Pros and cons of underdrive pulleys
 

I need to know about them before i buy

Have them on my Vette and it doesnt like to idle for long periods. The battery looses charge! They made a slightly noticable improvement. The real improvement came at the dragstrip when I took the belt off completely.

I put a set of March under drive pulleys in my 91 T 15 jimmy. They had just come out and I fell for the hype.
First, my alternator wasn't charging enough, so I put the smaller pulley back on. Then the water pump couldn't keep up when the A/C was running, so I put the smaller pulley back on. Each pulley replacement required a different belt.


All said and done, there was a slight improvement at first. But by the time I'd replaced 2 of the 3 pulleys back to stock, the improvement was gone.


The factory engineers seem to have gotten pulley sizes right. I've been looking into those new flex fan blades that flatten out as rpms increase to reduce drag. I'm not ready to leap yet.