This is a more descriptive instructible than the last one about making a faster radio control. This instructible is only for RCs that are 2 wheel drive or only have 2 motors. (If you are not entertained by it as a slow rc and don't mind the possibility that if you mess up too much then your rc could be broken then continue. If you reduce your RC's value I am not responsible.) First measure the gear box and get an approximate length of the area you have for the motor. If you can not see if you DO have about an inch and a half from the motor edge of the tire then go to this instructible, for instructions on how to mount a bigger motor; if you want a harder job that will make a faster RC if you use the right motor.(http://www.instructables.com/id/how-to-supercharge-a-cheap-radio-control/)
Step 1: MORE POWER
Then, find out the voltage the car uses also find the Mah rating if it is rechargeable. If the voltage is 6 volts and under then double the voltage in battery rating is required. If it is 6 volts and over then give it 5-6 more volts of battery rating. If the RC is alkaline battery powered you will need to make it rechargeable battery powered. If your batteries are not rated at at least 1000 Mah then double the Mah is needed of the batteries you will use in the car.
Step 2: Batteries in series
To get the voltage you figured you need you must put the batteries in series (do not do this with lithium batteries). Like the pucture conect the negative to the positive. Then connect an additional wire to the negative. And one to the positive if there is not already one there. The wires labeled b are the ones already attached to the circuit board.
Step 3: Soldering
Here is the relay I use. ( http://www.instructables.com/id/relay-that-clears-pulses/) Cut the wires connected to the motor and connect them to the relay's motor part. Connect the wires labeled r on the first picture, to the N and P on the relay second picture.(n to negative wire p to positive wire) The black lines on the second picture are the wires connecting parts of the same relay as in the instructible above. Then connect the wires labeled M+ and M- to the motor's negative and positive wires you cut.