It should have O2 sensors at the manifolds and just behind the cat's, the worst thing that can happin with no exhaust, your running hard one day she getts a little hotter than normal, you shut her off and COLD air gets pulled into the exhaust, bends the valves, you tag a piston or two, total engine distruction. Or your drivin around one day while its raining, you park the car shut her off and suck water from the puddle you just splashed in sucked up into yer exhaust, I have seen both happen.

Otto