It'll make it worse.
Actually, the voltage won't drop any lower (at 11.9 you're running on the battery, anyway) but it'll do it at higher RPM.
IOW, if it only does it below 850 RPM, it'll do it below some higher number, like 900.
I programmed another 100 RPM or so into my idle speed. Helped quite a bit.
Edit - You can probably find a smaller diameter alternator pulley in order to get the alternator RPM back up, but that will take away part of the benefit of even having the underdrive pulley.
Raydar88 4.9 Formula IMSA Fasback..........................88 3.4 coupe -soon to be something other than redRead Nealz Nuze! Praise the Lowered!
[This message has been edited by Raydar (edited 01-19-2008).]