More importantly than duration is valve speed. The more ratio that you run on the rocker the faster the valve travels. This allows it to open quicker and close quicker. My sbc has 1.9 ratio rockers on it, allowing me to take advantage of this.



I am not sure of a formula to compute the change in duration. It will not change the total duration just the curve of the duration at the valve. Changing valve lash on a solid cam will take away or give you more total duration though.

Bob