Underbody Panel Questions
 

I know it's been discussed here before but I do not believe anyone has attempted it yet. I don't plan to go all out but I do want to make one connecting the bottom of the rear well to the rear fascia. It looks like the most turbulent area on the car (91 Firebird) although I could be wrong.

So if you were to make this what material would you choose? I'm thinking something like 1/16-1/8" aluminum and using rivets to attach it. I've also been thinking that rolling some beads or louvers into it would help keep the strength up, and in the case of the louvers, allow air to escape from the top side of it. Aerodynamics is definitely not my field so I would welcome any advice or comments. Thanks.

Its called a belly pan. It will work fine, but an engineered diffuser would be much better. But thats gonna be a complex engineering project, costing lots of money and time.

Don't use rivets, use screws or dzus fasteners, you will eventually need to take it off.

I'd probably use 10-16 gauge aluminum.

How fast are you planning on going?

Yea, I've heard them called belly pans but I just don't like calling them that, thus I will call them underbody panels.

I don't see why I would ever need to take them off. Once they are on why would I need to take them off.

406TPI, this will not be any kind of super high speed car. It will just be a stock LS1 auto and I would like to improve some on the car. I figure it would not be difficult to do and may help with gas mileage, even if it is only 0.1 more miles/gallon. A diffuser would be nice but I do not have access to a wind tunnel for testing. :p

You will want to be able to remove the panel so you can work on the car and also clean out all the dirt and dust that will collect on the topside of the panel.

I would make the panel/s out of composite if it were me.

Yep. Wouldn't your exhaust be above the panel too? Not to mention access to the panhard rod, depending on how close you have the edge of the panel to the rear axle.
Tha'ts why you may want to remove the panel.

Mark.

Don't worry about the exhaust. It won't even make it to the rear of the car. This panel will start after the rear axle around the storage well.

I'm not sure if you're trying to minimize drag, create rear downforce, or just doing it because you can (which is worthy in itself).

You'd get a much greater improvement by putting a skirt below the front valence and under the rockers. If air doesn't get in there, it creates a vacuum, adding downforce. Without addressing the air management at the front, the rear pan won't be able to do much, as the air is already very turbulent at that point.

Air at the very front of the car is fairly laminar. As it passes by various protrusions (radiator airdam, k-member, oilpan, transmission), it creates a boundary layer of air that's pulled along with the car. That boundary layer grows thicker the further back you go, until it eventually touches the ground. At this point, much of the air is being pulled forward along with the car. The point of a diffuser is to gradually speed (or slow, depending on how you look at it) that air back up to the free-stream speed (car's relative speed, since the air is still, and car's moving forward) in a smooth method. Bernoulli's principal generally states that faster air has a lower pressure (vacuum), which is your goal. Unfortunately, a diffuser works much better with lots of smooth airflow, and if your boundary layer is at the ground already, there won't be much air to manipulate. If you have a nose and side skirts, that prevents air from getting under the car in the first place, lowering pressure under the car, and creating much more downforce.

The goal was to create less drag and because it didn't seem too difficult to do. The reason I do not want to modify the frontal area is that I do not want to lose any ground clearance in the process. The back area just seemed like a pocket where air could cause some drag. If preventing air from entering that section would have no effect then by all means let me know.