Well, 4gal -- ~0.5cu-ft and a typical 1/2" impact wrench will need
roughly 5 cfm@90-100psi (I was thinking of the big 3/4" jobbie earlier
needing almost 9). *So, not accounting for any losses at all would be
about 1/10th of a minute (6 sec) and poof!--tank's empty.


Is that a valid computation?

A 4 gallon tank may be 0.5cu ft in volume, BUT the air inside it is
compressed so I think a 4 gal tank holds a lot more than 0.5cu ft of
compressed air.

But maybe not... does anybody KNOW?

Mark