Iknow it has been brought up and would be something probably hard to tech for so I am NOT looking for an answer for competition. I want to regulate the voltage so I can use a Lipo to practice with....more laps using one battery, let me try changing set ups while charging the Lipo, save my 4200's for heats and races. Thanks Ralf

Just my opinion, but for the kind of currents our motors draw, you are not likely to find something small enough that it wouldn't change the balance of the car and affect your setups. That is even with the weight difference of the LiPo in your favor. A simpler appoach might be to find some old full-stack 27T armatures and run them in Mod cans with zero timing. If that doesn't get them as slow as you want, try cutting the brushes down.

I m running a brushless motor and just wanted to simulate a 4-cell output for practice runs. Thanks Ralf

I think what you need is one of these:

http://en.wikipedia.org/wiki/Buck_converter

I doubt one could be made to handle the power demand of an RC car motor and still be small enough and light enough to fit in the car without disturbing the balance and handling.