Why not just install a piggyback controller? Eric Hsu of XS Engineering says, "The bottom line is the only correct way to control your engine is with absolute control." A piggyback controller can do only minor adjustments since it uses a signal from the airflow meter and outputs a fixed signal using whatever settings you give it. The tuner doesn't have the control of where it will end up on the factory map in relation to what you are outputting. Therefore, the actual timing and fuel needed in part throttle acceleration and mid-shift acceleration may be way off. Another problem is how advanced modern ECUs have become. Manufacturers are implementing the use of wideband O2 sensors to give the fuel trim function more range. ECUs are so smart now that they can self learn the adjustments made with a piggyback. Therefore, when it comes to anything that requires an injector upgrade, you need to be able to manipulate all functions properly to get the right amount of fuel and timing.
The newer Evo and Impreza utilizes a flash-based ECU. As you can see a considerable amount of power can be extracted through tuning.
But why ROM tune? It sounds so difficult and bothersome. Kazuhiro says that the factory spends huge amounts of money developing a single ECU to match an engine. Since a production vehicle must operate in all conditions, the factory sets up an ECU to work in high altitude, sea level, summer heat, snow, high octane and poor fuel conditions. The factory takes this into account and builds an ECU to work accordingly. Many aftermarket ECUs have this capability built into it. The problem is that the data is standardized for all makes since stand-alones aren't really car specific. It is up to the tuner to tune for all different intake air temps, water temps and elevations. If all these functions are setup correctly then closed-loop control is effective, but this takes quite a bit of time and data. With a factory ECU all this data is already programmed in to match the engine. A skilled tuner can then come in and tune with less time set on building base maps or compensation settings. Similar to a stand-alone, the tuner can rescale ignition and fuel maps to match an increased range of rpm or load.
The misconception is that you're limited to stock parts on a ROM tune. This tune was performed on a SR20DET with upgraded cams, intake manifold, throttle body, MAF, injectors and turbo.
But the ROM tune isn't omnipotent. It has its limitations. It can only work according to the styles of sensors that the factory sets it up for. For example, you can't switch from a mass airflow sensor (MAF) to a manifold absolute pressure sensor (MAP). This puts a clamp on the maximum amount of power you can make before exceeding the capabilities of the MAF. This is where stand-alone ECUs are powerful. All the functions are open to the tuner: You can adjust fuel and ignition on the fly, different makes and types of sensors can be utilized, the number of injectors can be increased, no prior knowledge of factory coding is needed. For engines making big numbers or race engines running on the edge, this is great. But this comes at a high price. Many stand-alone systems cost upwards of $1,500. Higher end systems can cost $3,000 or $4,000 with the need for custom wire harnesses.
Units such as the Motec feature full control and compatibility on all platforms through user-set parameters. Tuners have many options for expandability with sensors and additional injectors. [Pic of Motec]
Now that we've discussed a little about ROM tuning, Kazuhiro went through the process of tuning an older style ECU. Since the process is the same for every Silvia chassis and generally with pre-flash Nissan ECUs, we had him tune an S15.
Potenza's S15 runs upgraded injectors and MAF all on a ROM tune.