The operation of Rhoads lifters isn't controlled by oil pressure, but RPM. From the Rhoads site:
"Q. Is it oil pressure or rpm that restores duration?
A. Rpm. Quite simply, at lower rpm Rhoads Lifters have more time to leak down. As the rpm increases they have less time to leak down and at approximately 3500-4000 rpm virtually full lift and duration are restored."
And see that little word up there, ^^^^^ "virtually"? That means "almost". Herein is why I DETEST hydraulic lifters of any kind. Because essentially, no one know exactly how much plunger movement there is. Even with standard hydraulic lifters, the plunger "floats" a certain amount and that amount reduces total lift and duration. The Rhoads lifters just do it more. How much more? Even the Rhoads company itself doesn't know, so they use the word "virtually". Hydraulic lifters were NEVER meant to be a performance lifter, I don't give a DAMN who or what article or forum guru say they are. They're not. They were invented for two reasons and two only. To get rid of the solid lifter noise and to eliminate the need for adjustment. PERIOD. There's really only ONE hydraulic lifter that'd I'd agree kinda goes "toward" performance and those are the "short travel" lifters. Even those still vary the amount of lift and duration. By how much? Who the hell knows? When you have solid lifters, you know EXACTLY how much play is in the valve train. It is EXACTLY the amount of valve lash.