They are made to tame radical high-lift cams at low rpm. Read more on Rhoads website. I was reading recently about Fiat's "multi-air" engine design (2.4L Chrysler 200). It is basically an electronic version of the Rhoads lifter, i.e. an electrically-controlled leak-down lifter. One good question is why Rhoads were never used in factory engines. My guess is that production cams were not wild enough to benefit greatly, and perhaps they worried about owners who let their oil get gunky which could clog the leak-down passages so they don't work right.
I put Rhoads lifters in my 273 but haven't run the engine much, plus a fairly mild cam (0.422/0.440 lift) so harder to notice an effect. I have another set I might put in my 383. I got both sets fairly cheap on ebay. Too bad I can't use them in my slant (no oil supply at lifters)
