There seems to be a little confusion about the effect of installing higher-ratio rockers. I have heard and read different opinions about it, so I thought I’d throw mine out there.
Camshaft Duration, (or just "duration):
The number of degrees of camshaft rotation during which the lifter is at or above a specified lift value. This is the duration you will find on the cam cards and specifications. This is not affected by rocker ratio, since it is based on lifter travel—or indirectly, cam lobe lift—not valve lift.
Valve-Open Duration:
The number of degrees of camshaft rotation the valve is actually off of the seat. Like camshaft duration, this is also not affected by rocker ratio, since the point that the lift begins and ends is ground into the cam itself. This does not change because of rocker ratio.