The 20R engine is a cross flow design and does not preform well without back pressure.
Back pressure is needed to slow the air flow and produce more power.
I recall removing the factory cat and driving my '80 with the open exhaust. It had no power.
Marlin
Wierd
My 80 came with no cat (alaska truck) and small exhaust tubing. I replaced the stock manifold with a long tube 4 to 1 header, glasspack and 2.5" tubing, and it really woke the truck up! It even had very nice low end response, which actually went away when I stuck a 2.25" glasspack in the system to quiet it down for deer season.
