Alternator pulley?

[jamie_eaton]

Hello guys quick question, i believe i have a set of underdrive pulleys on but have seen this
http://cgi.ebay.com/ebaymotors/Mazda-RX-100-RX3-RX4-RX7-Alternator-Pulley_W0QQcmdZViewItemQQcategoryZ38656QQitemZ8001230916QQrdZ1QQsspagenameZWD2V

i would like to get this, but replacing the alternator pulley with this one make any difference?
do you benefit more from the water pump pulley? i dont want to get this if it means the one i have allready will be better.

[MillSpeed]

Looks like that pulley is designed for two v-belts rather than the ribbed serpentine belt on the FD.

[oldone]

seems to be a double belt pully,def on that

[Martiny]

If you use your car on the road, I would not recommend underdriving the system .

The water pump cavitates at very high rpm, and the underdrive pulley is to try and avoid this. However, you then slow down the pump  when idling in traffic.