Rear valance acting like a 'parachute'?
 

A friend of mine and I were playing around on the Interstate today, light traffic - early morning stuff, made a few runs up to 135 - 140 and he said that at high speeds my rear valance (black piece under the bumper) would occasionally flare back by several inches. He said it looked like a parachute....lol!
And now that I go REALLY look at things, that area of the car is not near as 'clean' aerodynamically as the rest of the car.

Has anyone else noticed this?

Yea it definitely looks like a parachute from the bottom of the car. You would probably make it more aerodynamic if you cut holes in the bumper.

Yeah, that's why they sell rear difusers...

I've noticed that on Rx7s and also on Miatas. That is why I got the new RE-A street diffuser from Shine. :)

I would love to see someone do a 100-140 mph test with both the stock rear valance and a "rear diffuser".

well some small differances can make huge impacts in the amount of lift and drag created. For instance, there was an article around here that showed that the R1 lip reduced the coefficient of lift by like 30+% while increasing the ce/d by barely anything. this test done in a wind tunnel.

UMMMMMMM. No.
I assume you are talking about the lip under the rear bumper. This piece is for asthetics and not aerodynamics.

Rynberg is on the right track. Porsche, as far as I know, was the first to put a functional "rear diffuser" on a production car and that was the 944 Turbo in late 1985 as a 1986 model car.
Porsche actually showed, through wind tunnel testing, that this "diffuser" lowered the vehicles drag coefficient which increased the 944 Turbos top speed to over 150. This may sound like a low number but a stock 944 Turbo came with only 217hp with a weight of just over 2900lbs.
I would love to see the drag coefficients of a stock bodied RX7 and one with an actual diffuser. Does anyone happen to have a wind tunnel we can borrow?

I'd like to as well. My guess: absolutely minimal difference.

There is a hell of a lot more drag created by other sections of the underbody than the stock rear valance. And the main purpose of an actual, functional diffuser is not to smooth the rear undersection of the car...

While the diffuser probably did lower the 944 Turbo's overall Cd, it didn't accomplish this all by itself, and it won't result in some sort of magical increase in top speed. A diffuser needs smooth underbody airflow in order to function, and most car's undercarriages are full of gaps, divots, crevices, and components hanging out in the airflow that prevent this from happening.

And the real purpose of a diffuser on a racecar is not to decrease Cd-- its purpose is to create downforce.

WRONG.
That is the absolute purpose of a rear diffuser. I have a little bit of experience in aerodynamics.
If you want to talk aerodynamics, a diffuser alone is used smooth out the airflow from under the car as it exits the rear to reduce the Cd. Increasing the downforce can only be accomplished by lowering the air pressure under the car which happens before the diffuser and ideally at, or before, the cars aerodynamic center of pressure.

Increasing the speed of the flow below the car results in lower pressure which results in downforce. Unless you're traveling supersonic, the effects at the back of the car absolutely affect the pressure under it's center. That's how a rear diffuser can improve downforce.

In any case, Mazda didn't reach a Cd of .28 on this car with stuff left flapping in the wind. If the rear valence is moving around, it's probably because the low pressure wake is pulling on it, not because of any 'parachute' effect. That low pressure wake is there regardless of whether the rear valence is bending. My guess is that the flexibility of the rear valence doesn't make a difference, and it's flexible because they chose to lighten it.

Dave

SO....now I would REALLY like to see a car with a rear wing vs a car like mine (no wing) to see if the 'parachute effect' is reduced or increased on a car with a wing, to see if what causes the effect is more from a low pressure wake or from turbulent air from underneath the car.....

If the wing is designed properly, it would not have the "parachute effect."

Wings create downforce at the cost of drag. This is why properly built race cars have smooth undercarriages combined with rear diffusers as it basically gives free downforce. Without the smooth undercarriage, a diffuser it basically for looks.