what is the main difference in the two. why does everyone say there swapping to obd1. just answer in the simplest terms.

 

One is older and one is newer.

OBDII is far more advanced and has more diagnostic features.

OBDI is good for tuning

 

The reason people switch to OBDI is because OBDI ecus can be chipped with a flashable eeprom. This allows one to run custom fuel maps based on your setup. OBDII ecus are harder to chip and program which is why people switch.