All of this information varies from pack to pack due to size and other qualifications. Mass location or where to add more weight is a very controversial subject. Since Gravity is the main power source, the more weight one can add onto the car the better because of forces pulling down on the car. One of the most used ideas is to put the weight in the rear and slightly to the one side so that only 3 out of the 4 wheels are touching the track. Other people say to completely center all the weight.
New ideas develop every race.
A research study from the University of Michigan engineering school shows that pouring molten lead into the wheels themselves has the best results. This school had used several different ideas when it did it's research. Of course, the average kid, or even the average father does not know how to use molten lead. One more idea is to put .38 caliber bullets in the car. When using normal weights from the BSA, people usually put them on the top of the car, where they are more easily accessed, or in the bottom of the car.