Motors and voltage
If you Google Graupner with the motor model number you should be able to find the spec for your motor, including the max volts. The Important bit is the Power rating (ie Watts which is the product of the voltage and amps). if you exceed this your motor will overheat and could therefore fry. You can buy Wattmeters (Giant Cod) that allow you to measure watts, amps and volts. Useful when selecting the optimum prop for your motor.
The other consideration is the speed controller(s) max voltage rating. They would need to be suitable for greater than 15 volts (LiPo are 14.4v). if you exceed the max voltage rating you run the risk of cooking the ESC. May not happen Immediately but I believe the damage is cumulative and in my experience usually goes at switch on.
Good luck and please post your experiences 😀
Dave