Originally Posted by brewstaedt
Does the size of your grain bill have any effect on conversion efficiency? Will fifteen lbs of grain convert more completely than seven lbs?
In theory, the size of the grain bill doesn't matter. At the right temp, the enzymes will convert starches to sugar and a kernel of grain doesn't care if it is alone in the tun, or with thousands of friends.
However, in practice there can be a significant variance in the efficiency, especially if you are brewing in the same equipment. Your efficiency is based on how completely your sugars are rinsed away by the sparge water. If you use a small grain bill in a larger tun then the grain bed is thin and wide. This decreases the contact with the sparge water and could decrease the efficiency. A taller, narrower tun will change the way the water flows through the grain bed.
Stirring vigorously can help to overcome some of this and increase efficiency.