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Use the chart below to determine drive ratios for 6-71 1/2'' pitch drive superchargers. Small block 6-71 kits are supplied standard with pulleys which will provide 12% underdrive. Big block kits are supplied with a 9% underdrive. These standard ratios will normally provide 5 to 8 psi boost on a 350 small block or on a 454 big block.
Note: top and bottom pulleys are interchangeable.