* Engine speed: Timing will advance at higher RPMs.
* Engine load: Timing will be retarded under heavy load.
* Engine temperature: Timing adjustments are made based on coolant temperature.
* Sensor inputs: Various sensors (e.g., oxygen sensor, MAP sensor) provide data that the ECM uses to fine-tune timing.
Therefore, there's no static "degrees BTDC" value you can use. Trying to set a fixed timing will likely damage the engine or lead to poor performance and drivability.
To check the ignition timing on a 1991 F-150 5.0L, you'd need a timing light and a scan tool capable of reading the data stream from the ECM to see what the actual timing is under different operating conditions. Even then, you wouldn't be adjusting it manually; any adjustment would require recalibrating the ECM. Unless you're very experienced with automotive electronics and have the appropriate tools and knowledge, this is a job best left to a qualified mechanic.
