I have 3 Grover horns on my Expy. A dual pair of 13" horns paired with a 30". I have a small 1-liter tank and a vacuum-operated compressor from an old air-ride system. An electric solenoid wired to a button next to my driving position to activate it. It has saved me a time or two. Like when a dump truck was backing out of a driveway and didn't see me. He sure heard me though. When I hit the horn, he hit his air brakes. Another time, I passed a truck on the freeway and, for some reason, he fwapped his horn at me. I just fwapped him right back and continued on. My tank and compressor are mounted on my firewall and the horns are behind the grille pointed straight down. I painted them flat black so they are not so obvious.