[ByronRACE]

If the car overheats sitting still at idle, the dam will make a minimal improvement. Yes, it will tend to block hot air from the engine bay from being sucked back into the face of the radiator, but over-all, most of the air will be drawn from the front of the radiator where the cool air is anyway. If it overheats sitting still, I would look at the radiator quality (total surface area of internal cooling fins...fin density.). Modern aluminum radiators have 20+ fins per running inch. Older and/or cheaper radiators are far less dense.

When moving down the road, the dam will have a profound impact on how much air goes through the radiator. In fact, a chin spoiler hanging directly under the radiator (assuming it's vertical mount) can drop water temps 20deg F when the car is moving. The spoiler is enough to make the path of least resistance "through the radiator" instead of around it. Look under most Ford passenger cars and you'll find one of these. If a chin spoiler does that much, just imagine what a complete dam can do.

Can you post pictures of your fan/radiator arrangement? That might reveal something. If you don't have a puller fan and a complete shroud covering all the surface area of the radiator with at least a 1" gap between radiator and shroud, your radiator isn't nearly as efficient as it could be. In addition, there's a whole bunch of new puller fans available that you may not be aware of...some OEM, some aftermarket, and some as high at 5000CFM in a single unit. With a picture, I might be able to make a suggestion.

Byron