Hi!
I'am racing the 1/12 GT Stock Class and using 1S 3,7V LiPos and using the Hobbywing 13,5 Justock motors in our class.( LiPo max charged to 4,2V )
Would a motor tester/analyzer do any differance for me and my setup, and which information would I have from such of a product that would be usefull for my 1/12 GT Stock Class racing,,?

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The Justock motors are fixed timing and there's not much variance between them. An analyser won't really tell you anything useful.

I have found that shimming the rotor will effect degrees of phase to close the gap which in turn improves efficiency so that you can hold punch longer in a race so I think there is some benefit there

I am not familiar with Justock motors so if you can't shim the rotor then I would agree there wouldn't be any benefit investing in a motor analyzer.

Thanks!
But as I understand the rules, we are not allowed to open the motor in the stock class and change anything....but i will ask ..

This guy discusses shimming the rotors (and other mods that people are doing) on the Justock here, seeing videos like this makes me appreciate that I no longer race stock classes, shrug...

As others have mentioned, a tuner would not be beneficial with a Justock. However, it would be beneficial to have one if you went down and purchased a few of them, tested them, keep the best one, then return the others. However, the shop wouldn't like that much. If you get one to compare motors, I recommend the HobbyWing.