I know this is a train horn forum but I don’t know where else to ask questions about semi truck horns. I have an FJ Cruiser and want to get a semi horn for my truck, the only problem is that I don’t know what compressor I should get (I’ve been looking at the Viair 400c 150psi compressors), but don’t know if I need a 2.5 gallon or 5 gallon tank for this. I only want to use one horn (or i think most people only have one horn for a semi sound, correct me if I am wrong) and also will a 5 gallon tank (if I need it) fit under the car? Lastly, where do I get a compressor switch (does it come with the compressor?) and a horn botton.
a 2.5 gallon will be plenty for one semi horn… it works well for the 4 horn set too. i had a grover semi horn myself, all though they are no where near as loud as the black 4 horn set hornblasters sells. if your looking to scare people or really move people out of the way i would look into the 4 horn kit. the kit is not big rig sound but still has a deeper sound to them and are perfect sound for a big truck!
I don’t think you’re gonna have enough room for a 5gal tank under an FJ. Most semis use at least 2 horns. I would agree with Version; go with a HornBlasters kit. Unless you already have a set of horns. Before you buy anything crawl under your ride and see how much room you have. You’ll find tank and horn dimensions on HB’s page.
Ok yeah I think I will go with two grover horns. Does anyone know where I could find a set? I’m new to this so will I need bigger than a 2.5 gallon tank for two horns? Also I have looked at tanks and what is the advantages of more ports on the side of it?
When mounting a compressor and tank under the car, is there a certain way I should go about mounting it? E.g. upside down, sideways.
ive got a hadley 24" semi horn, its not crazy loud but its legal 
5 gal for even 2 grovers is overkill, look for some grover stuttertone horns they use on firetrucks
Look on ebay for semi horns. More ports are for easier suspension bag installation. Never install a pump upside down. Make sure you mount your tank with a port facing down so you can drain condensation.
i bought my grover from a peterbuilt dealer in their parts shop for $90 a horn
Ok yeah I might go to a dealer to check that out. I found these horns online:
They have a 28" and a 24" so you can mix tones. Would I just join the hoses that go to each horn and then to the tank?
dont get that thats a no name brand horn you want either a hadley or a grover
mine was a 24" grover stuttertone you can hear it in the intro of my video http://www.youtube.com/watch?v=rIXcM1Vegj8
Ok thanks a lot for the help guy.
Oh nice thats sounds awesome, hahah great video too 
my semi horns
hadley:
cheap set off ebay, very high pitch but loud!
its the 2nd horn in this video
http://www.youtube.com/watch?v=Y3fvcMPZgFg