You can determine the amperage draw of the horns by connecting it to the battery, with a multimeter (set to read amperage) connected in series in the positive leg of the circuit.

 

Adding a relay can't hurt and it would make me feel better if I were changing it out. Just be careful that you hook the wiring up correctly. If you reverse the polarity that horn will suck instead of blow

 

Well I used the old Mercedes horn as it has a 2 blade connection like the stock horn. It sounds at least twice as loud than the stock horn.
You have me wondering now about relays. Where can I pick one up and where would be a good place to mount it?
I did not check the amperage draw on this one so maybe I better do that before I go too far.

 

I don't know if I did that right with the picture but here is a shot of the horn installed. It's really not much bigger than stock but is a lot louder.

 

2 horns will need a relay. I ran two horns without a relay (for testing) and it did not work well. Definite overload.

Installed relay and horns work great. This is stock Harley horns on my fxds. One is a high tone and one is a low tone like on 60's and 70's cars. Nice and loud.

I wanted the cages to think they are merging into an old buick!

If you google 4 pin or 5 pin relay wiring, tons of info will come up. You don't even have to cut any stock wires, just ad more to your current system. My relay is under the seat, and the bolt down tab of the relay (plastic) is just zip tied to one of the frame tubes.

Best of all, everything looks stock. You can even get the stock color coded wire by the foot on fleabay (for mine it is one tan wire and one black wire).

Trigger or switch is your stock switch wire, power would be a red wire from the breaker panel or fused and direct to the battery, load would be your stock horn wire color and a ground to the actual horn(s), and then one other direct ground lead to battery or frame.

Now make it pretty with the proper connectors, some vinyl wire protector sleeves, a few zip ties, and your done! YD

 

Thanks Yankee Dog, this horn is loud and does get attention. It made my ears ring while riding after I honked at someone. I like the sound from the '57 low tone horn but there is only one blade connection. I think if I attach the ground blade to the body of the horn it will work. I'll give it a shot one day soon.

 

Ok, it took awhile but I finally got all the parts together and wired the old Chevy horn up with a relay. Nice. That horn is freakin loud. Now I'm thinking about how to mount up the other half of this pair..