These are wired into the car’s ECU loom by either and additional wiring loom or directly in to the cars wiring loom.
They work by modifying the signals that the ECU sends and receives.
Below are some of the types of piggyback ECU's avalible.
Apexi S-AFC/S-AFC II - This nice looking unit with a blue screen acts on the cars MAFS signal allowing you to trim fuelling +/- 50% at a number of RPM and throttle points. Can be used to install larger MAFS and/or injectors. It has no ability to alter ignition timing, nor any “fuel cut defencer” function. It is however quite cheap and is very popular worldwide.
Greddy E-manage – this is a relatively new system. Out of the box, it is a very basic Air Flow “trimming” device similar in capability to the old model S-AFC, with 5 screws on the front to adjust fuelling at 5 RPM points. With the optional software and harnesses it becomes a fully mappable piggyback capable of controlling ignition timing as well as fuelling. It also has built in drivers for 2 extra injectors, “fuel cut defencer” function, and an idle stabiliser. An optional pressure sensor allows it to operate when the cars AFM’s capacity has been exceeded. There are a few tuning companies in the UK who are currently familiar with the product.
Dastek Unichip - Commonly offered by tuners in the UK. This operates in a similar manner and offers similar capabilities to the E-manage. Some reports suggest it may be less able than other systems when it comes to dealing with larger injectors due to a limited range of adjustment.
HKS Fcon Pro V - This takes the concept further in that it is actually a complete management system installed in the manner of a piggyback. It is actually capable of running the car without the original ECU according to HKS. This system is used in many “big power” cars in Japan. Custom fit looms for many applications. Specialist (expensive!) set up required. It is also only avalible from 3 tunning companies in the UK
Stand alone ECU's
Full management systems are available, such as those from Haltech, Motec, Omex, DTA, GEMS, Apexi Power FC, Link etc. These completely replace the cars standard management system. They generally require a large amount of custom re-wiring and may require other changes, such as to the ignition system. They are getting more competitively priced now, and it may be possible to get one installed for less than £1100.