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there is a difference between flat tappet and rollers in that on a flat tappet setup, the cam to lifter contact is a line tangent to the lobe/tappet and the line moves across the face of the lifter---- on a roller set up that is a constantly changing contact point/angle as you have two curved objects getting friendly---------
On a flat tappet cam you are limited by the edge of the lifter reaching a point where it would hook the edge into the cam(sorta like a garden hoe into the dirt as you scraped it across the surface------

On a roller setup that is a constantly changing line angle with no hook and it allows a much faster opening rate which allows the valves to open much faster to there max and with the radius of the lifter roller allows it to stay near max opening much longer where a flat will go up there and slow down stop and then start lowering the valve sooner

These are factors why the cam companies use .050 from base circle to be a measuring point for the cam cards numbers
A roller cam can open the valve so fast that we have to watch for tappet flying off the cam nose as the deceleration near full open occurs while on a flat its necessary to run really high spring seat preasures just to keep the valve from bouncing----------