A good page explaining why lead was added to petrol in the first place, what engine knocking is, and why unleaded petrol should be used.
http://www.chm.bris.ac.uk/motm/leadtet/leadh.htm
And a good page explaining engine knocking.
http://www.stanford.edu/~bmoses/knock.html
And the ultimate gasoline faq.
http://www.repairfaq.org/filipg/AUTO/F_Gasoline.html
