W, I gotta tell ya- there is no going back! I changed a Speed 600 geared with a JETI 15/4 in a 2 meter sailplane and was very impressed. A 2:1 geardrive on it turned a big prop and climbed like crazy while getting long runs due to its far better efficiency.

I upgraded to lithium poly batteries and found that my S400s didnt last nearly as long as when they were run with nicads and nimh batteries but man did they fly! I got a bunch of them very cheap so changing one out once a month or so isn't that big a deal.

2 lipos on a 6V S400 doesnt exactly sparkle since its like 6 nicads- but 3 cells- with a 7.2V motor will rock them but good! The carbon motor's 50% efficiency (that's when its cold!) is to blame. The hotter it gets, the worse the efficiency and so it generates even more heat in relation to the power it is putting out. The overvoltage takes it's toll but that's the racer's edge isnt it? Heatsink it.

An FMA ad I saw showed 3 cell lipos powering a 7.2V S400 motor- and I just had to try it- and love em- just expect the motor to be shorter lived than you might expect. I compensated for this on my twin by using redrives so I could run at lower throttle with the bigger props and still have good climb. These motors are nearly fully exposed to the airflow and remain reasonably cool- unlike the motor in my Ascent electric glider which I replace all too often with its poor cooling.

A brushless is a much better choice for efficiency- 80% or better- and increased power- but throttle management is critical once you have the option of stomping it and going 3 times the S400 power level, and cutting your flight duration but showing some real spunk up there! Even your nicads would fly that much longer- and handle the extra amps it can take- but be sure to limit the amps for your lipos or double- or triple- them up.

The only way you can know exactly how many amps it draws at full throttle is would be to use a Wattmeter of some type. I use the Astroflight meter and can be sure I'm not going to fluff up a precious lipo- or burn the motor-and it lets you perfect the prop selection. Determining the maximum amp level by the amount of smoke coming from the windings is not the shrewd move.

A key point for brushless is that you get the same amount of power but at a reduced amp level since the eff is higher. A gearbox is nearly always helpful to increase prop disc area by adding gears- but they reduce overall efficiency. One way around this would be the outrunner motors that require no gearing. Simplicity is half of what direct drive has going for it (high thrust speed for racers is the other) - now you got that and the advantage of a larger prop too.

Brushless are often lighter than the motor they replace. So are lipo batteries. No brush noise for better radio reception. Maintenance free. Add up the advantages and there ya go.

Lipos transformed electric flight. This takes it all the way. These are the motors that lipos need to really shine and transform electric to a whole new level.