Pre Ignition or Detonation? Not easy to distinguish at times....

Below is a basic diagram of the activity inside a cylinder that is considered to be pre-ignition or detonation. It is important to note that these problems can often be caused by things outside the motor.   Over-propping, poorly ventilated engine room, wrong gear ratio for setup, high altitude operation, and faulty wiring in the boat are often good places to look  when you are trying to solve a problem of this type. Also, remember that you may not hear this type of associated problem unless you open the engine hatch and or get down in the bilge and listen to the engine while it is operating at or near full operating rpms.