Just to be sure this is clear, it's the outlet of the trap that needs to be measured. If you look at the trap as a utube manometer, you want the u to be deeper in inches than your negative. You need a 6" u of water, I say 6 because you want to allow for the filter getting dirty, you could even go 7 or more.
Pneuma is right about the depth of the trap. If you increase the diameter and the problem gets better, the trap isn't working right anyway. Only depth will make the trap work right with water. Increasing the diameter just allowed air to be pulled past the water without spraying that water into the unit.
Originally Posted by big johnson