MINI Cooper S rear diffuser w/ underpanel
 

From the rear valance, my design extends back 26" to the rear subframe and is nearly 4ft wide. From the subframe, the underpanel begins a gentle rise at the same angle another 12" back. So we are talking about nearly 12 square ft of area acting on the air.

http://img72.imageshack.us/img72/4629/venturism8.jpg

A and B of diagram above, the best angle for drag reduction (CD) is from 2 and 4 degrees.

A (like mine), the best angle for reducing lift (CLR) is 6 degrees.
B, the best angle for reducing lift (CLR) is 11 degrees (at the expense of increased drag)

Made out of black coroplast, total weight with strakes, just over 4lbs.

http://lh6.ggpht.com/_tPeOcA9yGbA/TM...Y/IMG_4341.JPG
http://lh6.ggpht.com/_tPeOcA9yGbA/TM...M/IMG_4342.JPG

Visual indication of how far back the 6deg rake goes back to combine with the flat underpanel:

http://lh3.ggpht.com/_tPeOcA9yGbA/TM...mNBY/angle.jpg

Now for the shocking results!

I drive the same commute to work everyday and usually average about 26-27mpg. After logging 106 miles with the diffuser, I filled it back up with 3.538 gallons....that's now an avg of 29.9mpg!!!:eek:

Joe

Awesome! Beautiful diffuser. How did you close out the coroplast edges, and how did you mount the strakes?

Thanks!
Just used painters caulk to close the trailing edge...I may revisit to clean the edge a bit more. For the strakes I had a brainstorm...they are coroplast pieces folded in half so bottom edge is perfectly flat...which also made them much more rigid. They reside in aluminum C channel adhered with Gorilla Glue.

Here is a view of the top side:
http://lh5.ggpht.com/_tPeOcA9yGbA/TN...I/IMG_4390.JPG

I'm jealous! I'm certain my Fit would benefit from a rear diffuser, and it would be relatively easy to do. Please let us know if your improved results hold up over time.

Another 302.1 miles, this time 26.9mpg (some of which was stuck in traffic, so i'm happy).

Nice work! It looks great and I think you will continue to see good results. I did a similar thing from just behind the catalytic converter and I am amazed what it did for my mpg. I may have to add some strakes like yours. I used some thin aluminum roof flashing to fill in between the fender liner and the diffuser. I'm in snowy Colorado and I didn't want that filling up with slush and snow.

The area where you put your diffuser was just open chassis before? Presumably it meets your belly pan on the leading edge...?

It looks like a really nice job, I hope I can make one just as nice for my Beetle.

Really nice work! Very clean, it looks life factory car!

After more logging, on average, my freeway mpg has increased 1mpg to 27mpg. I'm very pleased with the results!