ARD owner and tuner Robert “Lucky” Arnold writes
When I started tuning years ago the only thing approachable was using a piggyback ECU to adjust fuel and timing on my LH fueled RWD Volvos. Of all the cars I put piggyback on, my 242 was my favorite and after the swap from the mechanical K-jet injection to the electronic injection of LH2.2, I was ready to start playing!
Increasing the boost by adjusting the wastegate, or in my case using a manual boost controller, was the easy part and instantly netted a nice power increase. However it was over fueled badly in upper RPM’s and detonated in the midrange, since the power increase was substantial the ignition computers default timing map was now too aggressive for my setup. So a piggyback was the answer!
To understand the use of a piggyback ECU or a SAFC (super air flow convertor) we need to understand a bit about the factory ECU. In this case we’re talking about the LH versions of Bosch motronic fuel system but ultimately this applies to just about all electronic engine management systems. The ECU receives a varying voltage from the MAF and converts that to a particular air flow value. From this information and other inputs like RPM, coolant temp, and throttle position, load can be calculated. With load known the ECU can look up a table to compare the load and RPM and apply a proper amount of fuel. The ECU then compares the projected air fuel ratio, load, and RPM to calculate an optimal ignition angle and then fire the ignition coil to make a spark that will ignite the mixture. This happens incredibly rapidly and hundreds of times per second.