Hy-lift "S" lifter is their top shelf lifter with close tolerance bleed down rates. It's their only performance quiet lifter..... They won't make them for our Pontiacs.
R lifter is considered a quick bleed lifter with consistent bleed rates. Noise is considered normal when used with performance valve springs and quick ramps.
951 is only recommended for use with stock/factory valve spring pressures due to uncontrolled bleed rates that can cause individual noisy lifters when use with fast ramp cams and stiff springs.
But supposedly the got their $1500 HR lifters right.
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