First off, I want to talk about the different kinds of engine management systems available for the Subaru Impreza WRX STI.  To do this, we need to understand how the factory set it up and what abilities the factory engine control module has.

The Stock engine control module is a fully capable unit with a multitude of engine sensor inputs to deal with emissions, performance, economy, and safety.  It is capable of running in “closed loop” which means it uses the car’s oxygen sensor feedback to modify the air fuel ratio in realtime, in addition to the normal inputs of engine load and rpm among other things.   It uses closed loop for the majority of the time you’re driving – at idle, cruising on the freeway, putting around town at very low throttle inputs.  This mode is generally configured to yield economy and low emissions.  However, when the driver wants more, he communicates this to the car by depressing the throttle and the ECM responds in kind by switching to an open loop mode.  Open Loop refers to the idea that the oxygen sensor feedback is no longer part of the loop in determining the parameters to control the engine.  The advantage of this is speed, in the computational sense.  Running in closed loop means applying fuel, checking the O2 sensor, adjusting, checking again, and so on.  This takes time and although it’s pretty good, we don’t want sensitive things like ignition timing and air fuel ratio to be anything other than accurate when we are accelerating hard and loading up the engine.  So to prevent this lookup/check/adjust cycle, the ECM just relies on preset lookup tables referencing the engine load and engine speed for its main parameters.  Of course, this is not all it does.  There are literally countless tables of information in the stock ECM’s memory which reference all kinds of things to improve the drivability, response, and safety of the engine.