Not to be a bummer, but the loudness of the horn will be limited by the max amperage of the relays. If the horn relay maxes out at 15 amps for 12 volts, you get about 180 watts of energy to the horn. Most horns are just a coil that vibrates at a set frequency to move the rubber or vinyl diaphragm inside the horn. That’s the noise.

Short version, you can put really any horn in there, but the total amount of watts will be the limiting factor. A more efficient horn (aka a new one) will make more sound for equal input wattage.

The other option is to make an array of junkyard horns and set them up with more relays to lift the limit. You’d need to have multiple fused connections unless you just want to be a hillbilly and run them right off the battery with a giant ass relay.

You might start a fire but it will be loud as shit for a few minutes before it burns down.

Find one that plays “La Cucaracha”!

Don’t do a train horn. As someone with ptsd and a heart condition, those things are an absolute menace.

The cars that I’ve owned with loud horns in order of loudest:

1970 Chevrolet Impala / 1969 Chevelle. I think any late 60’s to 70’s GM car will have a loud horn.

2003 Acura TL. This car’s horn actually surprised me. It was very nearly as loud as Chevys’.

2019 Toyota Camry XLE Hybrid: My current car and it has a surprisingly effective horn. It’s a lot louder than my previous car which was a 2014 Camry LE.

I’ve seen a hidden camera clip where they installed first a truck horn, and later a large train’s horn on the underside of a very small car, something like Mercedes A class or Smart.

Two innocent looking Japanese ladies drove that car, creeped up behind a car stopping at a traffic light, and HOOOOOOONK!!! all while looking completely unconcerned.

In the UK it’s illegal to have a car horn that operates at more than a single pitch/tone, but there didn’t seem to be a limit on volume… laws be wild.

https://www.youtube.com/watch?v=1kdbH40abnY