Air dam placement
 

I am interested in putting an airdam on the bottom of my Mercury Topaz
Obviously for better MPG, but it will have the side effect of looking good :)
I am planning to put a 3-4 inch air dam at the bottom of the bumper lawn edging then paint it grey.
My question is should the air dam be straight up and down, or tilted back (tilting towards the tires at the bottom). or tilted forward pointing away from the tires at the bottom.
It seems tilted back towards the tires would push the air under the car, forward tilt would push the air up over. i here both trains of though the air over the top is the only way to go and that some air needs to go underneath. Well what is everyone's ideas

first of all straigth is likely the easiets thing to do, as you'll have to conform to the curve of the existing bumper. if you angle it forward you'll trap a small buble of air above it so you'll move the overall stagnation point down a little . but you could compensate this by makeing the straight dam longer.

i wouldn't angle it back, exept perhaps in the middle to allow a little more air to go underneath the car in the midle... (some cars use this strategy, because allowing some air underneath the car gives less net drag than sending it all around it)

what i've found as an average on most modern aerodynamic production cars is that the airdam is about 20 cm above the ground... in front of the wheels and wheelwell this sometimes drops to 15-16 cm, these are not strict figures but they' might give you some reference

I would also go with straight up and down, since otherwise you will be creating a pocket of air or end up sending too much car under the air anyway.

I had a similar question related to air dam design, so I'll add it to this thread. What is the goal on the height of the air dam? Should it just extend low enough to be level with the suspension components? Or should it be a set height above the ground?

With the original poster's Topaz, I assume it's close either way. I want to add one to my 2wd 2003 Silverado pickup, and I'm unsure what a good starting point would be for the air dam height.

The purpose of an air dam is to direct the air flow toward the smoother outer body surfaces instead of the aerodynamic nightmare underneath. If you plan to put the dam just below the bumper then you are limited by driveway clearances. If you put it closer to the wheels then you can have it much lower.
Bottom line: lower is better!

Aside from trying to avoid creating a snow plow, I don't think there is a simple answer to the height question. From an aero perspective, I think that "lower" is better, but only up to a point. Drag forces are the product of your drag coefficient and the area. Your gains from a dam will be realized by reducing the Cd. The penalty can be from an increased A (and hitting stuff). Getting below the lowest point on your suspension is a given, but others, particularly AndrewJ, here have proven benefits by extending even lower.

starting point
 

Take a protractor and draw and cutout a 10-degree wedge of cardboard a couple feet long.Then get two friends to sit in the front seats of the car to "weight" it while you shove the template up to the underside of either of your front wheels(tires).---------- If you project the elevation of the airdam down to this template,you'll end up with the lowest possible height,which will not crash into the first driveway ramp you go up or down.

The idea that "lower is better" is true only to a point.
If you go so low that you are actually increasing the frontal area, you have gone too far since the drag force equation multiplies Frontal Area and Coefficient of Drag.

why not CURVE AND TAPER the air dam?
 

maybe have it higher in the middle, which would be at front of car, and it could curve back and get lower as it gets close to the wheels. that way you still get some air under the car, driveway clearance, and low air dam in front of wheels(is that kind of like a wheel spat?).