Hi Richard. I went out to the shop and timed my air compressor, using your
method, and I got between 6 and 7 CFM from 80 psi to 120 psi. It's on a ten
gallon tank. From what you said, that's about the right size, and I might
gain a bit by going to a 20 or 25 gallon tank depending on my typical
usage - big air or little air. I use it mostly for smaller jobs, and once
or twice a year I get into a big usage situation for a day or so.

Maybe I'll just think about this for a while longer. It seems I could go to
a 1.5 hp motor, and regain the flexibility of running it on 120 volts,
though. It came to me with a 1 hp motor that was fine in warm weather but
couldn't reliably start it in below zero temps.

Brian
"Richard J Kinch" wrote in message
. ..
Brian writes:

Generally, would putting
a bigger tank on be a good thing or a bad thing? any significant
down-side? Is there a rule of thumb for sizing tanks to compressor
head capacity?

As I explain (see http://www.truetex.com/aircompressors.htm), "the tank
should be sized to be at least 1 gallon of volume per CFM of the
compressor". Bigger is OK, costs more, more overhead, but lets you run
tools bigger than your continuous capacity for short bursts, and cuts down
cycling for lesser loads.

Most systems have some slow leaks, and if your use is only occasional,
then
you wind up refilling a huge tank and slowly leaking it out just to get a
puff of air for something now and then. The ideal would be to have a bank
of several sizes of tanks with valves so that you can select an optimal
aggregate tank size for the tasks at hand. But if like most you have just
one tank, one compromises with something a little bigger than what
optimizes the average task.