By "stem diameter" I mean the sliding part where the seal occurs in the valve body.... There is air pressure on the inside, and only atmospheric on the outside, so quite a bit of force.... In a Disco at 2000 psi, there is 45 lbs. of closing force (38 lbs. from air pressure, 7 lbs. from the spring).... Compare that to the rougly 130 lbs. force required to crack the valve, and then the unknown forces due to flow past the various parts of the poppet and stem.... I haven't given it much thought, but I was wondering if making the stem larger or smaller (and hence changing the ratio of opening force to closing force) could be used to advantage to "shape" the lift curve vs. pressure....

As an example, with a 1/8" stem (same valve spring) the closing force drops to 32 lbs. (from 45).... but with a 3/16" stem it increases to 62 lbs.... If the same throat size was used, the opening force remains at 130 lbs, so the ratio changes.... Of course the throat would have to increase a bit with the larger stem to provide the same flow area.... and it could be reduced with the smaller stem.... so more variables to look at....

Bob