How do you make flames blow out a mustangs tail pipes?

You can't reliably make flames shoot out of a Mustang's tailpipes in a way that's safe and controlled. The idea of "flamethrowers" on cars in movies is usually achieved with special effects.

To get any kind of flame, you'd need an extremely rich fuel/air mixture (way more fuel than normal) and a significant backpressure. Even then, this is incredibly dangerous:

* Extremely dangerous: Doing this could severely damage your engine (catastrophic engine failure is possible), cause a fire, and injure or kill you or bystanders. The backpressure required could blow out gaskets and other components.

* Illegal: In most places, modifying your exhaust system to intentionally produce flames is illegal and could result in fines or other penalties.

* Unpredictable: The timing and intensity of any flames would be virtually impossible to control.

In short, don't try this. It's a significant safety risk and a recipe for disaster.