Since I've owned my 8, I've been searching the Web and beyond for parts that are deemed useful, and worthwhile. A while back, I was on RE Amemiya's webapge, and I noticed that they were selling a rear diffuser for the SE3P. The Japanese, who always give us two choices, had it in FRP and in carbon fiber. The question is, do these diffusers work? I see the term flung around like monkey poo, especially with the "ricers." By definition, a diffuser is a device that slows the velocity of an incoming fluid. These rear diffusers are supposed to slow down the air passing under the car, therefore adding some downforce at high speeds, right? How do these diffusers behave under low-speed conditions? Since diffusers such as the Amemiya's are not speed-dependent, in which they would adjust according to speed, they might hurt low-speed performance, even if slightly. In conclusion, are these diffusers even worth buying? If there is someone who can debunk the myths of diffusers, please, feel free to get your writing on .

 

In the case of designing a speed-dependent diffuser for the RX-8, I would like to see that. It could work in conjunction with an speed-sensitive wing/spoiler. The result: maximum downforce at all driving speeds...methinks. Well, first, I think it would be a better idea to extract more power to be able to get to those high speeds, on the circuit of course . I hope to see input .

 

there's not a single damned person in the aftermarket world, at least one developping body kits or "aero" devices that could actually come up with application specific, functional speed dependant diffusers. cost is a huge issue. understanding and ability is also a huge issue.

onto diffusers themselves... in this application (under a car, at the back) they're not to slow the air but accelerate it upward, and utilise the ground effect to increase the efficiency of generating a pressure difference between the top of the car and the bottom of it (on the back end, in this case).

now, as you say, there's a lot of real crap out there that isn't worth a dime of what they charge for it, unless you think it really does look good. it's tricky to get a real "diffuser" street ready for the RX-8 'cause of the way that the muffler is shaped (rather enormously, unfortunately) and situated (right at the rear of the car). i think, correct me here you owners, that the plastic black junk between the tail pipes can come off of the painted bumper panel, which is actually good news. solve the muffler problem, design someting which will acutally generate a sizable pressure difference, situate it close enough to the ground for it to work efficiently (clearing the ground at a 10cm level would be terrific i'm sure... the stock ground clearance is what, 16-18 cm??), and you're good to go.

 

Intersting. Thank you wakeech. You insight is always valuable .

 

The other problem, aside from getting an effective diffuser to generate that pressure differential (diffusers are more efficient than wings because there is less drag vs. downforce, but just like wings, they are speed dependent - they require significant air velocity to work), is that diffusers require clean airflow under the car to work at all. Ferrari are experts at using diffusers, first on the F355, then much better on the 360Modena and Enzo. For them to provide significant downforce, Ferrari uses a flat floor on the bottom of the chassis with air channels to direct air cleanly under the car to the rear diffuser. Without a specially designed floor bottom and air channels, the diffuser is mostly useless, and just a ricer pretention. You'll also note that race cars that use diffusers also use a flat floor and air channels.

Regards,
Gordon

 

I've noticed that about the Ferraris. underbodies look like those off of a Hot Wheels car (they have flat underbodies?). Though they're not analagous, the idea is there. I think without the flat floor and the air channels, the car's underside would pick up too much turbulence at high speed. Therefore, the car would be unstable at high speeds. If my guess is correct, even the most well-crafted diffuser wouldn't work because there's no air to be channeled out! A big deal of the air would still be stuck in the underbody. I haven't had a crash course in fluid mechanics yet. But, when I do, I promise I'll have a real technical explanation . Thank you Gordon for your insight .

 

i'm sure cost, cooling or the disapation of heat are reasons for not having a flat underbody on the 8 however it would sur look neat.

 

Why would 10 cm be ideal? Where do you arrive at that number? Just curious. And if you think the back end of the '8 looks too big now, just imagine what it would look like with the underbumper practically draggin on the ground...

 

Correct me if I'm wrong.

Isn't diffusers are used to reduce up-lift rather that creating down force? Yes I know in a way it increases down force, but funny hardly people mention what it really does - 'reduce up-lift'.

