changing from air diffusers to venturi?????

[nigel]

Anyone has experience with this change?
Here is what I will try, I have an old skimmer 4" acrylic about 30" high. It has 2 air diffusers. I would like to change it to a venturi skimmer.
The plan is to place a 1 1\2 diameter tube inside the 4" dia. tube. place the water inlet into the 1 1\2 dia tube. This tube will be used as a mixing chamber. I will purchase a venturi (kent if I can find one)and I will add it to a mag drive pump.
Anyone know how close to the cup should the 1 1\2 tube be. Any ideas on improving this experiment?