Chuck wrote:

I have an older Sears compressor that was given to me because it had
pinholes in the tank. I would like to replace the tank with a larger unit
(20 gal instead of the current 11 gal). I think I've located a suitiable
portable tank. But, in the process, I wanted to learn a bit more about the
actual compressor itself.

The compressor currently has a 3/4 hp motor. At first glance I was kind of
depressed thinking "Gee, just about every compressor I see these days has 3
+ hp. Will this thing even work well?" In asking around I was told that
amperage plays a big part in the actual torque of the motor. This is where
I get confused...

That is intentional. Compressor ratings are largely a vast conspiracy
of lies.
You'll see lots of home shop type compressors at the local Home Depot with
standard 15A 115 V plugs rated at 5, and now even * 6 * Hp! Well,
there's no
way to get 6 Hp out of a 15 A wall socket. You can barely get 2 Hp, and
if you
run so much as a 100 W bulb on the same breaker, it will trip after a while.

Anyway, my rule of thumb is a decent single stage compressor will
deliver about
3.2 CFM (at 90 - 100 PSI) per (real world) Horsepower. I also will
throw out
another rule of thumb, which is that 2 (real) Hp is about the lower
limit you
should try to use in a home shop. 3/4 Hp is just too small for serious
air tool
work. It would be fine for an air brush or tire filling, but almost any
standard
air tool, like a die grinder, air paint sprayer, (very small) sand
blaster, air chisel,
etc. will use at least 6 CFM, and some will use a lot more. Unless you
want to wait
5 minutes out of every 6, you will soon be looking for a bigger compressor.
Forget HP, as some compressors sold for industrial use DON'T lie about it,
so it can't be used for comparisons. Just look at CFM at 90 PSI (don't
be foold by ratings
at 40 PSI, either.)

The other bad news is that the oilless compressors tend to be EXTREMELY
noisy,
and will drive you (or your neighbors) nuts! Oil filled compressors
generally
turn much slower, and some good ones can be rather quiet.

Jon