From the dyno plot below, you can see that not all Imprezas are created equal. These tests were done within 30 minutes of each other on the same car - no other changes, only the ECU. Both ECUs are standard original equipment on the same model of car. This car is entirely standard with no modifications.
The two sets of lines on the graph. The lower line is 'power at the wheels' whilst the higher line is 'power at the flywheel'. Swapping an AE802 ECU for an AE801 on this entirely standard car caused the power to rise from 202 to 217BHP !
Checking your ECU Version
The ECU is situated in the passenger footwell of the car, whether RHD or LHD. To check the version of your ECU, simply remove the carpet clips holding the carpet down in your footwell. This will reveal the passenger footwell kick plate. This must now be removed by unscrewing the 4 retaining screws. Lift off the kickplate, and there is your ECU. You can now read the version number from the top of the unit.
When viewed externally, the ECUs may be split into two categories:
- Right Hand Drive - RHD ECUs are labelled ae80x, where x is the version number.
- Left Hand Drive - LHD ECUs are labelled ae78x, where x is the version number.
The physical mountings of RHD and LHD units are a mirror image of each other. Other than the mounting differences, and ae78x is functionally equivalent to an ae80x. I.e. for the same value of 'x', the circuit board, software, maps and pin connections are the same. That we have found so far, any version of either RHD or LHD ECU will work in any car, with varying performance results.
Note that STI ECUs will not work in standard cars due to wiring differences, so don't try it. You Turbo/WRX/GT car is also missing the air temperature sensor which STI rely on in their maps.
Aside: ProDrive ECUs for these model years of car were only warrantied in the UK. I have spoken with people who have ProDrive ECUs in mainland Europe, and it appears that the brackets have been adapted from RHD to LHD for sale there. I.e. A left hand drive ProdDrive ECU was never made. Not that it matters though.
The following list may not be exhaustive. If you have seen an ECU variant that is not on this list, please contact us so that we can update the information shown here. We have not yet tested an ae780 or ae781, but from the table below, you can see that the pattern is obvious:
|Version||RHD ECU Name||LHD ECU Name|
|1||ae800||ae780 (to be confirmed)|
|2||ae801||ae781 (to be confirmed)|
The software in an ecu can be split into two categories
- Code - This is the program running inside the ECU.
- Data - This is the maps that determine engine operation including ignition timing, fuelling and boost maps.
There are two code versions that we have seen so far. The ae800 has the earlier version, whilst the ae801 & '802 share a newer code version. The differences are fairly minor.
The are two main versions of data, and they are not paired in the same way as the code versions. The ae800 and ae801 share the same basic mapping data. The newer ae802 has very different maps. There are significant differences in these maps, which goes a long way to explaining the perceived performance differences between seemingly identical cars. The section that follows, highlights these differences.
The newer ae802 runs richer than the older versions. Since the early versions already run rich, this extra fuel will reduce power. The increased fuel will however increase cylinder cooling and move the knock point even further away.
The ignition maps on these ECUs differ. All ECU versions have the same high octane ignition map. However, on the more recent (ae802) version, the low octane ignition map is more retarded at high load and high RPM.
Boost control on Imprezas uses two main maps. Both are 8x8 in size, though their function is different. These maps are known as 'boost achieve' and 'duty cycle aim'.
The boost maps in these ECU versions are significantly different. In the '802, the maps are progressive. I.e. the higher the engine load, the higher the desired boost. This contrasts with the constant desired boost (for a given RPM, regardless of load) for the '800/'801. Also, note that the RPM scaling maps are significanty different for these ECUs, meaning that the actual RPMs that each row corresponds to differs between ECUs.
The boost achieve map specifies the desired shape of the boost curve. The map is two dimensional, and so desired boost can be varied according to both RPM and engine load.
Duty Cycle Aim
The duty aim map specifies the 'best guess' duty cycles required to produce the desired boost curve, specified in the boost achieve map. This map allows the ECU to take a guess at the required actuator duty before closed loop control homes in on the boost actually required. Get your aim map too high, and the boost will overshoot. Get it too low, and spool up will be impaired in lower gears.