The term “piggyback” is used to describe an engine management solution that works in conjunction with the stock ECM.  The most popular piggyback on the market for STI’s is the TurboXS UTEC.  There are some inherent strengths and weaknesses with a piggyback system, especially with the UTEC.  Namely, the UTEC offers launch control and static timing.  Some would say the static timing is good because of consistency, but I would contend that it is an absolute flaw because it offers little to no protection against knock if something happens that puts your tuning into the wrong side of things.  The UTEC will knock every single pull if you let it, whereas the stock system will knock once, realize it, and then not even attempt to run that timing combination again until much later, hopefully by which time, you’ve fixed the problem.  This highlights the inherent strength and intelligence of the stock ECM – its ability to learn and adapt.  Piggyback solutions are supposed to be cheap, easy, and fairly effective, but the UTEC is expensive ($1000), difficult (there really is not such thing as “easy”), and it is effective, but only if static timing or launch control is something you desire. 

Other piggyback solutions include the XEDE from Vishnu Tuning and Unichip from Dastek.  The XEDE is even more expensive than the UTEC if you want its best features, which is the adaptability upgrade known as SMART which isn’t even available yet.  The single most annoying thing about piggyback systems though, is due to the inherent design flaw that makes them ‘piggyback’ to begin with.  They utilize the factory ECM’s tuning for closed loop operation and switch to their own internal tuning for open loop operation.  This results in a persistent and impossible-to-tune-out hesitation at varying load/throttle positions where the system switches from ECM control to Piggyback control.