Question is, i have a V belt pulley 6 inches in diameter(power pulley), and it turns another pulley 8 inches in diameter (accessory),if mid way between these two pulleys, i add a double (pancaked together) set of pulleys, and then have the first, 6 inch pulley turning a 8 inch, the 8 inch will turn the pancaked pulley that will be 3 inches in diameter and then it in turn runs the original 8 inch pulley, wont that multiply power to the last 8 inch and also slow its rotational speed?

yes, I think you would get less than a 1/4 turn for every 1 turn of the drive pully, but I am rusty.

My Jeep is a work in progress, Happy Trails

A small corection, over a 1/4 turns .29

My Jeep is a work in progress, Happy Trails

Sounds like you added an extra 8" and an extra 3" double pulley. So, now you go 6", 8", 3", 8". If you go from 8" to 3" to 8" the two 8" pulleys turn at exactly the same speed, there giving you no advantage. If you go 6" to 3" to 8" then the 8" accesory pulley will turn slower than originally, but will it have more "power"? Perhaps more torque but is that what you want. Certain accesories like alternators need to turn faster to work better not be turned with more torque or "power".

"When in doubt go faster"
'64 CJ5, 351W, T-18