stumbled upon that one - answers a lot of the questions on brushed motor categories and nomenclature
http://www.ezonemag.com/pages/faq/a414.shtml
What do the motor names (Speed 400/500/600 or 05/15/25 mean?
Q. What do the motor names (Speed 400/500/600 or 05/15/25 mean?
A. Steve Lewin
The numbering systems used to describe brushed motors can be a little confusing. There are 2 quite different schemes, one based loosely on the designation used by Mabuchi the company who make many of the small electric motors used in the world and a second devised by Bob Boucher of AstroFlight which is mainly used by American companies for the specialist rare earth motors.
The first numbering scheme is very simply based on the physical size of the motors. The most basic sizes are 360/380 and 540/550. These motors are 36 or 38 or 54 or 55 millimeters long. Easy ? The German company Graupner coined the Speed xxx naming convention and in the process rounded things up a bit. But basically if you measure the length of a standard ferrite electric motor in mm and add a 0 you have its approximate Speed xxx designation (in fact most Speed 400s are actually Mabuchi RS380s but that's near enough). These are now widely used as generic names so a Speed 400 motor tends to mean any of that size, not necessarily one from Graupner.
All you can say definitely is that 400s are smaller than 600s and 700s are bigger still. But, like all electric motors, there is a lot of variation within that general range depending on the exact wind of motor, usually expressed as a design voltage (6V or 7.2V) and also whether they have plain bearings or ball bearings (BB types), are designed as Race motors and so on.
The other naming convention used by Astro and others, 05/15/25 etc, was originally intended to refer to the equivalent glow motor size. I.e. 25 is in theory equivalent to a glow .25. The trouble with this is that electric motors are much more versatile than glow motors, which makes it a very vague designation e.g. an Astro 25 will fly most planes designed for a standard .40 glow motor. These designations are not based on physical size (e.g. 25 and 40 are the same size) but on the amount of power the motor is designed for.
The naming conventions for brushless motors vary by company and the motors themselves are so versatile that it is not possible to give much in the way of general guidelines.
Paul
