Camshaft tech by Dimitri Elgin

"Considerable information has been recorded about numerous aspects of the four stroke internal combustion engine. Nevertheless, only a small percentage of people really understand how it works and even fewer still know how to modify an engine to suit their needs. I will try to simplify this complex subject by discussing some basic principles that may be overlooked or misunderstood by the average person. First, it is very important to understand the relationship between piston travel directions and valve timing events. The reason this relationship is important is because it is one of the few things that is relatively easy to adjust/change. The camshaft which opens and closes the valves makes ONE complete revolution (360 degrees) while the crankshaft moving the piston up and down the cylinder rotates TWICE (720 degrees). Camshaft timing is usually expressed in terms of crankshaft degrees relative to the piston location in the cylinder. That is, relative to Top Dead Center (TDC) and Bottom Dead Center (BDC), respectively. Note that during the four strokes of a piston in an internal combustion engine the crankshaft will rotate 720 degrees and the piston will be at each TDC and BDC twice.