How do you calculate / convert the duration @ .050 lift to Advertised Duration?

AFAIK there is no conversion

If the lobes have steep ramp angles, the 50 thou duration will be closer to the duration, opposite for shallower ramp angles

So how is the adv. duration calculated from cam specs?

Advertised duration is just the duration of valve lift above some value. SAE specify advertised duration as lift above 6 thou but apparently this is fudged by lots of cam grinders. Duration at 50 thou lift is a better indication as it cancels out a lot of factors that might make the cam seem bigger or smaller than it really is.

I prefer to use the duration @ 0.050 lift myself, however, its amazing how many people refer to the adv. duration when discussing cam specs so I thought I'd find out what its all about.

advertised duration is the figure that the valve opens say 10 deg before to the figure it closes say 10 deg abdc which would be 200 deg adv , dur at 50 thou is the measurement at 50 thou opening to 50 thou b4 closing ,some cams differ, if difference is less between adv and 50 thou means valves open quicker like haveing longer duration more power better acceleration without neg side of longer duration that is the benefit of roller cams which are able to open valves quickly (ramp up quick)

Yep, this is one of the reasons why I love the roller cam. One of the down sides though, is the expense of them.

it could be worse ----flat lifters------plenty of wear!!!

it could be worse ----flat lifters------plenty of wear!!!