 

I'd imagine with the location of the radiator relative to the engine and the body, cooling might be the biggest problem preventing a flat underbody (depending on how low you're willing to go in terms of clearance.

To expand on gord's point about lift/drag relationships, it may not be so much that diffusers produce more downforce with less drag, but that reducing drag under the car would result in lower pressure below, therefore creating a net aerodynamic downforce (since lower pressures are achieved at higher airspeeds, at least according to Bernoulli).

With the cooling concerns on the 8, the best way to produce downforce might be to use some sort of a suction fan, ducted out the back (if the weight of such a system can be kept to manageable levels.

 

[QUOTE=bgreene]I'd imagine with the location of the radiator relative to the engine and the body, cooling might be the biggest problem preventing a flat underbody...QUOTE]

Do the engine heat vents behind the front wheel wells scavenge any significant amount of heat or pressure accumulating around the front end? Can they be made any more efficient? Is there any theory that could help D MENAC 7 when he gets around to making a side vent treatment? (see: [url]https://www.rx8club.com/showpost.php?p=490831&postcount=37 )

Thx

 

[QUOTE=MPG > HP]One thing is for certain, your bulletin board markup is below par. Perhaps you need a primer?

 

I don't remember Bernoulli's principle. Isn't it some sort of diff. eq? There's more than one that's for sure. I'm not in fluids so I wouldn't know.

 

Er, hi there. I'm sure you are absolutely correct.

Please be so kind as to explain. What is a bulletin board markup and why is it of concern? What constitutes "par" and how does one achieve and maintain such status? How can one determine another member's markup? Should members who are par or above be afforded special consideration by the sub-par? Should we clueless and sub-par members refrain from posting or otherwise curtail our participation? How does one obtain a primer?

I've reviewed again the RX-8 FAQ: Please Read Before Posting which does not mention posting or general bulletin board etiquette. I've also looked at the top of the Tech section, but could not find any guidance for us neophytes. Perhaps you can provide some directions to this and other newbie or occasional visitor as to what is proper behavior. What sorts of questions or posts are allowed or expected of sub-par members as opposed to par and above-par members? What are the consequences of noncompliance and who are obliged to enforce such rules?

Please advise, as I happen to be an ordinary rotary fan who appreciates the opportunity to learn and perhaps share a bit about these great machines and in no way wish to intrude upon the objectives or customs of this bulletin board. Perhaps it would be more clear if you could describe what particular in my previous post prompted you to point out that some sort of remediation is called for, on my part. I'm sure many others, on both sides of the "par" fence, would benefit, as well. Maybe your responses can be included in a FAQ as suggested by shellys_man_06.

Respectfully yours :o

cc:

PatrickB
FAQMeister https://www.rx8club.com/

shelleys_man_06 https://www.rx8club.com/showpost.php...42&postcount=5

 

Bernoulli's principle essentially says that the total of static and dynamic pressure is constant in a stream line. Since dynamic pressure varies with the square of flow speed, it means that less static pressure is exerted by faster moving ari when ambient conditions are held constant.

There's probably more to it than that, but it's been about 10 years since I took my fuid/aerodynamics courses, and a career in structural analysis has allowed that info to slip into the archives....

 

I believe that roberdot was making a comment about the mistakes in your "coding" in your post.

 

The MS kit was tested by Mazda in their wind tunnel to be functional and improve aero dynamic down force at high speed (Includes a rear diffuser)... At low speed the affects are minimal as down force generated is dependent on air speed above and below the car...

 

Well, I can't make a comment on that. I'll return to this thread after fluids.

 

Okay, I found a good definition of Bernoulli Effect, which is:

Here's Bernoulli's Law:



You probably won't be able to see it with the blue background.

I believe diffusers follow this principle, as what bgreene stated. Now, how to manipulate the variables is a mystery. I'll figure it out when I get home from PDE.

 

Shows up just fine on plasma Style background.

 

They look cool...