Ray writes:

On Mon, 24 Oct 2011 19:40:31 -0400, blueman wrote:


However, there are times when I would like to have a larger air supply
tank such as when I am blowing out my irrigation system. Of course the
steady-state is limited by the compressor motor to about 4-5 CFM@90 psi.

I am in the same situation. I also thought about the exact same tank.
Last year I paid the $20 for a commercial blow job. His compressor
put out 100 CFM. It took several minutes for the remote heads to stop
blowing mist. My observation is the 8 gallons of air will blow out
the near heads and is exhausted in a few seconds, another 11 gallons
may extend this to a fraction of a minute. The consensus on the
Internet is a min of 10 to 15 CFM is required. This somewhat depends
on the slope of the lawn. The steeper the slope the more CFM
required. In a previous house with a flatter lawn and auto drains at
the low spots I used my small compressor for years with no problem.

One more point, with 19 gallons (8+11) at 115psi, this gives 54 gallons
at 40 psi for the initial bolus -- which should be enough to fill any
reasonable residential style irrigation zone..