It's all about at what lift you measure the duration. In the car industry they tend to measure @ .050" open/close; you end up with a lot shorter 'advertised' duration that way as opposed to measuring at a point when the lobe just starts to lift, as this makes the measurements easier and more consistent. The cam in my old big block was sold as a 286/292 duration, but at .050 it was actually 248/254 (see link for reference, the cam card lists 2 sets of duration on there: 23-702-9(Three Bolt) - Xtreme Energy? Street Mechanical Roller Camshafts ) Calculating the duration from the moment the lobe moves the valve gives really cool, big duration numbers... but at that point you have to start figuring in valve lash and engine temperature if you want to be really accurate.... it's just easier and a bit more 'honest' to standardize the measurement at a given lift point.Question if anybody smarter than me looks at this. From what I could find on reading the two numbers added plus 180 should be close to the stock duration of the cam. From the race kit manual it says stock exhaust duration is 293 degrees but my numbers only show 247.5. I remember the last time I did this the numbers were similar but I didn't do as much reading last and didn't think twice about it.