Car won't stay running after new cat con installed

Both. It ran fine before I parked it and after I put a new battery in it and drained all of the old gas out. I never had any problems with it. I drove it around sounding like a helicopter an it ran fine, but as soon as the converter an muffler, etc was installed, it won't make it two blocks.

Try loosening the exhaust to create a leak and reply with results from that 'test'.