A lot of guys over think OBA, and miss some of the finer points...
FIRST, ASSESS YOUR NEEDS!
Do you just need one for an occasional trail repair, airing up your tires on the trail, ect.?
Or do you plan to use it for hard core automotive repair?
How big of an air tank(s) do you have room for?
How versatile do you need your system to be?
If you plan to just air up tires, and power the occasional air tool on the trail, the you will probably be fine with one of the larger volume electric compressors.
We're not talking the 'Wally-World' cigarette lighter compressor, but an actual industrial quality DC powered, oil-less compressor...
The good points of electric are, EASY TO INSTALL!
These things are a snap to bolt in and wire up!
No oil in the compressor means no oil crap you will need a filter for!
Larger tank volume will make up for a somewhat smaller displacement of the electric compressor.
Another large advantage is you don't need the engine running to make pressure.
These little compressors are easily portable, so they move from your off roader to your tow vehicle, to your daily driver pretty easily.
If you intend on larger projects, like a 'Repair Vehicle' for a club, or if you feel the need to power things like DA sanders or spray guns, then it's a good idea to use an engine driven compressor.
The advantages of an engine driven compressor are, they come in larger output volumes.
Depending on the compressor you use, you can power a high volume device continuously.
Since these belt driven compressors will need oiling for the pistons and crankshaft, you will need an oil separator if you intend to do anything but manual labor work with it.
Assess your needs, decide how much work you want to do on mounting and maintenance, and go from there.
Secondly, If you have a York A/C compressor, you have a very good start to things!
There were a TON of older AMC vehicles that came from the factory with York A/C Compressors, and they are (arguably) the best way to go if you already have one.
Since York is the ONLY A/C Compressor I know of that has an oil bath crank for lubrication, it's about perfect to convert to an air compressor, and if you have one in place now, or have access to one and all the brackets for you engine, it's a VERY good start.
You can use other types of A/C compressors, but you have to use a 'Pre-Oiler' so the piston and cylinder(s) get lubrication, and then you have to use an oil separator to get the lubrication oil back out of the air stream before it gets to the tank and plugs things up...
A York has a 'Wet' Crankcase, much like your engine, and is self lubricating.
This means you don't have to introduce oil directly into the incoming air stream, so there isn't an excessive amount of oil coming out of the compressor.
If you have a York, and your belts line up already, then by all means use it!
There are larger compressors than the York, designed to supply large volumes of air in the pressures that are usable for hand tools.
These are expensive, you will have to design and build mounts for them, you will have to get pulleys to line up so the belts will run true, and you will have to build the air system from scratch when you use them.
I would only use this type of compressor if it was on a dedicated 'Rescue & Repair' vehicle.
With reasonable storage volume, there simply is no need for something that big on a trail rig.
Electric compressors are a viable alternative for 'Hobby' off roaders....
Since that's what most of us are, it's a VERY good alternative.
Electric compressors can be mounted anywhere, so the valuable belt driven/front of engine space can be used for something else,
Electric Compressors are Very easy to install and wire up,
Electric Compressors are very cost effective,
Electric Compressors are very easy to move from one vehicle to another.
Electric Compressors take very little to no maintenance.
The one thing electric compressors are not is, High Volume, but you can make up for that with storage tank size...
Third, Give some time to thinking out your air supply lines/Air System.
Most guys cobble together a compressor from this, a tank from that, then spend a fortune in brass fittings, water pump switches that belong an AC water pumps instead of DC air compressors, and generally cobble up the install until the 'System' is barely functional.
With just a little bit of forethought and planning, you can come up with a rational, easy to understand and install, and very functional air system...
We'll start with the most misused component of any air system...
The pressure switch!
Most guys use a 'Compressor Switch' that is intended for an AC circuit, and is for a water pump (yes, most AC air compressors use a water pump switch)...
It's much easier and makes for a much cleaner install if you use a DC air pressure switch.
In the first place, it is built to switch DC, what you vehicle produces.
Secondly, it's designed for AIR PRESSURE, and it's not a 'Diaphragm' switch that was designed for water pumps.
