When I first saw the picture of the engine Bill C posted I was quite facinated by it. In searching the web I found this site that has many 4 stroke examples and short movies of them running with sound. Very very cool!
http://www.nvbackflow.com/engines
(sorry about this not being a hot link, I have not figured out how to turn on the HTML)
Bill if you have other examples you are working/have made I am sure everyone would be interested in seeing them.
Ken