Originally Posted by miamijim

Rev. you beat me to it. I know alot of autocross racers use foam to fill the frame rails. The main purpose is to stiffen the HOLLOW frame. It does work but theres one major drawback. It retains moisture. Honda used this technique on some cars in the past. The frames were predisposed to rusting. Once it started there was no way to stop it.

http://www.miata.net/products/chassis/bellco.html