A loud low-pitched train air horn requires a huge volume of air to operate it. Beside an air compressor you really also need a small 2.5 gallon or so air tank that can store that volume until needed by the air horn. I had a big low-pitched train horn on my previous TJ and it worked well off a 2.5 gallon air tank connected to my York-based OBA system.
For the OBA system for refilling your tires, buy the compressor with the highest CFM (cubic feet per minute) rating you can afford. Cheap air compressors will brag about their high PSI ratings which is meaningless. A tiny air compressor can produce high PSI ratings but will take all day to fill a big Jeep tire. I'd rather have a compressor with a 100 PSI pressure rating and 3-5 CFM rating than a compressor bragging about a 300 PSI rating but with only a .5 CFM rating. A 3 CFM compressor with a 150 PSI rating will refill a tire 6X faster than a 300 PSI compressor that only has a .5 CFM rating.
And cheap compressors will hide their low performance with a misleading (fake) CFM rating. For example, Smittybilt sells a compressor they advertise with a huge 5.65 CFM rating. The thing is about that rating is they're giving you a misleading CFM rating. For a CFM rating to be meaningful, the CFM rating must include into what air pressure it can provide that CFM into. Smittybilt's 5.65 CFM rating is into 0 psi of pressure. That means into open air. Your lungs can provide 5.65 CFM of air flow into open air. Believe me. But can your lungs provide 5.64 CFM into a tire that is resisting back? OF course not. A meaningful CFM rating will be given into something like 30 psi, like into a tire. Smittybilt's 5.65 CFM rating falls off to below 1 CFM into 30 psi.
What you want is a compressor that can put out close to a couple CFM into 30 psi. Quality compressors like from Viair always give their CFM ratings into a given PSI. Like Viair's 450C compressor puts out 1.52 CFM into 30 psi per their chart at
http://www.viaircorp.com/c-models/450c-compressor/ which is decent but not exceptionally high. I'd want nothing lower than that for refiling aired down Jeep-size tires. My York put out 7-8 CFM but then that's a big-ass compressor that is belt driven from the engine. An electric compressor capable of putting out close to 2 CFM or better into 30 psi would be fine.
And the lower the CFM rating, the more important the compressor's duty cycle becomes. The duty cycle is how long it can run without shutting itself off from getting too hot. Compressors get HOT from compressing air and the lower the CFM rating, the longer it has to run to run to fill your tires which means the longer the duty cycle has to be. There's not much worse than a compressor that keeps shutting itself off repeatedly from getting too hot from refilling your tires.
For electric air compressors, I'd stick with Viair, ARB, or similar quality. I personally would avoid cheap brands like Smittybilt.
That should help some....
