Yeah, so he needs a SMALLER alt pulley not that he's running an UD crank in order to spin the alt at the same speed... wasn't as tired as I thought.

 

op

at idle. If you turn lights, stereo, air, etc etc OFF. Does it still drain the power at idle or just with accessories ?

I know people running 42% on stock motors w/ no problems. Something is going on and I doubt its alt' rpm.

 

Underdrive pulleys are LARGER, not smaller.

My first post was a little misleading.

 

UD crank pulley is physically smaller than the stock crank pulley.

 

Im reminded of Abbot and Costello for some reason.

 

Guess I'm the little skinny guy.
Who's on first?

So let's get it straight:
UD crank pulley is smaller and spins things slower.
To spin the other accessories at proper speed (specifically in this case the alt to keep proper charging), you need to put smaller pullies of them.
By doing that, you are negating the power you tried to save by going to an UD crank pulley.

 

I got completely backwards. I'm a moron. I was thinking about it at work yesterday too and it came to me that I was wrong as ****!

 

Some interesting inputs here as far as underdrive pulleys. Some correct, some not.

Changing to an underdrive pulley on the crank does not change the rpms of the pulley. It can't, it's still bolted to the crank. What it changes is the distance to outer edge travels during one revolution. This in turn lowers the length of belt that passes across the pulley per rotation. This lower length of belt becomes a constant at the driven pulleys.

Reducing the diameter of a DRIVE pulley reduces the RPMs of the DRIVEN pulley unless the diamter of the DRIVEN pulley is reduced.

Conversly, If the diameter of a DRIVEN pulley is reduced, the RPMs of the DRIVEN pulley will increase unless the diameter of the DRIVE pulley is reduced.

 

 

My head hurts.