oh but you are mistaken WVRx7.

The rear diffusers from RE-A (and Rotary Extream replicas) are funtional aerodynamic aids. greatly redusing drag and turbulence at the back of the car.

That beening said, i dought most cars are driven in a manner where the effects would be noticed

 

Yea, some people do. And them some (like me) make sure it's worth every last penny spent on it

 

I am sure you could be an expert, or you could be just an enthusiast (like myself), but I am not wrong.
I have researched this subject.

A diffuser must be loaded from a point preceeding (or very close to) the center of pressure.

There is not a flat bottom on the FD. These "diffusers" are merely hung on the rear of the car and are not loaded in any fashion.

The racecar aerodynamic theroy book that I am most familiar with mandates that for any function at all, a diffuser must be:

Loaded from a point preferably before the center of pressure.

The tunnels should encompass at least 25% of the length of the car and the ramp of the diffuser should be a minimum of 17 degrees and be of sufficient length to properly evacuate the rear of the car.

Simply hanging a piece of plastic on the back of a car does not a diffuser make.

On the FD, in order for a diffuser to function, adequate loading ramps would need to start at approximately the area under the seats. The loading would continue a a flat bottom, sealed, to the rear of the car.

Those fashion pieces are merely wings mounted on the bottom of the car. They add drag and nothing else.

Of course, if you have any wind tunnel data or proof of any kind........ Statements of a vendor don't mean anything.

 

Ah, finally some sanity to this thread....

 

Thank you very much. Just made the decision much easier with this helpful knowledge.

Now who's got something to say???

 

Yeah, I've got something to say. First of all, I don't know why WVrx7 is so pissed off about diffusers. There are plenty of things on this forum that can also be deemed "useless" such as stereos, rear wings (including the stock R1/R2 wing) , big brake kits, etc. and there are threads galore devoted to these topics. It's about personal preference. If you don't like something, fine. But don't **** on someone elses fun. It's rude.

The RE Amemiya designs aren't simply chunks of plastic. Just because the design of the diffuser doesn't match the ideal parameters listed in the one book you've read doesn't mean that it isn't functional. It covers the open areas in the rear of the car (from in front of the gas tank to the rear bumper) which helps reduce turbulance. Ever look at the rear plastic lip under the bumper? It's scooped fwd like a parachute! And while most people will not be driving the speeds to need a diffuser, in a car that is capable of over 150 mph in stock form (yes with a precat and stock ecu) there's nothing wrong with an attempt to make it more stable.

 

WVrx7: what is your book that you've gotten this information from called, and how old is it. You are right to a certain extent. For a rear diffuser to create competent downforce most of those rules would have to be in effect, although the 25% the length of the car part should be thrown out the window. How the ramps would effect the air would be entirely dependent on vehicle speeds, vehicle size, how flat the bottom was to begin with, a whole lot of things that "25%" does not take into account. The most obvious possitive effect the diffusers could have would be from flattening and smoothing the airflow of the car. Air flows like water underneath a car, and the smoother the surface it's flowing over the less areas of high pressure you'll have and the less drag. So, by smoothing out any part of the bottom of the car, no matter how bad the rest of it is, your still elimintating drag, and elliviating lift.

Conclusively any aditional downforce that the vortex generators may provide would have to be proven in a windtunnel. But, any "diffuser" that flatens out the bottom of the car more and smothely delievers the air out the back will help alliviate high pressure areas and consequently reduce drag and lift.

 

I found that info VERY interesting...I had no idea. I always understood that - much like roadsterdoc said - the diffuser helps reduce turbulence at the rear of the car, plus I thought it was a no-brainer that the lower the rear bumper/end of the car is, the more aerodynamically stable it is (more or less), and the stock FD bumper ends pretty high up...

I know C-west and Mazdaspeed make front undertrays, and you're saying the diffuser would work if it was loaded right... does anyone make "loading ramps" as you called them to actually put the diffuser to it's maximal function?

I can't remember what car it was, but I believe it was either the new Ferrari or the Chrystler ME 4-12 that had the entire underbody of the car made of "runners" that lead to a rear diffuser.

I'm not saying you're wrong WVRx7, but I have lots of faith in our Japanese counterparts, and I've seen pics & move clips of the white Re Super G RX7 they have...w/ the huge GT wing and the RE Diffuer. Either they're running the diffuser alone because it IS functional as such, or they have runners/loading ramps for it as well, meaning we can get these from somewhere... The only question is where??

 

i was not aware that they were in fact so short in lengh. wouldnt one that is small like the old RE-A one just trap air between it and the car?

 

I am not trying to stir stuff up and absolutely admire your car and the way you use it. But I think no one on this thread is under the false pretense that these are major aero devices so they can drive upside down in a tunnel.

I race (or will continue to once it's finished) my car in autox too. Its exterior is purely for cosmetics...I have no other motive than decoration for what I've done.

What cranks me up is too many zealots popping into threads that just blab out...useless, ugly, get mods first. It's not up to the observer/reader to decide that unless prompted to. So for all you hardcores, save your Milliken, Fred Puhn, Carroll Smith, et al...for the techie threads. This one is just for answering the original question: "You guys know how FEED and some other companies have rear diffusers for the ReX"

 

I agree with WVRx7. Not so directly, but overall yes I agree with him.

There's a reason why all the undertrays of big Marques such as Ferrari and Porsche are completely covered.
To make a rear diffuser absolutely funtional and efficient you have to seal the undertray completely. And then add venturi tunnels as you get towards the rear of the car.

This isn't new stuff... It's all proven technology.

Slapping on a piece of plastic at the bottom of the rear bumper isn't going to do much. It might help a bit in letting the air pass through a bit cleaner. But it isn't going to make any real difference.
It's more for looks than anything else.

I've been looking into getting a replacement for my rear lip (damn those N1's get hot!!!). The muffler tips melted a bit of it. So I've been looking to replace it with the Mazdaspeed rear "diffuser". But at the price they want for it, I'm willing to live with a bit of the plastic melted...

Later,

 

It's not all about performance, I have the same issue with my exhaust, the black trim peice is starting to sag, and as with any beautifull frame, once sagging sets in, replace the parts that support
And if you're replacing, why not do it with a part that's better than OEM, that's the whole point of aftermarket. So if it Improves the flow between the gas tank and the bumper ---> BINGO ! It's improving OEM....