Anybody make a homemade exhaust flame thrower?

I've seen and done them a bunch of different ways. Some things to keep in mind. What you are looking to build is a "choker flame". I would not suggest doing anything pressurized. The guys that do that are pros. You need a spark source. Those boxes that are sold do work. Basically it will cut the ignition and fire the spark plugs that are drilled into the exhaust tailpipe. At the same time you will need to choke the engine to get the fuel source. Then poof you get the short choker flame. Chambered mufflers don't work well. You'll blow them apart and have issues getting fuel to them.
And since you are creating a crazy fat fuel condition you need to keep up on oil changes. Keep in mind also the fact the stress you put on the engine doing this. I wouldn't do it to an engine you like of have a ton of money in.