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Edelbrock just said their pumps are designed around OEM pulley specs. Was the 427 pulley setup a 1:1 ratio? That's about what I've got, though my water pump pulley may be a 1/4" smaller than the crank pulley. Its definitely not bigger.
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I don't know what CS originally put on the cars back in the day....Looking at a factory/OEM mid 80's truck (small block), the crank pulley is 6 3/4 inches in diameter and the water pump pulley is 5 inches in diameter..I would think similar pulley sizes for big block factory engines... I took the measurements from the outer rim of the pulleys, easy way when still in the truck.......as you can see, the water pump is turning much faster than the crank speed....Your set-up is about 1 to 1, slowing down the water pump from a stock/OEM set-up....I've seen crank pulleys up to 7 1/2 inches with water pump pulleys as small as 4 inches, in fact, when doing all my testing, I ended up with a box full of factory pulleys,both for the crank and water pump of varouis sizes, probably have a dozen to 20 pulleys....crank pulleys are for small block engine, not sure if the water pump pulleys would fit a 427, but if they do, your sure welcome to try a smaller pulley if I have one that will fit your motor......
I think you said your crank pulley was about 6"s in diameter, with a 5" diamter water pump pulley, you would be overdriving your water pump by roughly 15%, I think this should be a minimum.....
David