Why would low lift head flow hurt power?

92b said
Did you mean to say exhaust valve?

No but I do not and have never used a back cut on the exhaust. Never even tried it. I agree with what he says about tulips as well and would have added that you'd want to specific throat shape for that valve also. I have a lot of experience with stock iron heads and they are limited on the exhaust and what you end up with is going for balance between a good number and port speed/velocity. There's a natural tendency when you get really really good intake numbers to push closer to the ideal exh ratio and that can diminish the velocity/a potential reversion susceptibility with a low lift enhancing backcut if you go by this literature/theory. They don't like to do much over 200 CFM they like to perform in the 175-198/200 range. Casting vary.. but the thing is.. the intakes can flow in the 265-290's cfm quite routinely.
What does the fuel do when it hits the back of the valve with a back cut as opposed to with no back cut. That's a little more interesting to me at the moment because I'm not trying to use the wrong cam timing. Think about the back cut '30* or 33*' at overlap and its aimed, the exhaust. We collect data when we flow a head and there's different things to look at. That data is very important when choosing a cam and building an engine. Engines I would use a back cut on the intake would be in the range of idle to 5400.