Back cutting valves for improved air flow
This is a procedure done to help flow the air charge across the back of the valve and smooth the transition into or out of the combustion chamber.
Normally don on valves that are not fully machined or swirl polished.
Once the valve has had the face re-ground. Most common is a 44* or 45*
The valve machine is reset, and another angle is ground onto the back side of the valve towards the stem, most common is 30*
The valve on the left has the basic face ground to mate back into the valve seat.
The valve on the right has had the basic seat inked,(black) and the added angle ground. (sliver)
Most OE valves are not fully symmetrical, so the width of the valve face to seat varies around the valve, the added angle will help correct this and smooth the transition of air flow across the back of the valve.