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I've read about them for years and years too. I have no trouble believing their claims to increased power, and I can see small gains in efficiency (most pumping losses will still be there on a gas engine due to the throttle plate at anything other than WOT but there should be some gains w/o poppet valves).
I think the reason they haven't caught on is problems with sealing. Cylinder pressure forces the poppet valves shut and helps the intake charge from leaking past the valve. With these spherical valves, it appears the cylinder pressure tries to push the valve off it's seat and leak. I never see anything saying how long their engines last and how reliable they are.
Mike
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