A proper air compressor switch will have 'Thresholds', upper and lower limits...
Once it's turned on, it will air up the tank to say, 120 psi, or 135 psi, or 150 psi, depending on the switch.
When the system leaks off a little, or the air in the tank cools down from being compressed, and contracts a little, the pressure will drop some.
A true air pressure switch will allow for that pressure to drop some without turning the compressor on and off all the time...
On when the pressure is below 90 PSI, and off when it reaches 120 PSI.
Or on below 130 PSI and off at 135 PSI, or On at 135 and off at 150 PSI...
The second most common mistake is not enough reserve tank total,
Remember, unlike liquids, air tanks can be linked together just about any old way.
If air will go in the tank, it will come right back out through the same line...
No reason for gravity feeds or any of the other silly stuff you have to do with liquids...
So you can single line link as many tanks as you can get mounted in your vehicle to increase the total reserve!
If you have room for 5 one gallon tanks, then that's 5 gallons of air reserve!
And in case you don't know, 5 gallons is a respectable amount of air reserve on a vehicle.
If you have a 'Portable' or 'Lend' tank, you can easily double that, and reclaim the room if you need it for something else!
Third most common mistake is running rubber air lines like like a crazy person everywere!
There is a 'Hard Plastic' line used for brakes in big trucks.
(neither 'hard' or 'plastic', but you know how common terminology is...)
We call it 'Push Lock' line, and it is a great way to plumb things together.
The line is cheap, so if you pinch one, rub one through, or melt one, it's no big deal.
Push lock fittings are litterally a one hand, 10 second change, and you can reuse them over and over again.
This isn't something 'Experamental', this is the same line and fittings required by D.O.T. for big trucks, so they go BILLIONS of miles every year...
And every reasonable parts store (NAPA and the like) and every big truck parts store will have this stuff on hand everyday.
Fourth most common mistake I see is using a bunch of brass fittings to make a manifold.
Industral grease manifolds are a great way to get a manifold cheap that isn't made of brass 'T's and looks like a kid got in to a box of fittings.
If you are handy with a drill and tap, a simple piece of aluminum is a good way to make a custom manifold to fit your specifications EXACTLY, and look good doing it!
Remember, a drill bit is the cost of ONE brass fitting, and the pipe tap is the cost of ONE brass fitting...
So, for the cost of 2 'T's, you can have a custom manifold with a little work...
Fifth most common thing I see done wrong is underutilization of the air supply.
If you have the air, USE IT!
I use my air with a regulator. Regulator make your air system 20 times more useful and easy to use...
Set the regulator for your tire pressure, hook up the hose with a self locking tire chuck, and walk away.
The regulator will air UP AND DOWN with out you having to 'Babysit' with an air gauge all the time.
Set it for 10 PSI when you get there, and it will air your tires down to 10 PSI.
Set it for 35 PSI when you are done, and it will air your tires back up to highway pressure... All without the $150 'Air Down' tools, or having to babysit each tire with a pressure gauge...
My regulator allows me to blow up air mattresses, float toys, camp chairs, spray paint, ect.
My regulator allows me to pressurize my distributor so I don't drown out when in deep water.
I have it hooked to my transmission, transfer case, and axles now also, so my vital components blow bubbles under water, instead of letting in water and dirt/grit.
Don't forget a removable tank!
I use an under body storage reserve, but I also have a removable tank in the bed of the jeep most times...
You never know when you will want to help some one you may not want to get close too...
Or 'Lend' air to someone.
You can either pack around 100' of large bore hose, or have a removable tank you can drop off and get back later.
Since the tanks are dirt cheap at Wally-World, and you can get them out from under any big truck or trailer in the salvage yards, there really is no excuse NOT to make your air compressor a 'SYSTEM' with 'Lend' capabilities...
Here is a simple air system diagram that may help you figure things out...
I'm sure I missed some things, and I'm sure there will be people disagreeing with one point or another all over the place...
And in their particular situation, they may be correct...
This is just an overview of what has worked for us several times in the past, and on our current vehicles, and is a pretty good guideline of what an 'Average' off road vehicle might need.