Here are tips to help get the most speed out of your stock or modified motors. Let us start of with getting the most speed out of the stock Mabuchi 540 RS motor.

Entry level kits usually come with the Mabuchi 540 RS or the Johnson motor. These are the "stock" and "closed end bell" type design. You cannot adjust the timing nor change the brushes to gain more speed.   
The best way to gain power from these motors is to properly break-in the motor brushes and the bronze bushings. One quick method to break-in the brushes is by using the "water dipping" method. Simply dip the motor in a jar of water, then attached a 3 volt battery supply to run the submerged motor. 
You can make a 3-volt battery pack by putting two 1.5-volt batteries in series. DO NOT use a battery charger or electronic speed control (ESC) as a power source for the submerged motor. You might damage your charger or ESC. 

This "water dipping" method will also break-in the bronze bushings, which will make the armature rotation smoother, giving you more rpms and power. Another method to break-in the bronze bushings is to attach the motor armature to a drill or Dremel tool. Just be sure not to over do it.

Let the motor dry out, and put 1 or 2 drops of lightweight oil on the bushings. Now your motor should be 20% faster than before.

Hot Tip: Serious racers in the stock motor class usually break-in several of these stock motors. They then test for power by using a dynamometer. The best motors are then used for racing. This is the reason why some stock motors seem to be faster than others.


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Modified Motors
It is easier to get more speed out of modified motors, primarily because you can change the brushes and advance the timing.



With modified motors, you can disassemble the entire motor. You can "true" the commutator to make it as round as possible and remove the carbon deposits left by arcing and the motor brush.

The washers are used to align the armature properly with the magnets and magnetic field inside the motor can. Washers prevent the armature from moving back and forth inside the can, which robs the motor of power. With the proper use of washer, you can gain power and speed advantage over your competition.

Brushes play an important role in providing maximum power. The composition (e.g. silver, copper content) and the brush "cut" vary. Also there are "hard" and "soft" brushes.
Hard brush - More power, more commutator wear.
Soft brush - Less power, less commutator wear.

Full brush - More power, more torque, less rpms.
Cut brush - Less power, less torque, more rpms.
 
Springs are used in tandem with the correct motor brush. In general:
Hard springs - More tension, more amp draw, more torque, less rpm. For off-road and 4wd touring cars.
Soft springs - Less tension, less amp draw, less torque, more rpm. For 1/12 on-road pan cars.

Warning: Using springs that are too soft will cause the brush to bounce around, causing excessive arcing, and premature commutator wear.
 
Adjusting the motor timing will have a significant impact on performance. In general:
More timing - More rpms, less torque.
Less timing - Less rpms, more torque

The chassis is the platform to which the Body, Radio Receiver, Receiver Antenna, Motor, Speed Control, and Battery are attached.  As you can see, there is a lot of fascinating technology here.  This is a picture of the Tamiya TL01 chassis.  The chassis is a monoque design, meaning it is a strong hollow shell without internal strength reinforcement members.  The stock chassis is made almost entirely of plastic with some steel parts such as screws and the drive shaft. The TL01 is a shaft driven Four Wheel Drive (4WD) with a ball bearing differential for each axle, four wheel independent suspension with Constant Velocity Drive (CVD) half-shafts, and shock absorbers.  This is an excellent starter chassis with lots of upgrade or "HOPUP" options from Tamiya and other many other manufacturers.  These improvements include exotic materials such as fiberglass, carbon fiber, aluminum, titanium, and Delrin.  Improvements are aimed at reducing weight and increasing spped or increasing durability.  However I found the basic plastic parts to be quite durable and lightweight.