Do i need to change over the ecu from my subaru legacy 1991 sadan if im changing from auto to manual?

Yes, you will need to change the ECU when converting your 1991 Subaru Legacy from an automatic to a manual transmission. The automatic and manual ECUs are programmed differently to manage the transmission functions, so using the correct ECU ensures proper engine performance and transmission compatibility. Additionally, you may need to address wiring and sensor changes associated with the manual transmission installation.