There is a club in Pottsville, PA that has many members that have flame thowers on their exhaust. One guy explained it to me probably 20 years ago at a parade I was at. Pretty simple, crude, and well I thought crazy, but what ever floats your boat. Look up geargrinders car club. I don't think they have a web site, but they are on facebook. I'm sure you could get direction from someone there.
www.facebook.com/pages/Gear-Grinders-HRA/256363261047945