In the mid 1980s, Boeing used a company-owned 727 to test a new engine for a proposed twin propeller-turbine transport that was to be designated the Model 7-J-7. A prototype General Electric GE-36 UDF (unducted fan or propfan) engine was mounted on the right rear side in place of the standard turbo jet engine. The UDF engine was a hybrid between a turbofan and a turboprop. It had a significant weight reduction and a 45% increase in fuel efficiency, however, Boeing did not develop the 7-J-7 that was used to test it.