much improvement going the other way to get lower CD. Maybe you can if you're willing to remove your antenna, mirrors, and door handles. Lowering the car should help CD a little, but I can't guarantee it.

Vented hoods hurt your overall coefficient of drag some by pay great dividends in bay cooling. Switching to a more open front bumper skin and tightening the air guiding in the front end also make great improvements.

 

I thnk you should be asking Scrub from the forum, he has alot of stuff that has to do with this on his whip

 

RXracer -> I'm trying to build a faster street car that I can use for track days or maybe SCCA SOLO I competition occasionally.

dgeesaman -> I could go for front canards possibly, or at leasr canards that attach to the outboard sides/bottom of my front splitter.

SWATS1 -> I'll try to find Scrub's comments. Which forum? tech?

Finally...does anyone know if there has ever been any instrumented track or wind tunnel testing on what really DOES work?

 

You definitely want to lower your car, because that lessens the amount of air that runs under the car, reducing lift. Sideskirts also help in this as well (if you want stock, you can always go w/ the stock Mazda skirts). You wanna have a spoiler (stock or otherwise) to increase downforce in the rear. A larger front airdam helps direct the air more into the engine bay vs back under the car. That requires an undertray however, to prevent the air from escaping back underneath the car. The RE diffuser definitely works, yet not 100% by itself. To do it correctly, you need to completely seal the underside of the car, using the undertray in the front, and from about where the seats are, have loading ramps running to the rear, which end in the diffuser. That way, you're actually smoothing and channeling the air to streamline its flow, reduce turbulence, etc.

If you wanna get crazy, ask Damian. He was working on some brake ducts and what not...911 Turbo typa technology hehe. He's actually also in the process of making an undertray for the FD.

Mazdaspeed products are actually wind-tunnel tested. C-west claims its products are wind-tunnel tested as well, but apparently it's labeled as "up to 90mph" lol.

 

I think a Wangan Racer

 

If you're serious about setting the car up for downforce, go over to the race car tech forum and ask around or search over there. They'll be able to tell you which parts have been wind tunnel tested and actually help the car rather than hindering it, like that diffuser. I'm not sure I've ever seen proof that it actually does anything for the car other than create drag. For a rear diffuser to actually work well with our cars, the whole bottom would have to be completely flat and flow into the rear diffuser, kind of like a Ferrari 360.

 

I just want to reiterate what was said above me. Diffusers don't work without a flat underbody.

Be careful when adding on front "aero" things and doing nothing to the rear. Significantly more downforce (or less lift) in the front will cause an oversteer situation at speed. Diveplanes are generally worth a lot of downforce and usually need to be balanced with something in the rear.

 

Rent a wind tunnel and get back to us.

 

I wish...

 

i'm building one in my garage

 

LOL. Joey, you're kidding, right?? (That's my glimmer of wishful thinking...)

 

Dumb question, but what would you do to make the bottom flat? trim and bolt in sheet metal or something?

 

Not sheet metal, but something lightweight yet sturdy. I believe Damian is making his undertray out of Teklam. I'm not sure what the Enzo, McLearen SLR and Carerra GT use, but I imagine it's some strong yet lightweight plastic type of material, much like our stock undertray is made out of.

 

Carbon fiber.

 

There are very few, if any, aerodynamics experts on the Rx7 forum.


It is minimally helpful to ask such a question here as you will get many more jibberish and half-truths than facts.

Part of the problem is that aerodynamics as it applied to cars is quite complex. It really requires wind tunnel testing to prove any improvements made to the body.
The real problem, in this forum, is that most have no idea what they are talking about.

There are, however, some basic concepts of aerodynamics that may be helpful in deciding whether any of the aerodynamic aids may be helpful in geting you where you want to go.


For instance, a rear wing does not necessarily increase drag although most self-proclaimed RX7 Forum aero experts will state as gospel.
Although it is generally true that downforce comes at an expense of drag, it is not absolutely the case in all situations. A rear wing can clean up the aerodynamics of a car such that it can increase downforce and decrease drag at the same time.

