i have been told by my local hobby shop not to run more than a 4000kv motor on 3s in my mini 8ight as it will rev too high and damage the motor,any suggestions please.

Doubt that's true. I only run 3s in my m8. Lots of speed no issues with drivetrain or any of the motors

6800 and 5400kv motors by the way

what brand of motors do you run and are they two or four pole?

have run a 3 cell on a 8000 kv motor b4 without damage .. ate a spur gear after 25 minutes ..but that was plastic.. switched to a aluminium..

Castle 0808 5300kv 4 pole
Castle cm26 6800kv 2 pole

All motors have a max rpm. Some post the spec, some don't. They could be telling you the truth for a particular motor. Castle used to post their max rpm as 60,000 pretty much across the board but have not seen any max rpm specs on the 4 pole 0808.

60,000 is about the max I have seen for a 2 pole brushless car motor some heli and plane motors go higher, but not by much.

Tacons rate around 50000 rpm max

So basically you can take the voltage of the battery and multiply it by the kv of the motor and if it is more than the Max RPM, you are over the limit of the motor. Probably safer to stay around 40000 to 45000 to improve motor life.

It would more efficient to pick a lower kv motor and gear appropriately for how you want to drive, speed runs can gear up, long run times without overheating, gear down.

Castle published an article back in 2006 that stated that 120000 rpms was about the practical limit of how fast a brushless motor could be driven due to several factors, inductance of motor feedback loop, overall motor efficiency, fet cycling time, processing time of esc. At 120000 rpms they noticed the motor is actually delayed or retarded by 11 degrees so they had to build in an auto advance to keep up. Things may be a little faster now, put probably pretty close and there doesn't seem to be quite as much information now as then. You have to half that for 4 poles, basically the esc is pulsing twice as fast to fire 4 poles at the same rpm speed as a 2 pole. So it seems that a 4 pole RPM limit will be 60000 or less based on the motor.

Also the torque per amp is an inverse function of the kv of the motor, so lower kv motors provide more torque for less amp draw than higher kv motors, so will give longer run times and be less taxing on the battery.

I thought the max rpm was also influenced by high centrifugal force combined with reaching a resonant frequency that caused the rotor magnet to shatter? I know I blew one up on one of the first novak ss5800 motors that used non-sintered neodymium magnet on a speed run with 3s.

Also, I've seen smaller motors rated for higher rpms compared to the same quality ones from the same manufacturers. I always attributed that to the smaller rotors having less rotating mass, lower centrifugal force than a bigger motor at same rpm, and therefore a higher max rpm.

It sounds to me like they gave you a big safety margin. Not like they blatantly lied, but you can actually safely run 3s on a higher kv motor. The main issue is there are too many kinds of motors made to different standards so no one answer will be right. If you stick to brand name motors, you should be safe with anything right under 6000kv. Now, this doesn't mean you can't damage a motor by overheating due to gearing incorrectly so that should still be checked.

Sorry, should have been more specific, I was talking from an ESC perspective and working into what the 4 pole theoretical max rpm could be since I couldn't find the spec. I'm not sure resonance comes into play with the magnets, but glue bonding strength and the substrate used to bond them to wil factor in. If your magnets aren't balanced, I see what you mean, I was thinking of resonance from an electrical standpoint not a mechanical. I'm a EE and sometimes get stuck seeing things from an electrical/signal standpoint first.

I would like to see more lower kv motors for minis. You can use them on 2s and still have good usable throttle range for running in tighter areas, and throw in a 3s and push the boundaries of controllable speed. The higher kvs seem to shine in speed runs but really decrease the drive-ability of minis in other than straight line runs.

As I know that 3S will improve your run times.
But according to my experience you will need to use a lower KV.
you wont ever need a 15.T on the 4600 or 4000 KV motor that's for sure, more like a 12 or 13 would be heaps. Keep your mind in struggle.

What? I've got a castle 1406 4600Kv that says 100,000 rpm max... all the stuff I've seen says 50,000 to 60,000 is a REASONABLE limit. But lets say I wanted to run 4s occasionally? (4600x14.8=68080) Oops... wrong thread,not really micro.

I believe that max rpm is based on the rotor speed. On 4s, the stator would be taken out of it's efficiency range so heat build-up would be an issue, hence the 3s lipo max rating. Also, you would just have to make sure the esc can switch between phases fast enough for the goal rpms (escs also have max rpm ratings that vary between the amount of poles in the motor, the 1406 has 4) while also taking into account the amount of current in amps the esc can handle. Same motor, same gearing, same load, higher voltage = higher current draw. Motors also have a max current handling so going over that means heat and crispy parts. Geez...electrics seem more complicated than nitros now...