[chevyguyase]

It sure sounds like you have a clogged or restricted exhaust. When your converter came apart a piece may have came loose and and clogged your muffler/exhaust. If you can get a hold of a vacuum gauge (they are very inexpensive) hook into manifold vacuum, I prefer using the vacuum at brake booster hose if you have one. Watch the vacuum at idle and rev it up to about 2000 rpm if the vacuum readings start dropping off your exhaust system is highly suspect. You can also check for exhaust gas pressure by removing an 02 sensor and hooking up the vacuum/ fuel pressure gauge to the O2 bung hole. Running, the pressure should not be over 1/2 lb. Also leaving the Y pipe on and the 02s connected remove the rest of the exhaust and test it that way. Be aware that Cat. converters don't die, they are murdered, meaning something else caused the converter to fail. Rich mixture, spark plugs, wires. etc. Good luck.