Ecomodding by Cadillac
 

I was inspired by the taillights on 90's Caddys for a kind of a partial boattail. I noticed the rear end on my Tempo is slightly angled inward, and I decided to extend that angle out to the edge of the bumper:

http://i3.photobucket.com/albums/y93...y/IMG_0653.jpg

http://i3.photobucket.com/albums/y93...y/IMG_0654.jpg

http://i3.photobucket.com/albums/y93...y/IMG_0655.jpg

http://i3.photobucket.com/albums/y93...y/IMG_0656.jpg

http://i3.photobucket.com/albums/y93...y/IMG_0657.jpg



As you can see, I cut out a couple sections and covered them with clear packing tape so the brake lights can be seen from the side. I don't know if you legally must have at least some reflective red lighting on the side or not, but just in case, I didn't want it totally blocked.

I also added mini-spoiler like extensions off of the top and bottom edges to better detach airflow, but I'm not sure how much it will help.
I'll be testing the effects of this tomorrow.

...waiting to hear what your before & after "numbers" are.

Pictures of the bumper stickers?

IIRC, your rear glass is at a somewhat steep angle. You might benefit more from have a full blown boattail than the sonic the hedgehog things on the rear of the roof.

Now that I think about it i may have just stated the obvious.

Don't hold yer breath. This isn't gonna show up in any real numbers. :rolleyes:

The first coastdown test hasn't shown any measureable effects. Then again, it's starting at 45 MPH, so reducing drag by a couple points isn't going to show up much. Starting at 45, I'm still hitting 40 as I come to the Speed Limit 35 sign. Then again, it was slightly cooler, since it was about 6:30 PM, and today was colder than it has been recently.

Though I am starting to think about a way to add a kammback and still keep use of the trunk, since I do use it often. Maybe when I get some money to look at materials that aren't laying around, I'll start working on a design.

I think what you're looking for is Groar's Uglyback?

I was thinking more of a complete kammback with a hingd mechanism to keep the trunk functioning. He never did post the results of the "uggyback", though. I'd also want to get some clear material so I can still see out the back window.

Even though the coastdown test was unconclusive, I finally was able to reach 150 miles on a half tank. Sometime next week I'll be able to see what my mileage numbers look like now.

You could do the full Bassjoos deal and end up with a bigger trunk, too. If I had the money to spare I would totally do a boattail on my minvan and build it to be an auxilary trunk.

There's the biggie right there. I have no money to spare, so I'm working with what I already have on hand for now.

I was going to try another coastdown test, but traffic got in the way. Monday is the next chance I'll get. The way I see it, going out just to test mods just uses up the gas you'll save. Better to consolidate trips and test while you're already out.

Even a short Kammy is better than none, as long as you get the angles right. In fact, the first foot will help more than the second. Diminishing returns.

Maybe make a permanent shorty for short trips, and a removable extension for long, high speed trips? The short Kamm need not be transparent - in my case the only thing it covers are the headlights of trunks behind me at a traffic light, other than that everything's OK. In fact, it's better, since it keeps the sun off the rear window (doesn't "light up" the dust and keeps the temperature down). The extension could be clear, and since it would hook up to the existing Kammy, it would be shorter, lighter and easier to work with.

Filled up the gas tank, and the average is 26.3 MPG. It seems to be staying pretty consistent now, which is nice. I'm going on a 330 mile round-trip drive today, so that'll use up a tank.

I also thought that if the kamm was too short (such as simply detaching airflow off of the roofline), then it would just make it seem like the rear end was larger and produce a larger wake, since the airflow does reattach to the trunk right now. I guess I wasn't thinking about the flow around and above the vehicle as well.

A short roof extension wouldn't reduce your wake as much as reduce Cd (Cx) and lift (Cz). As ShadeTreeMech mentioned, your rear window is steeper than it should be. As the rear angle becomes steeper (starting from 0°), Cd gets smaller, with a minimum between 12° and 14°, and then grows until about 28°-30°, when airflow detaches. Lift increases with steepness, also until about 30°. For nice charts, see this post.

So what you want to start with is a short roof extension that will direct the airflow in the right direction, instead of allowing it "stick" to the rear window, increasing drag and lift. With a longer Kammback you start to get the added benefit of wake area reduction.

A second option is to raise and extend your trunk line. I believe that Frank Lee was interested in doing this with his Tempo, and Moonmonkey played with it on his Echo. Here are two threads on the idea:
Reattaching wake and drag penalty
Decklid mock-up photos

And that's perfect, because I'm looking at reducing both drag and lift.

Come to think of it, I noticed a small spoiler on the roof of a BMW once.

http://www.seriouswheels.com/pics-20...r-1024x768.jpg

Also, would it be a good idea to straighten out the sides as well? The C-pillars curve around to the rear glass, and I'm thinking a clean, sharp edge to detach flow would be better.