Piggy-back Fuel Controllers
Piggy-back fuel controllers are popular because they allow users to modify stock fuel injection without replacing the entire ECU. They operate in various ways. Some of them modify the injector duty cycle control signals as they travel from the ECU to the injectors. Others modify input data to the ECU (like MAF for example), effectively “tricking” the ECU into delivering more or less fuel at a given RPM. Regardless of how exactly the piggy-back works, you’ll want to measure the results of any modifications you make to your fueling map.
Even if you can’t tune the piggy-back (some are fixed and not user programmable), you’ll want to make a log before and after you add the piggy-back. This helps you evaluate what exactly it’s trying to do, and avoid dangerous lean conditions or wastefully rich conditions. If you can tune your piggy-back, use the LM-1 to carefully tune every cell of your fuel map.
Here are some links of the more popular piggy-back fuel controllers:
APEXi PowerFC & AFC-II
HKS F-Con and AFC