Q: What is cause of the well-known problematic delay with the USDM 04/05 WRX from the factory? Are the delay values too high? Why doesn't the USDM 02/03 WRX have the same issue?
Note: The following examples apply to the USDM ECUs.
High delay values are not the issue with 04/05 WRX. Depending on which delay value is being used, the 04/05 WRX may actually have the same or lower delay values than the 02/03 WRX. For example, looking at the delay examples above, for a manual transmission around sea level, the selected delay groups would be the following:
USDM 02/03 WRX - 122/732/366/3662
USDM 04/05 WRX - 366/732/366/366
The delay value chosen depends on the time since engine start. So, after about 8 minutes, the last delay value is chosen for both ECUs, which is going to be the delay value used most of the time when we are driving (unless you are taking extremely short trips). As you can see, the delay value is much higher in the 02/03 WRX (3662) than the 04/05 WRX (366)
So what causes the known issues? The problem is how the 04/05 WRX has the 'CL to OL Transition with Delay (Calculated Load)' and 'CL to OL Transition with Delay (Throttle)' setup from the factory. For the 02/03 WRX (and all other 16-bit ECUs besides the USDM 04/05 WRX), the load and throttle thresholds are referenced against rpm in a 2D table. For example, with the 02/03 WRX, the calculated load threshold is 5632 from 0-3600 rpm and zero at 4000+ rpm. It is similar for the throttle thresholds (69.4% from 0-3600 rpm and zero from 4000+ rpm). This means that above 3600 rpm, the threshold for load and throttle will begin to drop (due to interpolation) and will have zero threshold at and above 4000 rpm. However, with the 04/05 WRX there is only a single threshold for calculated load and throttle. Although the single threshold matches the lower rpm thresholds in the 02/03 WRX (5632 load MT, 69.4% throttle), there is no drop off above 3600 rpm since they are just single value thresholds and are not referenced by rpm. That means that, even above 3600 rpm, the 04/05 WRX will maintain the same thresholds for load and throttle until one of the "misc." tables clears the delay (rpm, veh. speed, etc.). The 'CL Delay Maximum Engine Speed' table in the 04/05s also has higher thresholds (5100/5200 rpm) as compared to the 02/03s (4200/4300).
So, in the 04/05 WRX, this can result in remaining in closed loop longer from over 3600-5200 rpm, whereas with the 02/03 WRX, the transition with delay would occur much sooner (lower or zero load/throttle thresholds) or there would be no delay at all (above 4300 rpm due to the 'CL Delay Maximum Engine Speed' table).