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Discussion Starter #1
Well I really really wanted one so... I decided to build one.

At the moment I am using cardboard as a mock up. My rear diffuser will attach to the bolts that face downward near the diff and along the bumper. The angle of attack is 12.5 deg. The only thing is I ran out of light and couldn't continue to hack away at the material near the wheels nor trim the excess from around the bumper so, I will update you all when I finish those parts. As well as when I finish doing the math to find out how much airflow with tray could be seeing.. Of course its all theoretical as I have no wind tunnel. Also if someone with a yard stick could check the clearance between those two bolts and your bumper (with the stock exhaust) I would like to know if this design would accommodate the stock rear muffler system with a modified cutout for the tip. If so I might make a few of these.

My final material will probably be lexan or something similar becuase it will be flat. Would anyone be interested in one as well?

Without Diffuser attached
http://www.miataspeed.com/images/miatadiffuser/122-2214_IMG
http://www.miataspeed.com/images/miatadiffuser/122-2216_IMG
http://www.miataspeed.com/images/miatadiffuser/122-2217_IMG
http://www.miataspeed.com/images/miatadiffuser/122-2218_IMG




With Diffuser attached


http://www.miataspeed.com/images/miatadiffuser/122-2219_IMG
http://www.miataspeed.com/images/miatadiffuser/122-2220_IMG
http://www.miataspeed.com/images/miatadiffuser/122-2221_IMG
http://www.miataspeed.com/images/miatadiffuser/122-2222_IMG
http://www.miataspeed.com/images/miatadiffuser/122-2223_IMG
http://www.miataspeed.com/images/miatadiffuser/122-2224_IMG
http://www.miataspeed.com/images/miatadiffuser/122-2226_IMG
 

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Interesting project, keep us updated.

Should look something like this:


One thing to consider is making sure air won't get trapped in the bumper, which will cause drag.
 

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Note: Front diffusers are called dog-catchers. Get it right, people!
 

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Are you guys having your exhaust cutt off there so that way it just exits from the bottom? This seems to be a good idea... but for later on. I've still got some work to do
 

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Im working on a project like this, although its taking much longer then expected.
 

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Discussion Starter #8
The Math

determining how much HP is needed to push a stock miata along at a given speed. vs my under tray.

F=(1/2)(Cd)(A)(p)v2

F= Force
Cd=Coefficient of Drag; ours is .37
A= Refers to the frontal area of a miata; ours is 16.5sqft
p= The density of air; which is .08 roughly
v= MPH

This equation gives us force in lb-mass /ft we need in seconds so we use 22fts/15mph(v(mph)2<< this gives us LB mass in FT per sec.
Lastly to account for gravity we divide by the acceleration of the earth gravity 32.1

So for a stock miata traveling at 15 MPH this gives us......

F=(1/2)(.37)(18.55)( .08 )= .2745v2

for lb per sec we....

.2745(1.46*15mph)2= 131.65

And the acceleration of earths gravity

117.12/32.1= 4.10

This means while traveling 15MPH a miata is pushing against 4.10 Lbs of force which uses horsepower to overcome. (More on that later)

I


v MPH 15 30 60 90 110 130 150
Force 4.1 16.4 65.5 147.3 220.0 307.3 409.1
RWHP 0.2 1.3 10.4 35.2 64.2 106.0 162.9
% of total RWHP 0.1% 0.8% 6.7% 22.7% 41.4% 68.4% "104%"
[email protected] 155 RWHP

Which means in a stock miata @ top speed 68.4% of your HP is just trying to move your car forward!

To test my Cd I will be using this formula and If you dont like it or think its right suggest another equation! Don't just criticize. Although In all fairness I dont have a wind tunnel so these are "approx" #'s)

Drop down speed test from 105Kph to 100Kph
(weight in Kg of my Car)[( 1.389mps X (1/ (my elapsed time is seconds))](.224 (to convert from N>Lbs))
(1/2)(18.55)(.08)(1.46*60MPH)2(1/32.1)

all of which =Cd

And I have not started working with the engineer yet and when I do she is merely to check the approx. figures. and give general suggestions regards to flow. As there is no CFD or wind tunnel.

p.s. the density of air I am using is in (lbs mass / ft3)

Final Protoype

http://www.miataspeed.com/images/miatadiffuser/122-227_IMG
http://www.miataspeed.com/images/miatadiffuser/122-228_IMG
http://www.miataspeed.com/images/miatadiffuser/122-229_IMG
 

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I'm not going to even pretend to know what all that means... I just thing the DIY diffuser is a kick ass concept, while not really looking "homemade".

-Jeremy
 
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