There is a thread on the larger (I can't remember if it is referred to as an underdrive or an overdrive) water pump pulley.
Using good experimental procedure it was shown to bring water temperatures down as compared to the stock pulley.
IIRC jb1490 did the demonstration, among others.
Page 3 - Test Results
I have one, but my car didn't have cooling problems before, and the larger pulley is just one of many features to my cooling system, so I can not say that it has given me amazing results. I believe as a part of a well designed cooling system implementation it gives me margin on my system.
OBTW - I live in Tucson AZ, and it was pretty hot here at the end of the summer when I got my car back on the road.
Rocky
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