Speedmaster head hypothetical

Earlie A said
Case in point. Look at the difference that going from a 35 degree backcut to a 32.5 degree backcut on a 2.08 valve with 50 deg seat made. 10 cfm negative in part of the curve to 8 cfm positive. I haven't drawn it out, but I know this change is only a few thousandths of an inch.

What I haven't done yet is try to figure out why it changes so much. This is where drawing things out on the computer helps me. For example, consider the 0.200" lift position. That can be drawn out and the air flow through the curtain visualized. On the computer you can blow things up to a much larger scale and look at flow patterns. It also helps to take the head off the flow bench, open the valve to 0.200" and look at the curtain area from all directions. There is something to be learned from this one test alone. That learning will unlock some other understanding.

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As a side note, the 50 degree cutters I use show the best numbers with a 30 degree back cut.

I know that flies on the face of conventional wisdom but that’s what it likes. Also the under head radius and the angle affect what back cut it likes.

On a set of custom valves I had Donovan make for me the BC hurt flow everywhere. So I used them without the BC.

It’s like Morgan says in that you are tuning the valve job up with the angles below and above the seat angle.

I know the cutter I use (for the intake side, the exhaust is a radius) has two wide bottom cuts. I forget the angles. If you start taking too much out of the bottom cuts the port starts getting a bit chaotic around .25 l/d.