Rocker ratio changes duration?

[CFster]

We all know what effect rocker ratio has in valve lift. What happens to duration, and is there math to calculate that change?

[68Formula]

I believe advertised duration is based on somewhere between 0.004 and 0.006" valve lift depending on camshaft manufacturer. The of course, the 0.050" lift that a lot of people compare. When you increase rocker ratio, the point at which those lifts occur happens sooner, so effectively the duration between those valve points is longer. But I doubt you can calculate it, as you'd need to know the lobes ramp rate at those points. You'd have to physically measure it.

[ford396]

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

[Skip Fix]

Comp used to have a chart for roughly every .1 ratio increase over 1.5 you get the equivalent of 2 degrees @ 0.050. Seat still the same so idle not as affected but motor will still notice the increase somewhat at idle.