Rogue1970's cylinder was a small reed.

The easiest method to find the RPM where the power valve needs to close on the LT250s and LT500s is to make a dyno run with the valve fully open through the whole RPM range and then make a dyno run through the whole RPM range with it fully closed.  The point where the two graphs intersect is where the power valve needs to close.

I do not agree with Mat Shearers theory.  There is only one optimum closing RPM on the LT power valves.  You cannot arbitrarily close it or hold it open to build a smooth power band.  If you open or close it too early or too late it will make a distinct zig zag in the power curve.  If you close the valve before the intersection point (RPM) on the graph, you loose power from that RPM to the RPM where the two graphs intersect.  If you leave the valve open past the intersection RPM of the two graphs, you loose power from the intersection point of the two graphs to the RPM where you finally close the power valve.


The 1st graph shows:

Green line a run with the PV closed the whole run.
Red line a run made with the PV fully open to 7600 RPM.
The two graphs intersect at approximately 5700 RPM.


The 2nd graph shows:

Blue line is the optimum setting where the valve closes at approximately 5700 RPM.