Well you guys that think a back-cut on an intake valve totally disagree with Darin Morgan’s thinking. His thinking is it will help in most applications and never hurt. Again I will say this is talking about the intake side.
IIRC, he does the back cut when the shape of the valve is perfect for his valve job.
I can think of maybe 5 times in my career when I got to order valves with the shape I wanted, and yes a back cut on those valves helped forward flow and didn’t increase reverse flow.
Every single time I’ve tried a back cut on a shelf valve no matter what valve job I use (a couple of which I designed) reverse flow went through the roof.
Thats why I say I almost never use a back cut on either valve.