Electric motors EXTREMELY simplified: An electric motor has basically two parts: a 
'can' and an 'armature'. In reality it has a whole bunch of other parts, but those 
are important mostly to hold everything together. The can is what you see on the 
outside; it is made of two (sometimes more) magnets. The armature is on the inside; 
you can usually see it if you look through the went holes in the can or if you take 
the motor apart. It is made of some magnetic substance like iron and has wire 
wrapped around it; this makes it an 'induction coil'. When electricity is run 
through the wire, it creates a magnetic field. This electro-magnetic field is 
opposite to the magnetic field provided by the permanent magnets; the two repel 
each other, and the armature rotates.
As a user, you are generally concerned about how fast the armature spins when a 
given voltage is applied to it, and how well it can retain those revolutions when a 
load is applied to it. The first is normally given by the manufacturer in 'RPM' 
(rotations per minute), and the second is referred to as 'torque'. Two motors (even 
the same make and model) can have wildly different values. Therefore motor specs 
have been generalized in terms of 'turns' and 'winds'. Turns is the number of times 
a wire is wrapped around the armature; winds is the number of strands the wire has. 
For example, you will see motors specified as "17T2"; this means the wire is 
wrapped seventeen times around the armature, and the wire is made of two strands. 
Assuming that everything else on the car is kept the same lower number of turns 
translates to higher RPM. Winds deal with torque. In simplest terms, a single wind 
will give you generally more punch and a higher wind will give a progressively 
smoother pickup and a bit more top speed. Both winds and turns also affect run time 
- higher number of turns and single wind will give you more run time.
Note that RPM does not always translate directly to speed! There are other factors, 
plus a better driver will always beat out a faster motor with an unskilled driver.

In R/C car racing motors come in two categories: 'stock' and 'modified'.
If you enter into a stock race, you will need to do it with a stock motor. The idea 
of stock races is that all the motors will perform the same, so the winner of the 
race is determined by: 1) the skill of the driver, and 2) the setup of his car. 
This is not necessarily always true, but that is the idea anyway.
Modified motors come in wide range of winds and turns, and are usually made so they 
can be rebuilt and therefore are modifiable. They are generally (but not always) 
higher performance motors as compared to the stock.

Significantly more detailed information can be found at Tom's R/C Page 
<http://www.csc.uvic.ca/~tyounger/hafh/rc/>.

9.1. How to speed up electric motors?

Keep in mind that several factors contribute to the speed of your vehicle, the 
motor is only one of them. Following is a post to rmrl, slightly edited for grammar 
and legibility:

From: popeye <//groups.google.com/groups?selm=news:9ep918$41q$1@news8.svr.pol.co.uk" target="new">news:9ep918$41q$1@news8.svr.pol.co.uk>
Tuning modified motors is not that easy, first of all clean the thing.
1. Mark the end bell and case so as to keep timing position.
2. Remove brushes and springs.
3. Remove the top screws twist end bell and pull up; be sure to not loose the small 
shim washers.
4. Pull the armature out of the motor again, look for the small shim washers at the 
bottom.
5. Clean the inside of the case and end bell with motor cleaner.
6. Wash the armature with the cleaner, do not touch the armature com (copper bit) 
with your fingers.
7. Use bearing oil to oil both case and endbell bearings. Reassemble motor.
8. Don't forget to put the shims back in; if you have them right then there should 
be a very small amount of play when reassembled.
9. Brushes should be replaced when about 1/3 worn.
10. Line up the timing marks, and hay presto one clean efficient motor.
Sounds difficult to some but it is not. If you want to get more speed etc, then get 
com skimmed, put softer brushes, better springs, renew bearings when armature feels 
gritty (to test bearings before reassembling place armature in housings from the 
outside and spin). Also as motor gets old timing may need adjusting. Never throw 
away a motor, it can be reused remagnetised even 15T4 turned in to 12T2 or any 
other motor is usually cheaper than the cost of a new motor.

For additional information on motor maintenance see: 
<http://www.wildhobbies.com/news/default.asp?cmd=view&articleid=412>, and 
<http://rcvehicles.about.com/library/rc101/blackbook.htm>.

