[GT40FIED]

Ok...on a cam you can measure duration really anywhere (i.e. how long the valves are open in relation to crankshaft rotation) but most are done as advertised duration or as duration @ .050" of lift. Some manufacturers in the '60s would use weird lift points to make their cams seem bigger or more streetable, but advertised and .050" are the widely accepted ones now. So, if you measure 260 degrees of advertised duration, the cam will keep the valve open for 260 degrees on each stroke (intake and exhaust) assuming it's a single pattern cam. I've read how they measure duration @ .050" and I think it's pretty similar, but I can't remember at the moment. Either way, you can figure out the streetability or power potential of a cam and choose the best cam for your engine by looking at duration and lobe seperation angle. Big duration and lower LSA = big high rpm power but shit for vacuum (brakes) and horrible low end. Lower duration and higher LSA = more streetable and better low end and vacuum but it'll lower your powerband.