As far as compressors go, you really cannot beat the volume or pressure put out by an engine-driven air compressor. This type of compressor can be bought or even built from an old A/C compressor, and runs off your serpentine or V-belt pulley drive system on your engine. It puts no amperage load on your battery, and a unit like this York unit from Kilby Enterprises (below) can exceed 250 psi if so desired (your air tools, lockers, and air tanks will not handle this much pressure). All engine-driven compressors we've ever come across are 100 percent duty cycle by nature. Since they are run right off the engine, the amount of air pushed out is directly proportional to how fast the engine is spinning. At idle, an engine-driven air compressor will put out less than an electric compressor, but once the rpm (of the engine) are ramped up, so is the compressor air output. At 100 psi, a unit like this York 210 will put out anywhere from 3 1/2 to 8 cfm, depending on your engine speed. No onboard electric air compressor we can think of will do that.
The downsides to having an engine power your compressor? They will require modifying your engine's pulley drive system, plus you will need to have the engine running when you're trying to use it. If you can't start your engine, then you can't run your compressor.
Air Tank Think of an air tank as a surplus or reserve for your air system. An air tank will fill up via the air compressor, and give you a pressurized volume of air that you do not have to wait for the air compressor to pump out. For some duties (not small ones like engaging air lockers) you will use up the surplus air in the tank before the compressor has a chance to refill it. At that time, you are then running straight off the compressor to power whatever you are powering.
This new 3-gallon air tank from 4WD Products has four 3/8-inch NPT ports on the side, three on the end of the tank, and one port on the bottom. If you need more ports than that, then maybe you should be running two of these tanks together.
Pressure Switch Whenever you're running an air tank with an air compressor, you'll need to use a pressure switch that will shut off the air compressor once it gets to a certain pressure. This will ensure against any unfortunate happenstances like compressors melting down, breaking, or worse. Make sure that the pressure switch has a lower shut-off pressure than the maximum air pressure of the air compressor.
Check Valve You should also have a check valve whenever running an air compressor and air tank together. This is basically a one-way valve that allows air to flow one way through it, but not the other (in opposite direction). This will prevent any feedback in the line, and will hold the tank at pressure.
Distribution Block This 6+2 universal manifold from Powertank will allow you to split your air system from just one line to multiple lines. Maybe you have more than one air tank, two compressors (one for backup), or want to run everything off your Powertank. You'll need a distribution block like this one to plumb all the lines together. This particular one has six 1/4-inch NPT ports and two 3/8-inch NPT ports.
Super Couplers These are super couplers from Powertank, and are worth every penny you spend over the price of a typical air coupler. The super couplers accept both automotive and industrial air fittings and have a black sliding sleeve that acts as an on/off for air pressure. With the sleeve slid down, no air will flow past the coupler. Slide the coupler up, and air will flow freely. Shown here (from left to right) are the male super coupler with black sleeve down (air off), the female super coupler with black sleeve down, and a female coupler with the black sleeve up (air on).
Air Tool Oiler This is an air tool oiler. The safe rule of thumb is to oil an air tool every time you use it. Do you do that? Nah, we don't either. But oiling an air tool will provide it with a much longer life with better reliability. Powertank has this inline air tool oiler that automatically puts oil into an air tool as it is used.
Pneumatic Switches (like for an ARB!) For the ultimate in reliability and performance, some individuals choose to cut out any electrical mumbo jumbo from their ARB Air Locker systems, and instead just run fully pneumatic switches. This is a great idea, as long as you do it right. This air toggle switch from Powertank has fittings (for the 5mm blue ARB line) for an airline from the compressor to the air locker, and one for exhaust (air exhalation when you shut the locker off). Powertank recommends that you run a line to the exhaust fitting and run it to the outside of your cab so you do not get a gear oil smell when the locker blows off.
Pressure Release Valve It's important to run a pressure release valve inline to your ARB Air Locker if you are using pneumatic switches. The valve will blow off pressure at 125 psi, keeping your ARB Air Locker safe. Running a straight line to the ARB (without a blowoff valve) could potentially allow the pressure in the air locker to exceed 125 psi, subsequently damaging the air locker.