Preignition or “knocking� is just what it says. The gas ignites before the spark plug fires. Temperatures can rise in the cylinder to the point that the gas ignites without benefit of a firing plug. First, the upward bound piston must fight the pressure created by preignition and second, there is inadequate pressure left to form a true power stroke to force the piston down, therefore, the bike is effectively slowed down. Thirdly, and more destructively, the rising temperatures created by �knocking’ could lead to a seized motor via the piston and cylinder wall bonding and attempting to become one, or, the piston might simply melt! Like preignition, detonation is caused by excessively high temperatures and/or pressures. With preignition, think “heat� and “meltdown� while detonation is synonymous with violence, explosiveness and destruction. A controlled, expanding burn is met with the shock waves of an uncontrolled explosion and it inherent pressures that bounce off cylinder walls, causing a rattling sound like metal hitting metal, or pinging Detonation is really bad as it shortens engine life by pounding pistons, con rods and associated bearings. Some causes are manifold air leaks, lean carburetion, low octane gas, wrong ignition timing and too much compression


Source: http://www.heavydutycycles.com/n8techf.htm