Mike,
It's been done already.
A few years back here in San Jose there was a Gold Wing rider with a set of trumpets on his bike. He had 2 of them mounted on the left side and the large one mounted on the right. The big one was about 3 feet long and maybe 10 inches dia. at the noise making end. He fed them with a custom 3 gallon tail dragger air tank.
I met him at the Santa Clara Convention Center at a machine tool show. He parked his bike next to my little Ascot. He came out when I was getting ready to go, we got to talk a little bit. He said they were from a diesel train locomotive, he had friends in the railroad industry. The air tank was good for about a 5 second blast. Recovery took about 45 seconds with his on board air compressor. The compressor size limitation was due to the available amps on the bike.
When we were leaving he lit them off. :scare: We were on the ground floor of a 3 story parking structure but you'd swear that Union Pacific had a set of tracks running through there! :sleepy:
I always wanted to get a set up like that but it would be kind of harry in the turns. Those suckers add a lot of weight!
Brodie