You do need to be careful with a cracked exhaust. When an engine is turned off outside air is pulled through the tailpipe all the way into the engine, but because the complete tailpipe section is hot the air is heated and by the time it reaches the engine they are both about the same temperature. When air can get in closer to the engine it may not have time to heat properly and this can cause a HUGE problem if it's very cold outside. Cold air on hot valves can and will cause valve warpage. Just asked the "old timers" that used to run their hot rods with open headers.