Often times, car manfacturers will program the stock ECU with a known amount of “play”. This amount of play allows the manufacturer an extra level of safety and/or the ability to utilize this at a later date so they can have an increase in HP in later years. An example of this is where a 2001 car has 200HP and a 2002 car has 215HP. Some manufacturers couple this with additional parts to get increased HP levels, but this should give you some idea of the concept. From a marketing and sales aspect, this ensures that the public will continually be interested in the new year models, even if the body style does not change. While this FAQ is not about the science and art of auto sales, this may give you an insight as to why the stock ECU is not 100% perfectly tuned from the factory.
What are the least talked about benefits of engine management? Engine management solutions to one degree or another can reduce or eliminate black tailpipes, improve drivability throughout the powerband, eliminate the open/closed loop delay in 04+ Subarus, and increase MPG.
What Subarus NEED engine management? While every Subaru will benefit from engine management, the 04+ turbo models (except STI) “require” it. The reason for this is the EPA mandated greater restrictions on ECUs for 2004+ and allow manufacturers to exclude one model. Subaru chose the STI as the exclusion, so the restrictions are not on the STI. The restrictions have to do with the open and closed loop fueling. To put the restrictions in a nutshell without being technical, there is a delay between open and closed loop fueling that can allow your 04+ Subaru to run lean during this crossover point in how your car gets fuel. Lean is dangerous as it produces detonation which is the #1 factor is blown pistons. In stock configuration, 04+ turbo models are fine, but modifications such as exhaust work or higher really necessitate the use of engine management as those mods cause the fueling issue to rear it’s head. Yes, you can run certain mods for a short time until you get engine management, but you should never kid yourself on 04+ turbo models that bolt on modifications are fine without engine management.
What about manual/electronic boost controllers or air fuel controllers?
A manual/electronic boost controller or air fuel controller really isn’t engine management. MBCs are fine when used correctly on 02/03 WRX and 04+ STI by a judicious user. The issue gets further clouded on 04+ turbo models less STI, due to the open/closed loop delay. One can lump in the mix air/fuel controllers as well for the same reason. Usually those two devices are used by either cheap skates, n00bs, or advanced users who combine them with other forms of real engine management.