Is there a chart somewhere that shows the amount of boost for the different pulley size's, how do you figure what the ratio is with regard to underdrive and overdrive?
Thanks
Larry

www.holley.com has some info about pulley sizes for their Roots type blowers.

To figure the ratio of under or over driven, simply divide the diameter(or teeth#) of the lower pulley by the diameter(or teeth #) of the upper pulley.

If the top pulley is 3" and the lower is 6" then its 2:1 and 100% over driven. If it was 4" and 6" then its 1.66:1 and 66% over driven.

If we are looking at ratio of teeth on top pulley to the other, then maybe we should be looking at circumference of one pulley to the other, not the diameter. I am not sure about this, I should have paid more attention at school.

Thanks for the Info: On superchagers, what does the size stand for, CFM, CI, ect; and what would a 12-71 be?

Your 12-71 example would have originated with GM/Detroit Diesel. It was for a 12 cylinder, 71 cubic inch per hole 2 stroke diesel engine.
10-71, 10 cylinder, 71". 14-71, 14 cylinder, 71" & so on all the way down to 6-71.
As for all the rest, I'll let someone who is up to date on this stuff tell you more.

It makes no difference. It would be the same since to get the circumference, both diameters would be multiplied by 3.14. The ratio would remain te same.

I should have paid more attention at school.

Thanks for setting me straight.