In that type of valve, dwell is between horizontal valve opening and vertical valve closing....You can adjust spring tension from the wheel under the chamber so you can control how fast vertical valve will close.
If someone can fit a bigger vertical valve chamber that would be an improvement for power. its already pushing 100fpe in 25 cal. semi auto to boot.
its true that 100fpe is not all that but "semi auto", my friend. 100fpe is enough to kill anything medium sized up to 100 yards in the right hands. super quiet too. i am okay with 100fpe semi auto. i am even okay with 80fpe as long as its accurate like the NSA in the impacts reaching out to 250 yards by Utah airguns on utube. not to start an argument, but he inline airforce valves in .257 probably yields 10 shots at the most. the Huben valve is also efficient, i got 3 mags worth of shots at 80fpe.