DIY CO2 Reactor

I am looking for advise to improve my DIY CO2 Reactor gas output for my
40 gallon tank.
I use a 2 liter soda bottle for my CO2 reactor and mix 2 cups of sugar 
and 1/4 teaspoon of yeast and water filled up to 2" below the cap cover,
so far the CO2 generated is only about 2 ~3 bubbles per minute 
.... Is this sufficient ?

1. How can I increase the rate of CO2 output and what is the normal 
   rate of CO2 generated from this type of DIY system ?

2. I also experience very low CO2 release pressure into the tank when I place
   the soda bottle higher than the water level of the tank. Where should the
   soda bottle be placed to get a reasonable gas release pressure into the tank.

3. What are the methods use to connecting the CO2 line into the tank ?
   .... to diffuser, or power head ?

    KS Chan