I came across this article that explains the difference very well. Many techs use these terms interchangeably when they are actually two different things.

The first half of the article explains detonation and the second half explains pre-ignition. There is a lot of useful info as to the theory of these phenomenon, and the cause and effect.

http://www.contactmagazine.com/Issue54/EngineBasics.html

Engine Basics: Detonation and Pre-Ignition

Written by Allen W. Cline

Reprinted from Issue 54 of CONTACT! Magazine, published in January, 2000

Steven from Washington