Originally Posted by SouthArk370Z

Not sure why one would want to do this but:

Flames out the tailpipe are the result of having flammables (primarily unburnt fuel, elemental carbon, and carbon monoxide) in the exhaust gases. These will ignite when they hits the oxygen in the atmosphere - if they are hot enough. If the gasses have cooled below the auto-ignition point when they reach the exhaust exit, there will be no flames.

Cats turn the flammables into carbon dioxide, which doesn't burn.

So, if you want flames, ditch the cats and go with short pipes (less cooling). Detuning the engine to run rich will "help."

Keep in mind that if you are running rich and get air into the exhaust you have a severe fire hazard. Plus there's the risk of drawing carbon monoxide into the cabin. All-in-all not a great idea but it's your car/life.