The test vehicle was an Australian delivered Mazda MX5, 1990 model. 98 Octane fuel was used.
Firstly, it is widely accepted that the Japanese version of the MX5 engine produced about 10kW more than the Australian version. The actual reasons behind this are often based on conjecture and assumption - for example it is well known that Japanese fuel was far superior to that available in Australia in 1990. Hence it is likely that the Japanese engine was "detuned" for Australian delivery. This may be in retarded ignition timing, or reduced compression, to compensate for the low octane rating of fuel at the time.
The myth was that this detuning was all in the ECU. That is, one could get a Japanese-spec ECU from an MX5, plug it into an Australian-spec MX5, and immediately gain power.
This myth was easy to test, with an Australian MX5 and a Japanese ECU.
The dyno graph below shows the results (the highest peak power obtained from each configuration is shown):
As well as testing Japanese vs Australian ECU, I thought I'd test the Japanese ECU with some more advanced ignition timing, to see if that made a difference to power output.
As can be seen from the graphs, the Japanese ECU consistently admitted more fuel to the engine, however there was clearly no extra power to speak of. Although this graph shows an extra 1 kW or so, there is really not that much in it.
We do notice that advancing the timing by 5 degrees makes a benefit to midrange torque, however no real difference to peak power production.
The test was repeated using an Adaptronic ECU to control fuel and ignition timing. Both were tuned to achieve maximum power. No hardware changes were made to the engine, other than the addition of a MAP sensor, and in the green graph below, the removal of the airflow meter:
As can be seen from the graph, the factory ECU is tuned quite conservatively at the upper end of the RPM range, and an increase of almost 10% at peak power is achievable without any mechanical changes to the engine or exhaust system. Furthermore, gains when compared to the stock ECU are clear right across the rev range.
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