For standard, or refurbished motors, your engagement strength should remain between 8 and 14. If you have Supermotors, your engagement strength should be much lower with a range between 3-6. If you require a higher engagement than this, there may be something wrong with the motor alignment, wheels, or your track.
For videos showing how to get the correct strength setting, see: http://www.iqdemos.com/search/label/Engagement%20strength
If you have update 8.20.15 or later, there is an automatic engagement strength setting feature which can be found in Main Menu> Utilities> Configuration> Motor Control Setup/Test. A video describing this feature can be found here: https://youtu.be/p1evl0yMgkA