Therefore, if you really want answers, I suggest that you read 2 books(both available at Amazon.com):

"Competition Car Downforce: A Practical Handbook" by Simon McBeath

"New Directions in Race Car Aerodynamics: Designing for Speed" by Joseph Katz

The first is a prettty easy and quick read. But it will give you much more knowledge on aerodynamics than almost all of the members of the RX7 Forum.

The second book has more in-depth treatment of the aerodynmics. It is longer and may be more than what a arm-chair aerodynamicist want to get involved with. But it will give you much more expertise.

 

Hell yeah i'm building one. It'll be completely built from parts from Wal-mart and Autozone. I'm going to use a TI-88 graphing calculater for the main computing mechanism.

 

And that is really the problem - the manufacturers of wings and body kits claim to do wind tunnel testing, but don't disclose any information about it. If you ask them, of course they all claim to lower drag, increase downforce, everything.

Plus, once you modify the body of your car, there is no way to know if it helps or hurts except by wind tunnel testing or track performance.

Dave

 

Kento, I was going to say CF, but I've personally seen them (minus the Enzo, but add the Ford GT), and while lighting under the car isn't exactly the best it didn't look like CF to me. I couldn't make out the weave pattern. Plus, it wasn't very glossy like clearcoated CF. It was almost like a flat black... One of the cars (I forget which) actually looked like it had aluminum/sheet metal, in many many small segments, all bolted up together (vs. it being one large peice). But like I said, I couldn't tell for sure...

Mr. Stock, you're probably right, and I speak for myself when I say the "advice" I was giving was using basic concepts, and in isolation. Indeed aerodynamics of a car are very complex, and how various parts work together at certain speeds is a totally different topic. I don't think it makes the general advice (such as lower the car, reduce air underneath it, seal the bottom) useless, however. Yes?

My other concern is that I agree w/ what you're saying, but how will the book help me in the sense that even if I understand the aerodynamics 100%, if I can't TEST it, I really am still working blind. I can't take a bumper and say, theoretically, this should reduce lift and decrease the drag coefficient, put it on, and test the theory against real scenarios. So we're kinda left in a no-win situation, am I correct? (Unless you're a baller and wanna buy Mazdaspeed parts, which are indeed wind tunnel tested).

It's interesting that you just said this, because I was watching Car & Driver TV only 5 min ago, and they were airing a segment on various SCCA (I believe) cars, and the IS300 they had, the team (I forget the name) actually created their own spoiler via wind tunnel testing, and it's unique shape (a very FAT tear drop shape) actually increased downforce while reducing drag. They said they did that because of the lack of a large market for aftermarket bolt-on parts for their car, so they did the design and testing themselves.

Joey, lol...I can't tell if you're serious or kidding. Either you're totally putting me on, and I'm definitely being suckered by it, or you're making a dream come true: designing a poor man's wind tunnel. If so, I say we get that sucker up and running, make it a forum group effort, and you could prob. even rent it out to various organizations/individuals, at a HUMANE price (not the $50K an HOUR that Mazda asks for at their Irvine center!)

 

Both the Enzo and Carrera GT undertrays are carbon fiber. Well-made, structural quality CF often doesn't look like those glossy-finished hoods and showy body parts that you're thinking of; it often has a flat finish because they're concerned more with actual performance instead of car show looks.

 

Thanks Kento

 

Knowledge is power.

The books will help you in that you will have a better understanding of aerodynamics. This knowlege will allow you to decide for yourself what may work and what may not, instead of posting on the RX7 Forum for answers, which undoutedly will lead to many wrong directions.

 

If it's a street car with maybe occasional track use, why the hell are you worried about improving the aerodynamics? The stock car is very stable well into the triple digits. Since you shouldn't be regularly driving over 120 mph on the street, that shouldn't be an issue. If you are really concerned about downforce, mount a stock rear wing, it reduces lift (contrary to popular opinion...). Since you have a front lip spoiler, a rear wing is definitely necessary to balance the car back out.

Again, aero for an almost purely street-driven car is pretty useless. And listen to Mr. Stock. Part of the reasons Ferraris cost so much is that they generate several hundred pounds of downforce WITHOUT obvious body parts.

 

We do have a windtunnel (2 actually) here at work. Justifying putting my car in it however would be slightly hard to accomplish.