[chuckm]

I've never had this happen to me, but I've seen a backfire big enough to do it. A buddy of mine used to have an old car he could make backfire pretty much on command. We were driving under an overpass and I was in the rather, umm, exciting position of being the car behind him when he did his thing. In a quick sequence, he went from ~30% throttle to idle, which let it lug for a second, punched it to the floor, then back to idle. Unburned fuel made it through into a exhaust system that was a bit rusty (read: there was at least one hole eaten through), where it didn't remain unburned for long. Let me tell you, this underpass made a VERY impressive echo chamber and it was dark enough to clearly see the fireball erupting from the rear of the car. It sounded like being next to a double barrel 12 gauge shotgun.

If you know anybody with a welding rig, clamp it closed and weld it shut. TIG is best here, MIG second, and stick is just a maybe (and only if you have the right type of rod).