Horns are a safety item. Generally, when I honk the horn I want the person I am honking at to hear me. The amount of energy which is transported past a given area of the medium per unit of time is known as the intensity of the sound wave. Stock hrons are labeled as being 110dB at 12V, which while fairly loud is still orders of magnitude less in intensity than 118dB.
Hella Supertone 118dB horn vs stock Z31 horn:
Horn positioning on the Z31 is in a very shrouded area. The horns are mounted on the front structure of the car and direct their sound straight at the bumper, which is entirely inside the bumper cover. Simply relocating stock horns to a less obstructed location will make them sound louder, but the supertone horns will be even louder when relocated by the same method.
Extension bracket mouting the horn further down from the stock location, and angled forward and slightly down for maximum sound without more extensive mounting fabrication.
Stays of obstacles by being higher than the front spoiler:
Be sure to wire in the relay trggered by the stock horn switch. Otherwise, you have more load than the circuit is designed for and the horns will not have enough available voltage to sound correctly and they will actually be worse sounding and lower intensity than stock horns. Don't believe me? Try it for yourself.