9.2. Brushless motors

This is still quite new, or at least not very widely accepted concept as of yet. 
The idea is something like an electric motor with no brushes and no speed 
controller. Read all about it here:
<http://www.gtdodd.demon.co.uk/brushless/intro.htm>
<as_umsgid=a8mod3$28p$1@reader06.wxs.nl">http://groups.google.com/groups?btnG=Google+Search&as_umsgid=a8mod3$28p$1@reader06.wxs.nl>
<as_umsgid=20010225001710.04970.00002026@ng-cd1.aol.com">http://groups.google.com/groups?btnG=Google+Search&as_umsgid=20010225001710.04970.00002026@ng-cd1.aol.com>
<as_umsgid=19980129044301.XAA29614@ladder02.news.aol.com">http://groups.google.com/groups?btnG=Google+Search&as_umsgid=19980129044301.XAA29614@ladder02.news.aol.com>
<as_umsgid=uOIwO9qlBHA.227@net037s.hetnet.nl">http://groups.google.com/groups?btnG=Google+Search&as_umsgid=uOIwO9qlBHA.227@net037s.hetnet.nl>

9.3. Speed controllers

There are two types of speed controllers:
1) Mechanical Speed Controller, MSC - This type of a controller is basically a big 
variable resistor, controlled by a servo. It can normally achieve three different 
speeds (three-steps), and does not have a reverse option. Advantages: there is only 
one that I am aware of: cost!
2) Electronic Speed Controller, ESC - This type of a controller is a mysterious 
black box (to the end user). This means that it has a lot of electronic components, 
all of which are not visible to the user. It supplies the motor with pulses of the 
appropriate voltage. Since it is electronic, and the insides are digital, it has 
many more steps / speeds (up to 255), which makes the running the car seem much 
more life-like. These also do come with a reverse option. Advantages: better 
control of the car; runs much cooler; can handle higher battery cell packs; can 
handle hotter motors without melting. Most drivers today run with electronic 
controllers. It is actually quite rare to see a car with a mechanical one today.
Most manufacturers will have an explanation of all the terms that go along with the 
ESC; unfortunately each manufacturer uses a different term to name the same thing, 
and further, each manufacturer will stress a different thing on their product (it's 
called 'marketing'). The things that you should concentrate on, to start of with: 
reverse (this is a yes / no type of thing - for off-road go with a 'yes', for on-
road you could go either way); number of cells (this is generally a range - you 
will want one that can definitely handle 6 cells, which is almost all ESCs on the 
market today); motor limit (the number of turns on the motor that an ESC can handle 
- remember: the lower the turns = the hotter the motor).

Way detailed info on ESCs: <http://www.math.niu.edu/~behr/RC/speed-ctl.html>. Wanna 
build your own? See Mike Norton's Hobbies at <http://home.HiWAAY.net/~mjn/>, or 
Stefan's Electric R/C Web Site <http://www.capable.on.ca/rcstuff/escbec.htm> 
<http://www.capable.on.ca/rcstuff/esc.htm>.

9.4. Gear ratios

There are two gears on your car. A 'pinion' is generally the smaller gear attached 
to the shaft of the motor. A 'spur' is the bigger one connected to some drive 
mechanism of the wheels. A 'gear ratio' is the number of teeth on the spur divided 
by the number of teeth on the pinion. This ratio represents the number of rotations 
the pinion makes per every rotation of the spur. Note that one rotation of the 
pinion is equal to one rotation of the motor, since the two are attached by a 
shaft. Depending on the drive mechanism in your car, there might be additional 
geared wheels (like differentials) between the spur and the tires - this is almost 
a certainty for 4WD cars. The manufacturer will generally supply you with the 
'final drive ratio'. This is the number of rotations the pinion makes per rotation 
of the wheels. If you have a two wheel drive car and the spur gear is connected 
directly to the wheels, then your drive ratio is going to be the same as the final 
drive ratio.
Taking the circumference of your wheels, divided by the final drive ratio, 
multiplied by the RPM of your motor, would give you the theoretical top speed of 
your car - the units will be same as your circumference per minute. This is only 
theoretical since it does not take into account friction, and other factors such as 
how well your tires stick to the pavement, and the skill of the driver.

So what gear ratio should you use? Chris Dugan 
<//groups.google.com/groups?selm=news:jx_j8.48632$y76.5834820@news6-win.server.ntlworld.com" target="new">news:jx_j8.48632$y76.5834820@news6-win.server.ntlworld.com> offered the following 
advice: With pinions you use a larger one for more top end speed (the reverse for 
the spur), but only change either the spur or the pinion not both. Most people 
change the pinion and leave the spur at the factory supplied size, if you change to 
a smaller spur and start to use small pinions you might find trouble meshing the 
gears (the motor won't reach the spur). Same thing goes for a larger spur than 
standard.

There are different sized teeth on the gears. This is called the 'pitch'. Lee Cao 
<//groups.google.com/groups?selm=news:a24aqn$gjv$1@lore.csc.com" target="new">news:a24aqn$gjv$1@lore.csc.com> offered the following definition (edited for 
spelling): Pitch is the number of teeth a gear has per inch of circumference. So a 
32 pitch gear would have the tooth sized and spaced in a manner such that if the 
circumference of the gear is exactly 1 inch, the gear would have exactly 32 teeth. 
Similarly, a 16 teeth gear would therefore have a .5 inch circumference.

Also have a read through S. Varah's info on gearing: 
<http://www.schumacher.clara.net/gearing.htm>.