1515 vs 1518

Thereoretically nuz69 is right. The setup only draws as much power as it needs for a certain speed. If the output power required to go 40Mph is 2500W, then for the same vehice it will ALWAYS be 2500W to go 40Mph. What changes however is the input power from the battery based on the efficiency of the system.

nuz69's theory is based on the assumption that the efficiency of the 1518 will be the same as that of the 1515, with a 4S lipo pack. This is incorrect as the 1515 will have the most efficient top RPM of 32,560 whilst the 1518 will have a max RPM of 26,640. At 26,640 the motor is in a lower portion of its power curve, and is not as efficient compared to an RPM of around 30,000. The 1518 motor is more efficient when run on higher voltage, in the right RPM range, compared to the 1515 on a lower voltage - as it is high current that increases the power loss in the system.

As a result, the lower efficiency of the motor will demand more INPUT power from the batteries. More Input power for the same voltage means a Higher current draw. Therefore, running the 1518 on 4s will draw more current from the battery resulting in shorter run times. Not to mention higher temperatures.

Xsmoker, I think there are only two options for you. higher capacity 4S batteries for longer run times or a 5S setup with a 1518 and lower gearing. Cheapest would be the larger 4S batteries, since the other option requires you to buy a whole set of new 5S batteries and a 1518 motor.

[nuz69]

Originally Posted by biddy

nuz69's theory is based on the assumption that the efficiency of the 1518 will be the same as that of the 1515, with a 4S lipo pack. This is incorrect as the 1515 will have the most efficient top RPM of 32,560 whilst the 1518 will have a max RPM of 26,640. At 26,640 the motor is in a lower portion of its power curve, and is not as efficient compared to an RPM of around 30,000.

No I am damn sorry, this statement is correct for two motors of same weight and different Kv due to different windings, BUT we are talking about two motors wich have same windings and different weight.
"lower portion of its power curve", This is not a nitro motor, the efficiency of a brushless motor is not measured by varying the voltage, but the LOAD or the AMP going through the motor, while the motor is under a given voltage.
The 1518 motor, with its longer rotor, produce more torque at the same amp, so puting an higher pinion on it will compensate the lack of RPM and will result of the same working point in terms of current, however the shaft of the motor will be strain of an higher torque. Moreover, the 1518 will have to handle the same power in a larger can and on a longer rotor, so the temperature will necessarly drop a little, and efficiency will go up.
That's exactly the reason why here in europe in our large tracks we use the 1515 2200Kv on 4S instead using 1512 2650Kv on 4S because it always overheats, too small motor...
On the other part, in our practical application, a lower Kv motor will produce lower amp spikes during acceleration, and amp spikes are responsible too of the heat and bad efficiency, that's where the 1518 win the efficiency test too.
I have done the test between a 1515 and a 1520 on 4S geared for the same speed in my mbx5T, I always had lower temp and longer runtimes with the 1520. I suggest you to do the test.
Anyway I race my truggy on 6S with the 1600Kv but with adapted ESC parameters to avoid as possible the amp spikes...

==> Xsmoker you can begin by increasing the punch control, and maybe drop one pinion tooth, I know by experience that most of the pilots have a too large gearing for the track they race.
After that if you have some dollars to spend, don't buy larger 4S batteries, it will add more weight, not good for the driving behavior and strain on the system. Buy some 5S 4000mAh and reduce the pinion of 4/5 x actual number.

[Pdelcast]

I have to agree with Nuz69 as well -- geared for the same speed, the 1518 will run a little more efficiently than the 1515. It will run at lower RPM, which will result in lower magnetic losses. Copper losses will be a wash between the two motors (because current will be very similar.)

In most cases, when geared to give equal speeds, the longer, lower Kv motor will usually be a little more efficient.

This assumes that the two motors have the same winding and construction (like the 1515 and 1518).

Thanx!

Patrick