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The concept of roller lifters is that they allow the cam to have steeper open and close ramps than flat tappets, with the same reliability. This is where the extra horses come from. The hydraulic roller offers the best of both worlds - quieter, less maintenance with more hp.
BTW, always bear in mind that the venerable FE's main bottleneck is breathing. When you hear the stories about only 10-15 HP roller gains, they could be true because high-lift, long-duration cams, whether flat tappet or roller, are possibly strangled downstream in the air flow of the heads anyway.
Brian
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