"Subaru STi: The Difference of Standard ECU and Piggyback ECU"

Blue Subaru STi Front Left View

What is the difference between Piggyback ECU and the standard ECU’s on Subaru STi?

Answer: A piggyback ECU is just a computer that sits in between usually the airflow meter and the stock ECU, which intercepts the signal from the airflow meter and changes it so the stock ECU injects the correct amount of fuel. This is essentially a crude band-aid fix for tuning a modified car, and I wouldn’t do it to an STi. Save up the money and do it right, by getting the stock ECU re-flashed by a reputable tuner, such as Vishnu, Cobb, XS Engineering, etc.